Starting in February, 1999, I've been getting the pieces together to start an 8" f/7 telescope, probably to initially receive a Dobsonian or Springsonian mount (the latter is a Dob type with the focuser mounted in the center of the altitude trunnion -- the fixed eyepiece height is reminiscent of the Springfield equatorial mount, in which the eyepiece is fixed on the polar axis). Below, I'll document the expenditures and time investment -- this is my first ATM project, if one doesn't count a similar mirror I started to grind in 1977, to become a scope for my high school. At that time, I started work (and completed the 80 grit rough grinding) on an 8" f/7 that was to receive a Springfield mount. This time, I intend to finish the job, instead of leaving school after rough grinding.
1 box Plaster of Paris
1 mat of "field tile" -- 1" square unglazed ceramic tiles, bound together in a mat (as you might use for a non-skid throw) with silicone adhesive.
Total cost for these items: about $8
March 5, 1999:
Total cost for this item: free
March 6, 1999:
UPS was here while I was out doing the grocery shopping with my wife. While we were out, we also went back to Home Depot, and I got a piece of melamine laminated particle board and a roll of iron-on edge tape, for use in making a moisture resistant grinding shelf, from which I can just wipe or rinse grit, to go across the open top of the barrel.
8" Pyrex Mirror blank: $35 plus $5 shipping
Melamine laminate and edge tape: about $9
Tile nippers: $11.96
Pilot drill: $4.77
Wood screws: $2.27
Carpet runner: five feet for $3.75 (enough for dozens of mirrors)
I cleared a space in the basement for the barrel, brought the barrel in from the carport, cut the prelaminated shelf to size and put on the edging on the cut edge and the unfinished one, then fastened cleats on the bottom to keep it from wandering around on the barrel when I grind -- seems pretty steady now, though I'll probably have to put something under one side of the barrel to keep it from going "thooom, thooom, thooom" in time to my strokes -- the floor is evidently not very flat.
After getting that done, I got the mirror blank from its packing, cut a square of 64 tiles from the mat of field tiles, then got the nippers and trimmed the tiles to match the size and shape of the mirror. At the last minute, I remembered to put the center of the mirror in the corner of a tile, instead of the groove between -- I'm not certain it would make a difference, but I was able to offset things by a little more than the 1/8" clearance between tiles without upsetting anything. After trimming the tiles, I stuck them, still matted together with the little dabs of silicone, to a piece of shelf paper cut to the size of the mirror blank. I then cut a strip of the carpet runner (rubbery sheet material about 3/32" thick, with a glossy smooth underside) three inches wide and full strip width long (seems to be about 28 inches, give or take), wrapped that around the mirror and tile, and duct taped it snugly in place. Then it was supper time.
After supper, I mixed and poured a batch of plaster, which set up in about 20 minutes and got quite warm over the next hour (but never warm enough to be uncomfortable to touch -- I'd estimate it peaked around 115 F). As soon as it was firm enough, I smoothed it out (like troweling concrete), then as soon as it was hard enough to handle, I peeled off the carpet runner strip, turned it over onto a plastic bag, and slid the mirror off the shelf paper. There was a small amount of plaster on the mirror (got past the shelf paper), but it was still soft enough at that point to wash off the glass easily. Before sliding the mirror off, I pressed down on it for a moment, hoping to push out any serious irregularities on the back of the tool; my success was marginal. I think I'll plan to scrape it flat before I seal it.
Total working time for today: about six hours
I also got two pounds of 60-90 silicon carbide lapidary grit at a local store, to be used in hogging -- I'll keep them in mind; they have graded grits from 120 to 600 in five or ten pound tubs for very decent prices, and if I get it in mind to make a lot of mirror, a tub of each size and a larger container of 60-90 would go well, and likely be cheaper than ATM sources.
Ballast sand: $2
2 lb. 60-90 SiC: $8.50
Time spent shopping: about a half hour
Total working time: about 20 minutes
First step was beveling the edge of the mirror, to avoid chipping. This was accomplished very quickly with a power tool Peter Hirtle has in his garage; it's a spindle and plate with a machined beveling attachment on it; you put some wet coarse grit (I think he uses about 40 for this) on the metal beveling tool, lay the mirror blank into place, and then power up the tool while holding the mirror. In about ten minutes, I had a bevel between 3/32" and 1/8" wide, which Peter pronounced more than adequate.
Using the 60/90 lapidary grit I had purchased, I had "first grit" at about 6:45 PM, working with a chordal stroke -- working the center of the mirror over the edge of the tool. Every six to twelve strokes, I'd rotate the mirror about 1/6 turn to the right in my hands, and take a short step to the left around the barrel; a couple trips around and the grit was ground to mud, making it time to renew it. After a short learning curve to get a feel for the right amounts of grit and water (this coarse grit requires a lot of both), I made good progress; I had the glass ground out to the edge in about an hour, and around 9:00 PM, when I knocked off for the evening, a spherometer reading indicated radius of curvature around 140+ inches -- I'm heading for 112 inches, corresponding to a 56 inch focal length, and I can probably get there in another hour, perhaps less. Once I finish the roughing and, if I can get it done this weekend, the 120 grit, I'll then make a new tool to match the mirror curvature, to avoid grinding into the silicone between the tiles later in the process.
One interesting phenomenon that occurred during the first few wets; if I let the tool get a little too dry, some combination of the silicon carbide grit, the hard ceramic field tile, the Pyrex (R) mirror blank, and possibly the nitrate dope used to seal the tool led to sparks (!) under the mirror. I don't mean a little triboluminescent glow, like crunching a wintergreen candy in a dark room, I mean genuine incandescence. Something was locally getting very hot under there, though the sparks disappeared after the first three or four wets. They only occurred at one point on the tool, and I was running on the dry side the first few wets, but they made me glad I hadn't sealed the working face of the tool with the nitrate dope; it would have been embarrassing to set my tool on fire in public, and the nitrocellulose composition of nitrate dope will burn under water, once ignited.
I also found the sound of grinding interesting. Most of the local ATM group attendees get their mirrors pregenerated to the rough curve, so they don't hear much rough grinding; one of them said the sound, with a tile tool, should be like a small dog growling, but with the size mirror and grit I was using, a fresh charge of grit sounded to me about like an 85 lb Rotweiler. When it started sounding like a lap dog instead, it was getting to be time for a recharge. Most interesting, and quite different from the sound when using a glass tool -- which, to my ear, sounded (in 1977, so I could have a distorted memory) more like feeding window glass into a crusher. The glass tool makes much more of a high frequency sound; the tile tool, especially after grinding the tool and mirror into good contact, has a lot more low frequency noise.
I also purchased the remainder of the grits needed to take the mirror to the point of being ready to pour a pitch lap, so I can continue work at home with the finer grits.
More work on Sunday!
Grit kit for 8" mirror: $15
Total working time: about 2 hours, fifteen minutes (probably about half of that actually grinding, remainder charging, washing, and checking progress)
I spent some more time with rough grinding, this time in my own basement. Over the course of the couple hours I worked, I began to see the radius come in, and I worked a few wets now and then with a W stroke, instead of the chordal I had been using, in order to improve likelihood of arriving at the end of rough grinding with something resembling good contact between mirror and tool. I think I succeeded.
After the first few wets, done with a W stroke to improve contact, I found the ROC (by spit test) to be about 138". Another half hour of chordal strokes brought it in to about 124", and another half hour, with more W strokes, finished the first pound of 60-90 grit and brought the ROC to about 120", at which point I declared rough grinding finished. I ground the last wet out to the a mere whisper, a thin, pale ghost of grit, hoping to reduce the pit size before switching to 120 grit, and I think I succeeded in that, as well as getting good contact. With the tool ground down almost to the point of contacting the silicone between tiles, however, I knew I'd need to either make a new tool, or put another layer of tiles on this one, before proceeding. That's for next weekend.
Total working time: about 1 hour, thirty minutes
Spray Bottle: $4
An hour later, the epoxy was fully set, but a little surface tackiness remained; to get rid of that, I put the tool (without the mirror) on a square of aluminum foil in the kitchen oven at 200 F for an hour -- with the exhaust fan on. The fumes weren't any fun, but they cleared out quickly enough once I took the tool out of the oven. This was just what the doctor ordered; after heating, and allowing the tool to cool for a couple hours, there was no trace of tackiness remaining in the epoxy. I then used medium thickness cyanoacrylate glue (Bob Smith Instacure +) to seal the edges of the tool face where the epoxy hadn't reached, sprayed with accelerator, and inverted the tool on a fresh piece of shelf paper, atop the mirror, and applied a heavy coat of the same nitrate dope I originally used for water sealing, in order to reseal areas where the grit had worn through the dope and trap the grit that had worked into irregularities in the plaster surface. After leaving the dope to dry for a couple hours, the tool was ready for fine grinding, but it was too late to grind more this evening. I'll try to start doing at least a few wets each day after work (now that I'm at least temporarily working a day schedule); I hope to be through with smoothing and ready to apply polishing pads by the first of May.
Total working time: about 2 hours
The technique of making the tool to fit the mirror was a resounding success -- in combination with the steps I took near the end of rough grinding to get a surface closer to a sphere than would normally be the case after a long session of chordal strokes, I had good contact over the whole surface of the tool almost from the beginning of smoothing. At first, there was a little tendency for bubbles to congregate near the center of the mirror, indicating a hole in the glass there, but after about twenty minutes, that went away, and I was left with a very even pattern of grit movement as seen through the clear back of the mirror, as well as a perfectly uniform appearance when the wet was ground to mud. Inspection of the tool when I rinsed it off at the end of the session also indicated that every tile was ground, and all were ground the same amount into the small bevel present in the tiles when purchased.
This session, a mere half hour of grinding, also saw the radius of curvature (henceforth ROC), as measured by spit test, come in to about 116", no more than four inches from the target figure of 112" ROC. I've still been working mirror on top, MOT, which tends to deepen the curve and shorten the radius (and hence the focus), but I'll soon have to start alternating MOT and tool on top, TOT to keep the radius close to the desired figure. The surface of the mirror already appears to be uniformly smoothed and all the 60-90 pits gone, though a closer inspection would be in order before declaring myself finished with 120 grit; in fact, however, the one bubble I was concerned about grinding into is about half gone now, and I'll need to continue with 120 until that bubble is ground out and the radius is as nearly as possible right on 112" before I move on to 220 grit. I can probably accomplish that in the next couple evenings, and make the great cleanup to change to finer grit (which will include resealing the channels in the tool, to trap grit, as well as a thorough cleaning of the work area), by the coming weekend, and start work in finer grits on Saturday or Sunday.
It's rapidly approaching time to start looking for a tube and think about whether I want to make a focuser or buy one.
Total working time: about a half hour.
I've put out a request on the ATM mailing list for advice on how to proceed; I really don't want to leave that bubble, but it's obvious I don't have nearly enough 120 to grind through it. There are a couple alternatives: opening the second can of 60/90 roughing grit, then restarting the 120 after grinding through the bubble with the coarse grit (without changing the ROC); finding a substitute for 120 silicon carbide in order to allow grinding through the bubble without returning to roughing; and ignoring the bubble, with the intent to black it after coating the finished mirror if it doesn't fully grind out (unlikely in the 220 and finer grits, and it'll tend to trap grit and lead to regrettable incidents, especially as I get into the finer grades and aluminum oxide grits).
Total working time: one hour.
The ATM list only got me one response, basically saying either to just ignore the bubble, which will have zero effect on the finished mirror (and if that were the only consideration, I'd have already moved on to 220), or to go back to the roughing grit to grind out the bubble and then restart the 120. Pretty much what I thought, but I wanted to verify before making a mistake that might take many hours to correct.
For myself, however, I came up with two alternatives. First, I've recovered what grit I can from the bottoms of the wash buckets I used (fortunately, I used a separate bucket when I changed to 120, with something like this in mind); it looks like enough coarse to grind out the bubble without opening the second can, and enough 120 to assure me that I'll be able to take out the pits from the coarse once the bubble is gone. The other alternative, which I would have applied if I couldn't recover enough grit to back up, was to fill the bubble with thin cyanoacrylate glue to prevent it trapping grit, and simply proceed. I don't think I'll need to do that in this case, but it's certainly one to keep in mind if the issue comes up again; the CA might or might not stick to glass well enough to do this job, but it's certainly no worse than just going ahead, and trying to keep from leaving grit in the cavity on each change (which, of course, will get harder as the grit gets finer). Of course, the super glue is softer than the glass, and will tend to grind out of the cavity, but that should be less of a problem with finer grits. Something to keep in mind for later, especially if I hit a bubble on some project that's bigger than a millimeter across.
Grit recovery was done by washing down the wash buckets with water, agitating well, letting stand for a few seconds to a minute, and pouring off the water. Anything that didn't settle in that time was much too fine, and I didn't worry about it; anything coarser was washed a second time, then washed out into a coffee can, and the excess water again poured off. The cans were then put on the kitchen range and the water boiled off as far as practical, with final drying in the oven at 250 F. No attempt was made to regrade the recovered grit; having finer grit mixed with the roughing won't do any harm, other than to cause somewhat faster grit breakdown while grinding; I can live with that.
Total working time: 45 minutes
To elaborate: The grit recovery worked well enough; I got back enough 120 to be reasonably confident of grinding out the pits from the coarse, and enough coarse for about an hour's grinding, but that hour wasn't anything like enough to grind away a millimeter over the entire surface of the mirror. The bubble is still there, still a grit trap, and I've used all the recovered coarse grit.
Part of the problem seems to be the deep channels in the tool. For the reface, I used just enough epoxy to attach the tiles, and the channels between them are the same depth as the tile thickness. That didn't seem to be a problem with the 120 grit when I was first grinding it, though I noticed then that the wets seemed on the short side -- but as I'd never ground with 120 before, I thought that was just the way things worked. When I got back to the coarse, I realized that wasn't the case at all. Wets that would have lasted one to two trips around the barrel with the original tool face, with the tile cast in place in the plaster, were now lasting a dozen strokes, and it didn't seem to matter if I was working TOT or MOT.
I also suspect this recovered grit is a little finer, on average, than the original (at least, the recycled 120 seemed finer than the small amount of 120 remaining in the film can). In addition, it seems to break down to mud a little faster, though that might also be because it has a higher mud content (fines that didn't wash out) to begin with.
So now, I'm back to the decision: fill the bubble with cyanoacrylate, regrind the 120, and continue, or open the other can of coarse grit and spend another three or four hours grinding off a millimeter of glass over the surface of an 8" mirror. Either way, I'm going to fill the channels in the tool with either epoxy or CA (or possibly plaster), then cut a relief to keep the filling out of contact with the glass; that should cut down on loss of grit into the channels as well as making them much easier to clean between grits.
Total working time: 1 hour
Forty-five minutes of vigorous grinding later, the bubble was no bigger than the other 60 grit pits. ROC now 106", corresponding to F.L. of 53". I'll be working the 120 TOT to lengthen that a bit.
Total working time: 1 hour, fifteen minutes.
And then I started to grind.
The reclaimed 120, and the small amount that had been left of the original, ran out after about an hour, with the mirror surface appearing uniform, no obvious large pits, and ROC about 117". Once more, The Great Cleanup, including rinsing and wiping out the wash bucket, and I opened the 220.
Another hour, near enough, and I was too tired to continue, but except for a half dozen pits that had probably evaded my eye when I ran out of 120, the mirror is ground out to 220 level, and ROC is about 111" -- which is well within my self-determined tolerance. I have some more work -- surely no more than another hour -- with the 220 to get those remaining pits out, and then it'll be on to the 30 micron aluminum oxide.
In order to finishg smoothing and be ready to start polishing on Saturday, I've temporarily abandoned my newsgroup reading, which was taking up almost two hours a day; I'm instead applying that time to grinding, at least until the weekend. Those of you keeping track might note that I haven't spent a dime on this in three weeks -- approximately since I started grinding.
Total working time: 2 hours
The 220 only took about 20 minutes to finish -- I used the technique of marking the deepest pits I could find, as well as marking a pattern on the mirror surface, with a Sharpie permanent marker. I then started to grind -- and every trace of the Sharpie marks was gone in less than five minutes. I kept grinding with the 220 for a bit longer -- I get a lot of variation in the focal length when I measure with the spit test (there's three or four inches just in how hard I spit!) just to be sure, and then called it good. The Great Cleanup took about fifteen minutes -- the laminate surface on the grinding shelf is paying for itself now, as it only takes a careful wiping down with a damp paper towel to eliminate all trace of grit.
The 30 micron was the first aluminum oxide I've used, and I was quite amazed at the difference in the sound. Where the silicon carbide sounds much like a dog growling, especially when the grit charge is fresh, the alox never gets louder than a soft singing sound. In addition, the grit seems to last forever -- where silicon carbide breaks down in just a couple minutes of vigorous grinding, the alox was lasting ten minutes or more, until the wet started to dry out and the friction level increased from mud buildup.
A half hour, roughly, was all that was needed with the 30 micron to develop a surface that had no visible pits. With only three more grit sizes to work through, and five days until the ATM meeting where I'll be able to get the pads and cerium oxide, it's looking as if I'll make it!
Total working time: 1 hour, thirty minutes.
The surface reached this state overall in about twenty minutes of actual grinding, or just over half an hour total time (including periodic washing to test the ROC -- still 112" to the limit of my measurements -- and inspect the surface), but I ground for about twice that time, total, to get rid of one deep pit (possibly left from the 220) and one shallow scratch.
In the process, I marked up the surface with a Sharpie again, as I'd done during the 220. I was amazed to see all the marks, right out to the edge of the mirror, completely gone in less than two minutes. I'd somehow gotten the impression that this was something I'd expect to see take several times this long. I was also gratified with the evenness of the disappearance of the Sharpie marks -- they disappeared all over the surface of the mirror, with less than thirty seconds variation from any one point to another.
I finished the evening with another Great Cleanup, and I'm ready to work 9 micron alox tomorrow.
Total working time: 1 hour, 30 minutes.
After working the 30 micron and 15 micron, I was rather expecting the 9 micron to be just more of the same, so imagine my surprise when, after wetting the tool and mirror, applying the alox, and gently putting the mirror onto the tool (slowly and gently, with a little movement, in order to detect the sound of mayhem on the glass if the grit has become contaminated) -- the mirror and tool stuck together as solidly as if the alox had been plaster! For just that one moment, I wondered -- 9 micron alox has an appearance and consistency similar to plaster of paris, and even smells similar, but then I realized I was seeing something that normally doesn't occur with tile tools: a vacuum lock.
This is apparently caused by leaving the tool and mirror in close contact without constant motion during fine grit stages -- perhaps for as little as a few seconds. In this case, I let them sit for fifteen seconds or so while I grabbed a paper towel to dry the back of the mirror, and that was too long.
Fortunately, I immediately recalled the wise advice for this situation: don't apply a lot of pressure and take a chance on the mirror shooting across the room (and possibly doing untold damage on impact), but instead put the bound mirror and tool into a bucket of water and let them sit for a while; eventually, they'll separate.
I did so, and they did so. In only about ten minutes.
On to grinding, and after a total of about half an hour actual grinding time (not counting clean up, adding grits and water, inspecting the surface, etc.); I decared myself done with the 9 micron. I arrived at this by determining at what point the surface looked uniformly smooth, without pits or variation edge to center to edge -- and then grinding for that length of time again. The resulting surface is beginning to look like glass again -- at a grazing angle, it looks polished, and I can keep a reflection of a light source as an image to about a 15 or 20 degree incident angle. I read recently that I want that figure to be more like 45 degrees before I polish -- and the 3 micron should get me into that range.
With my day off tomorrow, and all day Saturday before the ATM group meeting Saturday evening, I shouldn't (knock wood) have any problem finishing up the 3 micron in time to get pads and cerium oxide and start polishing Saturday night.
Total working time: 1 hour
I had a surface that was ground out after the first half hour, except for two small scratches. A half hour later, those were gone, replaced by one shallower, but much longer one. I took that opportunity to take a break, and came back to the mirror an hour or so later.
After another half hour, that long scratch was gone, and the one that showed up halfway through that session was nearly gone.
Yet another half hour, and all those were gone, with only a single, new one visible on the whole surface of the mirror. This one is only a quarter inch long, and close enough to the center that it'll likely be shadowed by the diagonal; it might even polish out before I find the parabola in this piece of glass. It probably came from either a stray particle of 220 grit, or something like a rust particle in the wash water. I can live with it. Of course, it helps in that regard that I'm completely out of 3 micron alox and due to get the pads and cerium oxide polishing medium in less than 24 hours.
The surface of the mirror is now transparent enough to see clearly objects across the room. If damp, the surface disappears, and the reflection from the damp surface easily projects an image of the sun onto a surface. When dry, a grazing reflection looks much like one from a finished mirror; the "redout" test has my basement flourescent light fixture reddening and fading away at over 30 degrees incident angle, possible as much as 40 degrees -- hard to measure, but a lot steeper than after the 9 micron.
Total working time: 2 hours
Total working time: about 1/2 hour
Polystyrene sheet: fify cents.
Today, I started polishing. Following the advice of the more experienced workers in the local group, I started by using a sharp utility knife to trim all around the lap, ensuring that there was no point where the edge of a pad hung unsupported beyond the edge of the tiles. That took about ten minutes. I then was ready to start the actual work.
Again on the advice of more experienced workers, I worked the first few wets with what would otherwise be a surplus of water and cerium oxide. The water I'm using contains three drops of Dawn dish detergent in a 24 ounce spray bottle of tap water, in order to increase wetting. The cerium is a dry powder, with a strong tendency to clump into balls as big as 1/8" across. Following the technique of someone I watched actually polishing on a mirror (albeit with a pitch lap), as well as the advice to work wet and rich until the lap is well charged with cerium, I first dipped the lap in my rinse bucket (which had been carefully cleaned after my last session, and freshly filled with cold tap water) to wet the pads. I then sprayed two pumps from the spray bottle onto the top of the mirror, which was sitting on a fresh pad of wet newspaper, and followed the water with about 1/8 tsp of cerium powder, which I then mixed with the water using my fingertips, spreading it evenly across the surface of the mirror. After rinsing my fingers, I lowered the tool onto the mirror and began to push it back and forth using a 1/3 to 1/2 W stroke.
As I'd been warned they would, the pads soaked up the water and cerium very quickly; a couple minutes later I had to add more of both; in fact, I had to charge the mirror about five times in the first ten minutes; after that, the pads had taken a uniform hue that matched the brownish color of the cerium (this stuff looks and acts very much like river bottom silt, except it doesn't have the glittering mica flakes I'm used to seeing in silt in the Northwest) and I was able to work ten to twenty minutes at a time.
From that point on, it was obvious that a different mechanism was at work than had been the case with even the finest grinding grit. The friction of simply pushing the tool back and forth across the mirror was three or four times what it had been with the 3 micron grit, even though I was applying less pressure to the top of the tool-mirror stack. When the wet began to dry out, which I've read is when it really starts to work the glass, this friction would increase by another factor of two or three -- in all, around ten times what I experienced with the 3 micron, perhaps twenty times what I had with 220 grit -- but in almost eerie silence; the only sound was an occasional soft squelch as a bubble in one of the hundreds of holes in the pads burst or something similar, and the very faintest of friction sounds.
After an hour of grinding, a careful wash and drying of the mirror surface revealed that it had taken a flash polish evenly over the entire surface, and I could see an image of the fluorescent light fixture even when I turned and put it directly behind me -- as close as I could get to a 90 degree incident angle. The surface was still obviously gray from scattering by the 3 micron alox pits, but it was clear the cerium was doing its job.
After a second hour of grinding, the surface was about what I might expect on the windshield of a car that was regularly driven in dusty conditions: clear, but with a faint haze. At a grazing angle, it looked polished evenly from center to edge, and what I had reported a week ago as a scratch was now just a single pit, almost in the center of the mirror -- clearly it wasn't as deep as it looked.
I'm told to expect to spend about six hours actual polishing time to polish out an 8" Pyrex mirror -- based on what I see so far, that should be about right. Next session, I expect to have to borrow my wife's laser pointer to examine the surface for haze, as I doubt another two hours will leave anything visible to the unaided eye.
Trimming pads: ten minutes
Polishing: two hours
Total working time: two hours, ten minutes
The alternative would be to find a way to cut an 8" diameter hole in the scrap section left over from cutting the shelf to length, and fasten that to the shelf with clamps or similar. That would be more secure than cleats, and wouldn't require drilling the shelf, but is much harder to accomplish and less amenable to adjustment for different sizes of mirrors in the future.
For the time, being, though, I can still manage a one hour session with a pad of newspaper, and did so. At the end, I temporarily absconded with my wife's laser pointer, and discovered that the surface, almost haze free when viewed under the fluorescent light in the basement, is quite visible by scattering of light from the laser pointer. I clearly have at least a couple hours of polishing yet to go, possibly as many as three or four, before the mirror is ready for pitch.
Total working time: one hour
About twenty minutes into the session, it was obvious that wet newspaper alone just wasn't going to do the job any longer, so I rummaged around in my scrap bins (full of leftovers from building model airplanes and rockets) for three suitable pieces of balsa, and one additional one with a wedge shape for use in locking the mirror into place. The virtue of a good scrap bin is that this sort of thing can usually be done without cutting anything, and this was the case today. I found three lengths of quarter inch thick balsa, and another length of the same material that had a taper trimmed in for use as a wedge.
First, I tried to attach the cleats with rubber cement, used like contact cement. This worked well for about five minutes, then the cleats began to pull away from the laminate. Fortunately, and the reason I'd chosen rubber cement for my first attempt, rubber cement readily scrubs away from most surfaces with a dry paper towel.
Second try, I used gap-filling cyanoacrylate to attach the same cleats to the melamine. I'm reasonably sure I can remove them with a sharp knife later without damaging the melamine (much), and as long as I'm working a mirror 8" or smaller, I should be able to simply add a shim or shims of appropriate dimension and work mirrors as small as 4.5" (important, since my next project will be to refigure the mirror from my Meade 4400). It's not likely I'll have room for a 10" any time soon, so there shouldn't be a problem with these cleats -- and they still didn't require drilling any holes.
Once the cleats were attached with CA, they stayed put, and I was able to put in a session with the minimal level of cerium and water that I understand to work the quickest. When I started, the surface over the entire mirror was slightly hazy in room light, and very visible with a laser pointer. At the end of two hours of actual polishing time, there was no visible haze in room light, and the laser pointer showed greatly reduced scattering, especially outside the 70% zone. The edge seems to be polishing out faster than the center, which I pretty much expected working TOT. I may have to do the next session MOT in order to polish out the center without spending too many more hours.
Also, now that the mirror no longer requires a coat of water to focus an image face-on, I was able to determine the focal length a little more closely. Based on careful measurement (and a serious dazzling in my right eye from the bright reflection of my pocket flashlight before I realized the reflection was bright enough to cast a good image on a white card), it appears my ROC is 109", plus or minus about an inch; that's equivalent to a focal length of 54.5" give or take a half inch -- or about an inch and a half shorter than I was aiming for; well within the error of the measurements I could make up through the 220 grit -- and from 30 micron on down, I wouldn't expect much in the way of radius change.
At this stage, it appears that about two more good sessions will see me ready for pitch. I hope to buy some of the parts for a Foucault tester today.
Experimenting with cleats: half an hour
Polishing: two hours
Total working time: two and a half hours
I continued to work TOT, wanting to avoid the possibility of major surace irregularities caused by thermal expansion from the heat of my fingers; the drag of the pads on the glass was too high to push the tool with my hands on only the back, and I was sure the same thing would be true if I worked MOT -- but it doesn't make any difference if I hang my fingers over the edge of a Plaster of Paris tool faced with tile and pads; hanging my fingers over the edge of the glass itself could lead to major irregularities in the edge, or even a badly turned edge.
In order to catch the center of the mirror up to the edge, for most of the session I used a different stroke from the 1/3 to 1/2 center-over-center (COC) I'd been using in previous sessions. Instead, I used a variation on the W stroke, with a side overhang of about 40% of diameter on each side, and six or seven forward and back strokes from one side to the other. While using this stroke, which with TOT would normally work the edge of the mirror almost exclusively, I applied strong pressure to the right or left side of the tool, whichever side was over the mirror at the time. By so doing, I concentrated most of the work inside the 70% zone, and by the time I'd been working for an hour and a half, the laser pointer gave essentially the same amount of scatter at the center, the edge, and all zones in between. I continued this stroke throughout the session, finishing the entire surface simultaneously. On the very last trip around the barrel, suspecting I was done from the continued increase in drag of the pads against the glass, I went back to a 1/3 COC stroke to get rid of the very slight tendency to stick when the centers passed over one another; that worked, and by the time I finished that last circuit, there was no variation in stroke effort along the length of the stroke -- which means the figure ought to be pretty close to a sphere.
I did have one moment of mild excitement; about an hour and a half along, I found a tiny scratch on the mirror during one surface inspection. It was short, however, and not at all deep, and the next inspection I couldn't find it. At this point, after spending three hours on the mirror today, the surface appears absolutely smooth, without any sign of scratches of pits, and when clean and dry exhibits no scattering from the laser pointer. Interestingly, the direct light of the Sun, which has been recommended for many years as the best source for testing surface polish, failed to show the surface a full hour before the laser pointer lost all its scatter -- this indicates to me that a laser pointer, at $20, is a worthwhile investment for the ATM who doesn't already own a laser collimator for use on the finished Newtonian. It also shows yet another way we can make better mirrors now than we could as little as ten years ago -- for how would I have known my mirror's surface was still not smooth before laser diodes were cheap enough to sell for the price of a good lunch for two?
I have a request in with the local ATM group host to see if I can get pitch earlier then the next meeting, which is June 24 -- the meeting that would normally have been in late May is cancelled because the host and several other group members will be out of town for the Riverside Telescope Makers' Conference, in California. If I can get pitch either the week before or the week after the conference, I can probably have the mirror figured by the June 24 date -- assuming I don't run into any major snags.
Meantime, I'll spend my work time on my Foucault tester, and on the tube, focuser, and mount. For the tester, I didn't buy anything last weekend because I realized that I probably have everything I need except a battery holder in my electronic junk box -- so I'll dig through there for LEDs, limiting resistors, and so forth before I spend money at Radio Shack on parts. I'll probably need to buy a pack of injector razor blades for use in making the slit and knife edge, and I'll need some nesting tube, threaded rod, matching nuts, a small compression or tension spring, and model aircraft plywood -- some of which, at least, I also have in the basement.
Total polishing time: three hours.
Shopping time: about two hours (between three stores)
10" x 60" galvanized heat duct: $10
Krylon Ultra Flat Black paint, 1 can: $3
Shopping time: about 1/2 hour
1/4" x 20 brass all-thread: $1.55
A little later in the day, I went to a local hobby shop and got two brass tubes and one more that slide fits over them; these will form the rails of the tester.
Shopping time: about an hour
Latches and hinges: $1.50
Tube cap: $3
Nuts and hardware: $1
Plywood circles: $2.50
Brass tube: $5
Shopping time: about 1/2 hour
Hexcell laminate: $10
Shopping time: about 1/2 hour
Medium CA: $7.00
Accelerator (refill size): $10
I also bought three drawer or cabinet pulls -- simple handles made of cast brass, to attach with two screws at each end, to be used for lifting and positioning the OTA when setting up and using the telescope -- and a panel cutter attachment for my Dremel to give it the capability of a Roto-Zip® without spending $70 for the real thing. This attachment consists of a base similar to the Dremel router base (possibly identical -- I don't have a router base for mine), and three bits that ought to last a great deal better than, say, saber saw blades. This tool will be used to cut the plywood for the Foucault tester, and possibly also (if it'll handle the heavy stuff) for the rocker box. In addition, if it can be used as a router base, it'll find use in making the mirror cell when that point arrives, and possibly in a sled focuser if I change my mind about the conformal.
Battery holder: $1.50
Drawer pulls: $6.00
Panel cutter kit: $15
Shopping time: about an hour
Unfortunately, I found that, despite the assurances of the help at Home Depot, the spiral cutter attachment from Dremel doesn't fit the old, heavy-duty tools made twenty-five or more years ago. Fortunately, I noticed the discrepancy in size and mounting modes before opening the package, and was able to return the unit, unopened, for a refund. Unfortunately, I then spent a good three hours visiting hobby shops, tool vendors, and hardware stores before finding one that had the original router base for the Dremel, which fits both the modern tools and my older Model 270. It was more money, and I had to buy the spiral cutter bit separately, but should do the same job as well -- and in addition, has full router capability, making it a more versatile tool for things like circle cutting, edge shaping, and grooving. The same shop also had a router/shaper table attachment for similar money; I may go back for it.
Along the way, I bought a package of fresh blades for my coping saw, thinking I might have to do the job the old fashioned way (which would have run to an hour or more just sawing out parts); I'm sure I'll have a use for them.
I also discovered the place to buy tools and tool accessories in Seattle, almost regardless of the age of the tool: Hardwick's Swap Shop, about 41st and Roosevelt in the University District, carries everything from wicks and mantles for Aladdin kerosene lamps to Japanese carpenter's saws to diamond and steel burrs suitable for a dental drill -- and, from the half hour I spent there, it seems like just about anything else you could possibly want in the way of tools and hardware. I'll be going back there from time to time...
Refund on spiral cutter: $15
Coping saw blades: $3.50
Dremel router base: $34
Dremel spiral cutter bit: $4
Shopping time: about three hours
The instructions for the router base mentioned that the feed should be oriented so this force is feeding the tool deeper into the work -- but didn't mention that if you're not using the edge guide, you won't be able to control it very well. Even with all attempts to maintain a slow, steady feed to make it easier to compensate, the parts I cut freehand had rather wavy edges. Fortunately, I was able to keep nearly all of the wave in the waste, and sand the pieces down to the critical dimensions after cutting -- and when it came time to cut the less complex straight pieces, I found I could do all of those with the edge guide, which makes cutting a straight line easier than ripping with a saber saw -- as long as you're cutting parallel to a straight edge.
All trials and tribulations aside, I did get all the wood parts for the Foucault tester itself cut out, and got my drill press stand assembled (my wife gave me this for Christmas and the box still hadn't been opened) and my drill mounted to drill the holes that will carry the 1/4" x 20 screw that is the heart of the micrometer movement of this device.
All of this took up two hours -- after which, it was time for a break, during which I went back to Hardwick's Swap Shop and bought the router table attachment for my Dremel that I'd looked at yesterday, along with a couple router bits. I'm not certain I'll use those for this telescope project, but I'm certain I'll use them for something.
Finally, in the early evening, I returned to the basement and spent another forty-five minutes starting to glue up the parts for the Foucault tester.
I just realized I haven't said more than a clue or two about the design of my tester. Initially, it's going to be a simple tester, consisting of a base box with two vee saddles supporting a pair of 3/8" O.D. brass tubes twelve inches long. Those tubes will serve as rails on which will ride a second, smaller box that will support the light source and knife edge assembly. The upper box has similar saddles that will house the next size larger brass tube, 13/32", which is a nice slide fit on the 3/8" tube rails. Between and slightly below the rails is a 12" threaded rod, 1/4" x 20 size, that will drive the light and knife edge back and forth. There'll be a spring around the threaded rod to keep tension on the assembly, and the rod drive will be set up such that there is no stress on the rather flexible Lite Ply box ends: the spring and drive nuts will be between the adjustment end of the base box and the near end of the rider box, such that the threaded rod takes all the tension of the spring.
All of this, in turn, will be mounted on an extension to get it high enough, and then on a photographic tripod to give a steady, but light and portable base.
Dremel router table: $34.00
Two router bits: $15.00
Two hours, forty-five minutes working time, including tool assembly.
Forty-five minutes shopping time.
Pressing on with the LED portion, I found the green jumbo LED I'd purchased was, in my opinion, nowhere near bright enough -- and it didn't improve (in terms of a more usable light pattern that might let me focus the light onto the slit) after I sanded way about half of the large clear plastic package. I also couldn't find a toggle switch in my junk box -- it's not very good, I'm afraid, since electronics isn't really a hobby for me, but rather a necessary evil for some other hobbies. That meant another trip to buy parts.
I first went to a large supplier of electronics and related tools here in Seattle, Radar, Inc. Nada. They had no high brightness LEDs, especially not in green. So, back to Radio Shack, where I finally settled on an orange high-brightness LED about 40 times as bright (on the same current!) as the green one I previously purchased. This one will happily throw a visible spot of light on a wall several feet away, in a dimly lit room, while the green one had to be shone on white paper from a distance of a few inches to make a visible display in the same room and lighting. While there, I also grabbed a toggle switch. Later in the day, I stopped at a hobby shop on the way home from the grocery store and bought a tube cutter than will handle tube up to 5/8" diameter -- that will deal with the tube cutting problem, and ought to last years.
Unfortunately, I discovered after returning home that I'd forgotten to grab a 9V type battery pigtail (to fit the connector on the battery holder I got last weekend); I'll have to get a package of those tomorrow.
Tube cutter: $4.00
Shopping time: one and a half hours
Working time: about an hour
9V pigtail: $0.25
Shopping time: about a quarter hour
Working time: about an hour
Working time: about one half hour
Today, however, I got somewhere again.
Early in the day, I set about separating the lap from the mirror -- not as simple as it should have been, because it's been four weeks since it was made, and the coating of soapy cerium oxide mixture that would have been fine for a mirror that would be polished within the week proved inadequate for preventing the pitch from grabbing onto the glass. Of course, it didn't help that the entire time acted like a cold press with the tool on top; there were a couple places where the pitch from an outlying button had flowed down onto the bevel at the edge of the glass.
When I took the tape off, it looked immediately as if I might have to destroy the lap to separate it from the glass, but after soaking the interface for fifteen minutes in slightly soapy water, then carefully trimming away all pitch that contacted the glass outside the spherical surface, I was able to slide the lap against the glass after only a half hour of work. In the end, no harm was done, as the pitch was already very well pressed and in intimate contact over the entire glass surface. Other than a little flaking of remnants that extended beyond the glass (and thus were "above" the main lap), nothing untoward happened.
After separating the tool from the glass, the first thing I did was given the interface a heavy coat of cerium oxide and soapy water, followed by five minutes of cold pressing with about 20 lbs of lead shot in its original bag, standing in an old coffee can. The can served to spread the weight of the shot over the surface of the tool, preventing localized pressure from deforming the plaster, but I didn't want to decant the shot into the can, since it's much easier to handle in the original bag.
After the first cold press, the pitch moved easily enough on the glass -- though with a very different, more "hard" feel than the pads had given. The pitch felt very much as if I was rubbing a solid on a solid, instead of the "soft" contact sensation resulting from the slight resiliency of the pads. Of course, it was that resiliency that most likely created the minor turned edge that was reported in my first Foucault test, but by the time I'm rid of the surface periodicity on the mirror, that turned edge should be gone as well.
I continued to work TOT for the time being, currently most interested in getting a smooth sphere, but I kept my stroke carefully close to 1/3 total movement, and center over center. That first session, with three recharges of the cerium each followed by a five minute cold press, lasted a half hour. Later in the day, I was able to spend another half hour with the same technique. At the end of that second session, I did a quick "sun test" and found the solar image projected off the surface of the glass seemed sharper and tighter than it did a few weeks ago -- which I optimistically took as an indication that the mirror's surface was improving. Even with my naked eye, I can see a reduction in the "soft" edge where the polished surface meets the edge bevel; that says to me that the turned edge is also going away, though I won't be able to tell until I can test the mirror with a knife edge and light.
In the evening, I was able to put in another half hour working on the tester, with assembly of the working mechanism complete, lacking only the calibration scales and the operating knob, though I don't yet have the slit, knife edge, or telescope mounted; nor have I made the stand off that will raise the tester to working height from the highest level my tripod can manage. Another hour or two should see it finished, if I don't run into any more snags, and preliminary testing indicates the motion on the rails is smooth over at least four inches; it's likely to be better when drawn by the threaded rod, since that won't distort the carriage with finger pressure, as happened when I was pushing it along.
I bought the last few parts today, as well; a quick trip to Eagle Hardware netted me a package of 5 carpet knife blades -- each of these has two straight edges over two inches long, along with a moderately wide slot in the center that can be used as a clamp and adjustment point. Along with the blades, I got a pair of 8-32 clamping knobs, with matching square nuts (for ease of manipulation inside the carriage of the tester) and lock washers. Lastly, I bought the smallest can I could find of gum turpentine, after finding the acetone I already had, for cleaning an airbrush, was completely ineffective in removing pitch chips from my hands.
Carpet knife blades: $2.00
2x 8-32 clamping knobs, nuts, and washers: $4.25
Shopping time: about fifteen minutes
Polishing: about an hour
Working time on Foucault tester: about a half hour
Working time on Foucault tester: about an hour
Testing time: about 45 minutes
Polishing time: about an hour
Testing time: about a half hour
Shopping time: about fifteen minutes
Shelves and brackets: $9.00
Later in the day I hope to make the mirror stand that will allow returning the mirror to the same location for each test; I'll also route a slot in the tester's shelf to permit using its tripod mount to lock it to the shelf after adjusting it for position; that will prevent bumping it with my nose or cheek from knocking it out of alignment. Once that's done, I'll retest the mirror as it is now, and then spend some more time figuring.
Putting up shelves: one hour
In connection with today's activities, I bought a couple 1/4" x 20 thumb screws -- one for the mirror cradle, and one to hold the tester itself, as well as some nylon washers that will go into smoothing the mechanism of the tester, and a flat steel washer to give a bearing surface for locking down the tester. I also bought a 1/4" router bit for my Dremel, before realizing that I wouldn't be routing a slot in the shelf under the tester.
The concept proved itself almost immediately; I was able to get a reading on my mirror in under ten minutes, instead of taking over a half hour simply to get the knife edge matched up with the focused image of the slit on an unstead tripod.
Thumbscrews and washers: about $1.00
Router bit: $8.50
Shopping time: one half hour
Finishing tester: one hour, thirty minutes
Testing: about fifteen minutes
After this polishing, I put the mirror back into the tester, and couldn't find any hint of texture on the surface -- to my admittedly inexpert eye, this mirror looks completely smooth.
What I'm less sure of is whether I've gotten rid of the turned edge. There's certainly no distinct shadow, as there was when I last tested -- but the diffraction ring around the edge of the mirror isn't a full circle when outside focus, and I'm not certain if that means I have a TDE, or if I'm just too far outside focus. I've posted a question on the ATM list, and hope to get an answer back before returning to the basement to polish and test more tomorrow evening.
Polishing: about one hour
Testing: about fifteen minutes
I continued working TOT, center over center strokes with a total length 1/4 the mirror diameter. After three sessions consisting of a ten minute cold press followed by five minutes of polishing, I cleaned up the mirror and tested again.
The TDE is much less on the second test, and the main face of the mirror is absolutely smooth and perfectly spherical -- when I find that tiny focal spot, the mirror darkens all over all at once (except for the edge); when I move inside or outside focus, so as to see the edge of the shadow cross the mirror, the edge is dead straight in all of the mirror surface where I can read it -- give or take about 1/32" to 1/16" at the edge. Once I have the TDE licked, I'll be ready to parabolize, and that should be by Friday (barring more things that keep me away from the mirror evenings).
Polishing: about an hour
Testing: about one half hour
That tempation comes from an attempt to determine the focal point of the edge zone, which in turn was prompted by noting, as the mirror started to dry out near the end of one polishing cycle, that the ceiling fluorescent light fixture, reflected in the polished surface of the mirror, was very noticeably distorted at the very edge, within a fraction of a millimeter of the bevel. This is the same region I had noted as looking "soft" after polishing with the pads, and I suspect it's too badly turned to polish out -- and I'm >NOT< going back to fine grinding at this point.
When I tried to find the focal length of the turned region, I discovered that it was well over two inches longer radius than the main face of the mirror -- in fact, after backing the tester off almost three inches, the appearance of the bright edge crescent was little changed (a bit narrower, but still far from a full circle). I only have about four inches of useful travel in the carriage on my tester, and after using most of it, wasn't anywhere close to the radius of the edge zone.
Other than that edge, the mirror is effectively unchanged from yesterday -- the main surface is dead smooth, smoother than the Foucault test can detect, and appears to be perfectly spherical with a radius of about 108.5" and no evidence of any flaws beyond the last outer fraction of a millimeter.
Polishing: one hour
Testing: one quarter hour
One session, twice around the barrel, with 4/5 W stroke at about 1/3 total overhand side to side left me with barely visible shadows on the mirror, and I took a set of readings as follows (working by eye for zones, since I hadn't yet made a pinstick):
Center: 108.375 108.368
70%: 108.406 108.387
Edge: 108.442 108.422
Sixtests called this about 1/6 wave overcorrected, which would have been already within my goal, but I wanted to get rid of the high zone around the 70% region, so I attempted a short polishing session, a single turn around the barrel, trying to work the 70% zone preferrentially. I also made a pinstick to the dimensions in Texereau, to allow reading four zones, instead of only three. That resulted in the following readings:
Zone 1: 108.375 108.391
Zone 2: 108.452 108.410 108.416
Zone 3: 108.474 108.456 108.480
Zone 4: 108.458 108.506 108.495
Sixtests indicated this was worse than 1/2 wave and overcorrected, with the zone at 70% wider than in the previous readings. Frustrated, returned to the 1/4 COC stroke, and even with MOT, this returned the mirror to the point where no shadows were visible in a single five minute session.
After spending most of the day waiting for pressing, polishing, modifying a cleat, making a pinstick, and taking readings, I was back to the sphere I'd started from.
Pressing and polishing: about two hours
Modifying cleat: about fifteen minutes
Making pinstick: about fifteen minutes
Testing: about two hours
Zone 1: 108.50 108.49
Zone 2: 108.55 108.54
Zone 3: 108.56 108.565
Zone 4: 108.565 108.57
Sixtests considers this overcorrected, with a 1/2 wave high zone about 60+ percent. Alternately, I could see it as having a turned edge a half inch wide and a broad hole in the center, but the zone has less glass to remove to become a parabola... B)
I now have to decide whether to try some method of taking down the zone, or whether to return to the sphere again and try to correct whatever defect in my stroke is causing this to occur over and over.
Pressing and polishing: one half hour
Testing: one half hour
Reading 1 Reading 2 Zone radius (h)
Zone 1 2752.7 2752.7 32
Zone 2 2753.4 2753.7 72
Zone 3 2754.1 2754.2 85
Zone 4 2754.5 2754.5 96.5
This, according to sixtests, puts me at about 1/5.5 waves, peak to vally, on the wavefront (the strictest measurement standard) -- and given that my goal was 1/5 wave overcorrected (this shows overcorrected with a conic constant b= -1.26, where -1.0 would be a perfect parabola) to allow for thicker deposition at the center in aluminizing, I'm pretty happy with this. Unless I hear something bad from the ATM list, or the local workers, I'm hoping to leave the mirror for aluminizing in just over a week. Meantime, I need to put work into high gear in getting the tube, cell, and mount built, settle on a focuser design, order my diagonal (which I've been forgetting to do for weeks), and find a good way to mount and adjust the diagonal.
Plenty to do yet, with the astronomical vacation coming up in less than two months.
Pressing and polishing: one half hour
Testing: one half hour
Later in the day, I dropped by the local used guitar shop (where they also do instrument repair) and purchased half a dozen inexpensive used machine heads -- these are the tuning pegs from a steel stringed guitar, with a worm gear arrangement to allow precise tensioning without slipping under tension. I also bought a half dozen .016" steel guitar strings -- these, combined with the heads, will allow me to adjust both the centering and the angle of my diagonal holder without requiring adjusting screws within the tube -- I need only allow for rotation of the diagonal relative to the mount, which shouldn't be hard. In addition, two .016" wires in line with one another forming each leg of the spider should give nearly zero diffraction spikes.
Laminating altitude bearings: about an hour
Shopping for guitar parts: about fifteen minutes
Six machine heads: $12
Six .016" steel strings: $6
With the zipper seam up the side, it was easy enough to open it up. First, I trimmed it to length; as it happened, crimped end (where it would have mated into the next section of duct), when trimmed away, left just over 58 inches of tube. With a 54 1/4" focal length, and a tube diameter of 9" (I think it was supposed to be 10", but nine it is and the cap fits) plus a couple inches clearance from the tube wall to the focal point, I need around 48" from mirror surface to diagonal center; coupled with around 3.5" thickness for the mirror cell and mirror, and at least 4", more likely 6" or more for the diagonal mount and it's adjustments, and I need a tube at least 56" long; this one comes to 58" after trimming. It just exactly fits crosswise in the '90 Aerostar on the back seat, and should go in nicely with one seat out when transporting it on the long trip -- and it'll still fit through one of the pass-throughs in the back seat of the '89 Sonata so I can transport it for local jaunts with the smaller vehicle.
After trimming, I clipped a lot of tabs at the cut end and then folded and crimped them into the interior of the tube to prevent injuries. The resulting end isn't perfectly square, but it's neither structural nor optical -- and I can probably make up the difference with whatever wrap I put onto there to soften the end some more.
After the edge roll, I opened the tube and propped it open, primed the interior with one coat of white spray primer (the cheap stuff, from Fred Meyer, which I already had around the house), then shot it with the Krylon Ultra Flat Black -- which earns its name! After spraying, and while closing the tube, I found some holes in the paint, as well as a section of the seam that had missed the spray, and touched up those areas, but the tube was already very, very dark inside.
At this stage, I crimped the ends of the zipper seam to lock the tube closed, then stood the tube on three bricks and primed and painted the exterior (being careful to avoid getting paint, especially the white primer, on the inside). Three cans of Forest Green later (of which I had one, almost full, when I started), I had the entire exterior of the tube a reasonably uniform color with good coverage, though the gloss left something to be desired -- it's very, very hard to wet out the surface of a tube this size with spray cans, especially when working in direct sunshine at 80+ degrees.
By this point, I was in serious danger of melt down, and I adjourned for several glasses of chilled water from the fridge, then called it a day when I realized I didn't have the materials for any other activities.
Two cans of paint: $6
Trimming and painting time: about two hours
Before going to the ATM group meeting, I gave the mirror as thorough a cleaning as I could manage without distilled water. When I arrived, there was one last test (because I had mentioned that I wasn't certain of my measurements, especially near the edge). Peter Hirtle, the local ATM wizard and winner of assorted awards at star parties for his optics and his mounts, ran the measurements through a program I wasn't familiar with on a notebook computer, and pronounced the mirror about 1/7 wave overcorrected, with the bulk of the error in the form of a high zone close to 65% -- which in fact agreed pretty well with my measurements, so I'm pretty happy with it. He also said there was some roughness, which I could also see, but based on the shadows I was seeing, I'd estimate it was no worse than about 1/10 wave. I note here that his tester shows shadows mine doesn't seem to -- either that, or the testing conditions were that much better in his basement than in mine.
After that, he took the mirror into another room for another thorough washing, this time with something called Alconox (apparently, a special detergent, perhaps for chemical glassware or similar) and distilled water. When he brought it back, it was visibly cleaner than I'd been able to make it. He then released the vacuum in his aluminizing machine, lifted the bell jar and an internal mirror support (that also serves to prevent aluminizing the bell jar into opacity), put the mirror onto the support face down, and closed things up. After that, we waited for approximately an hour. Peter said this was to allow water either absorbed or adsorbed (I didn't catch which) into/onto the glass during working to evaporate -- though I'm unsure whether this was to ensure good adhesion of the aluminum or simply to get the vacuum hard enough.
After a time, he came in and went to work. He describe the process as evaporating aluminum off the filament (by heating the filament to orange and even yellow-white heat) until the vacuum started to drop, then stopping to allow the diffusion pump to bring it back below 10-3 torr -- about one millionth of an atmosphere, or around one tenth the level that's in an ordinary vacuum tube. About the fourth cycle of this sort, he commented that he was running out of aluminum on the filament; one last cycle running up to nearly a full white temperature, and he said he'd need to let the filament cool (a process which would take several minutes because the filament could lose heat only by conduction through its ends or by radiation to the walls of the chamber), then release the vacuum and add more aluminum.
When he released the vacuum, however, and pulled the mirror out to examine it, he pronounced it finished; it was, he said, opaque enough that he couldn't see the ceiling fluorescent light through it, which he usually used as his gauge.
He explained I'd need to protect the coating, effectively pure aluminum (he uses 6061 alloy chips as the supply, but the principle impurity in that alloy is copper, which has so much lower vapor pressure that almost none gets to the glass), for a couple weeks until it builds up its clear protective oxide layer -- but I could see that the mirror was done.
I stuck around to watch him recharge the filament with aluminum -- a
process that starts with hanging suitable chips from his lathe on the filament,
then pumping down the enclosure again and gently heating the filament to
melt the aluminum and allow it to distribute itself on the filament.
By that time, however, it was well past my bedtime, with 3:30 AM coming
very early indeed, so I took myself and my newly coated mirror home.
The mirror will wait in its transport box for the two weeks needed, during
which time I'll continue to work on the mount. Barring difficulties
in locating a suitable diagonal, I should easily finish in time.
Later in the day, I stopped by Captain's Nautical and bought a focuser. I had originally planned to make my own, vacillating between a conformal (using hinges set at various angles in a plane to define a linear motion) and a Crayford variant. In the end, it was the latter than I bought (after finding no one seemed to have the less costly rack and pinion sort when I wanted one), a beautiful and highly adjustable teflon bearing unit made by Astro Dynamics. I spent more than I wanted to, but the end result is worth it.
PVC pipe parts: $2
Aluminum angle: $3
Shopping time: about an hour
I had intended to make a simple cell from wood -- I even had the wood around, some leftover hemlock shelf stock that had gotten wet on one end. The dry end had almost no warp, and no splits or checks, and I planned to cut a pair of rings from it to fabricate the cell. Unfortunately, I found out (after an hour of setup and attempting to make progress) that my Dremel, in the router base and with a spiral cutter bit mounted, wasn't up to the task of cutting approximately 3/4" thick hemlock; in cutting about halfway around the circle, the cutter unseated itself from the collet no less than three times (not to mention throwing a brush cover, which I managed to find in the grass of the back yard and reinstall). I have no doubt that a purpose built spiral cutter or a genuine router could handle this job with ease, but the Dremel clearly isn't up to such heavy work.
I then went to several locations trying to find precut plywood circles, from which I planned to saw out the center ring with my coping saw. No joy. Finally, at Eagle Hardware, I found myself returning to the heating duct area -- and realized that an 8" duct cap, nested inside a 9" duct cap (to fit my tube) would make a very serviceable and rather light cell. I'm not worried about mirror flex in a full thickness 8" Pyrex mirror, so the thinness of the metal doesn't bother me, and that very thinness makes it easy to work. A few moments in the specialty hardware aisle, and I had long extruded u-clip nuts, thumbscrews, and washers to use in collimating. I also grabbed some more PVC pipe (I'd realized I wanted something with a thinner wall than the Schedule 80 that had the threads in it, to go in the glue end of the adapter), clear silicone adhesive for mounting mirrors, a compass for drawing circles that would need cutting, some 4.5" pipe clamps (which later turned out to be incompatible with the too-large ones I bought a few months ago) and some sheet metal screws for mounting the cell into the tube.
On the way home, I stopped at Radio Shack and bought a nibbler tool, for cutting any shape hole in sheet metal starting with a simple drilled hole.
By that time, however, the day was pretty well shot. More to come tomorrow.
PVC pipe: $1
Silicone adhesive: $5
Assorted hardware: $8.50
Hose clamps: $4
Heating duct plugs: $5
Stainless screws: $1
Nibbler tool: $11
Shopping time: about three hours (!)
I started out cutting the holes in the pipe caps -- after a little fitting to get the larger cap to actually slide into the rear end of the tube (I wound up cutting three slots in the cylindrical section to provide the give needed to slide inside a tube the same diameter -- I'd have probably been okay if not for that darned seam!), I drilled a hole, put the nibbler jaw through it, and started nibbling. An hour later, I was done with the first six inch diameter hole. For the hole in the second (inner) cap, I got smart and used my tin snips, and was done in ten minutes. I also trimmed off about half the cylinder portion of the inner cap -- I needed some to keep the flat end stiff, but too much would limit the inward travel of the collimation adjustment. The time spent with the nibbler wasn't wasted, though; I got pretty proficient with this unfamiliar tool during that time, which will be a useful thing when I need to cut the hole in the tube for the focuser (which will be much too small even for curve cutting snips, much less my straight cutters). In addition, the nibbler left an edge that was much cleaner and less prone to cut than the snips, and such an edge is desirable for an exposed cut (as this one is). After two short trips to Eagle to get springs (along with more paint and sheet metal screws), then get the correct size of springs, I had all the holes drilled, primed and painted the parts Ultra Flat Black, and assembled the cell. All that's needed now to mount the primary is a set of blocks to hold the mirror away from the cell to prevent the collimation screws jamming against the glass -- and I have a piece of scrap maple that's exactly right for this job.
After a short break, I embarked on cutting the 45 degree angle on the diagonal holder -- first, I used an old rocketry trick, wrapping a piece of tape around the pipe to get a square edge to cut along; I then marked a second edge exactly one pipe diameter along the pipe, screwed the threads into the adapter in order to use the hex section as a brace, and cut the 45 degree (I don't have a miter box). A little careful sanding, and I had the cut within about 1 degree of the correct angle -- that much error can be easily collimated out. I then glued the thinner pipe into the interference fit end of the adapter, cut it to length and square, and ran the threaded 45 section in as far as it would go before priming and painting this unit with the same Ultra Flat Black.
By this time, it was 5:00 PM, and getting time to call it a day; I still needed to eat dinner, read my e-mail, and update this record before getting to bed, and 3:30 still comes awfully early. I should be able to mount both mirrors tomorrow, and the next day I can start cutting the spider slots (or drilling the holes, as the case may be) in the tube, as well as mounting the focuser; final assembly of the Optical Tube Assembly should be later this week, barring unforeseens. That gives me two weekends to finish the rocker box -- which shouldn't be all that difficult (I hope).
More paint: $6.50
Stainless screws: $1
Shopping time: about one half hour
Working time: about four hours
After some considerable time wandering around Eagle Hardware, I finally settled on a piece of 1" square aluminum tube -- which only came in four foot lengths. I've got lots left over, I'm sure I'll find some use for it. B) Unfortunately, by the time I was through finding it, it was time to start thinking about supper and my other evening activities (like e-mail) before bed.
Aluminum tube: $9
Shopping time: About an hour
I quickly found my mini hacksaw was useless for the task. Fortunately, I remembered the coping saw blades I bought a month or two ago (when I wasn't sure I'd find the spiral saw stuff for my Dremel, in order to make my Foucault tester) -- and sure enough, one package of blades was rated for cutting soft metal (like aluminum, say). A few minutes convincing a blade that had been in the saw for several years to come out, and then installing the new blade, and I was on my way. I then recalled why coping saws aren't commonly used for straight cutting: they simply won't cut straight, no matter what you do.
Eventually, I got the tube slices cut, and it was time to put things up and call it a day -- I only get about an hour of working time after work, most times, because of my early bedtime.
Cutting tube: about an hour
That went quickly enough, though, and after a quick brush down, change of shirt, and hand washing to remove the worst of the almunimum, I was back at it, disassembling the cell to allow drilling the holes to attach the standoffs -- then drilling matching holes in the standoffs, and using a blind riveter and the rivets I bought a couple months back, then intended for a conformal focuser, to attach the standoffs to the cell.
After that was complete, I took the upper part outside and spray painted it the same Ultra Flat Black as the rest of the interior of the tube (after first masking the upper surfaces of the standoffs). A short time for the paint to finish drying, and I was back in the basement, putting silicone adhesive on the standoffs, balancing pennies to keep a space between glass and metal, and gluing the mirror to the cell. I then covered it, to keep down dust accumulation on the coating, and left it overnight to cure.
Working time: about one and one half hours
Working time: about forty-five minutes
After that operation, I took myself out looking for scraps of laminate -- without notable success, though the expedition soaked up a couple hours easily enough.
After coming home, another trip to the basement yielded carefully measured and located 1/16" holes in the pipe portion of the diagonal holder (through which I'll thread guitar strings, retained by their barrel ends against the interior of the holes). I finished up the day by attaching the diagonal to the holder with silicone adhesive, then propping the assembly in a position where the diagonal would be level and covering the mirror with the tissue it had been wrapped in for shipping.
Working time: about one and one half hours
Shopping time (fruitless): about two hours
After dealing with that minor disaster, I made a trip to Eagle to get some more goodies -- a decent hacksaw, that will also double as a bowsaw for cutting firewood when camping or trimming the base of the annual Christmas tree, a miter box to use in getting decent angles when cutting the (visible after finishing) mounts for the guitar tuning machines that will tension the guitar strings to suspend and adjust the diagonal holder, and some shorter sheet metal screws that won't interfer with the inner cap of the cell when attaching it to the tube.
On my return, I cut the three aluminum angle pieces for the tuning machine mounts, then went inside and drilled and nibbled the holes and mounted the focuser -- and suddenly it's starting to look like a telescope. Amazing how just putting a focuser on the side of the tube can do that, but there it is.
Plastic miter box: $6
Shopping time: about one half hour
Working time: about two and one half hours
4-40 screws, nuts, and washers: $2
Shopping time: about one quarter hour
Working time: about three quarters of an hour
Working time: about an hour
With the mirror seated and once more covered to minimize the dust that lands on it, I then spent a few minutes blackening the edges of the diagonal with a Sharpie (R) marker, in order to reduce light scatter -- especially imporant since this edge is directly visible through the focuser, so that light scattered here can go directly into the eyepiece.
At this stage, I made a quick trip to Eagle Hardware (I've bought almost half the parts for this telescope there -- I should take it up there and show them what I've been building with all these weird little bits), and bought two more boxes of #6 x 3/8" sheet metal screws to attach the tensioners to the tube, as well as a set of shorter thumbscrews to replace the 2" long ones that lock the cell after collimation.
That done, I started work mounting the spider tensioners to the tube. Careful measurement and calculation was required to ensure that the diagonal will wind up centered relative to the focuser, in order to allow correct collimation; I then drilled the three holes for the sheet metal screws on each tensioner, and attached them to the tube.
At this point, I ran out of working time for the evening; still to do to complete the OTA is to drill the holes in the tube to pass the spider wires, mount the diagonal assembly, install the cell, and collimate. Even allowing for the fact that everything seems to take longer than expected at this stage, I should be able to accomplish that (possibly excepting collimation, which can be done in the field if necessary) in an hour to two hours, still leaving (I hope) time this weekend to complete the mount. I have the wood I need; I should only need to buy the azimuth pivot bolt and a drill bit to drill the hole for it and put in the labor and time to build the alt bearing attachment and rocker box -- which I've had to leave to last because I can't dimension it until I know where the tube balances.
Sheet metal screws: $2.50
Shopping time: about one quarter hour
Working time: about one and one half hours
I started in to mount the diagonal today and found that I either mismeasured or miscalculated somewhere, and the diagonal was over an inch too close to the primary -- this is not the same as the focuser being too close; that's (as far as I know) correct; instead, the diagonal couldn't be moved far enough up the tube to line up with the focuser, even with zero tension on the forward wires. The whole thing had to come out, which required replacing the guitar strings since they'd already been tautened on the pegs, curling the ends so they wouldn't thread back into the holes in the diagonal mount.
Since I had to get guitar strings anyway, I made a trip of it, buying what I hope will be the last parts for this project before First Light. I first stopped at the neighborhood second hand store and grabbed a Barry Manilow double album (on 12" vinyl) that will become my azimuth bearing. Then I went to Eagle, and picked up the 3/8" x 2" azimuth pivot bolt, matching nut and washers, and a drill bit to drill the holes for it. I also got three pipe caps that will serve as feet under the ground board, and bolts, nuts, and washers to mount them with, as well as a new combination square to replace my old one, which was gotten at a garage sale and has never been really square. Last at Eagle, I picked up a 16" precut plywood circle to serve as the base of the rocker box. Finally, I nipped back into the guitar shop, The Trading Musician, where I'd gotten the machine heads and the original set of strings -- this time, I took the opportunity to go a size smaller, and bought six .012" diameter strings.
Back home, and after drilling the lower holes in the diagonal mount an inch lower, and finding they were still too high, I was glad I'd gone to lighter strings -- on these, I was able to clip off the ring (that would normally secure the string at the bridge end) and reattach it by carefully twisting the wire around the ring. That meant I'd get three tries, total, on a single set of strings before running out of length. Fortunately, I didn't have to go that far; the next try (with the lower mounting holes still another inch lower, making the holes now four inches apart compared to a single inch between the pegs) proved to bring the diagonal high enough, and I was able to roughly align the diagonal, and final mount the cell, completing the OTA except for collimation.
Unfortunately, between the second and third tries at installing the diagonal mount, I had an eye appointment, and the dilation of my pupils, with accompanying blurred vision, and the several hours recovery time before I felt up to fine work again, meant I wasn't able to get anything done on the wood work -- but that's now the only thing left, save collimating the OTA.
I hope to start that work tomorrow, in the limited time between my model rocket launch and the local ATM meeting.
Double album: $1
3/8" drill bit: $5
Combination square: $8
Plywood disk: $5.50
Bolts, nuts, and washers: $1
Guitar strings: $6
Shopping time: about an hour
Working time: about four hours
In any case, I can now confirm from actual tests that the mirror forms
an image, though I wasn't able to keep it in field long enough to tell
anything useful about the collimation or figure.
After getting home, I retreated to the basement for a bit, to cut the wood for the tube box, which will fit between the altitude bearing circles. I was able, in an hour of work, to determine the needed dimensions, measure and cut the shelf stock that's being pressed into this use, and screw and glue two pairs of side and top/bottom pieces together. The remaining joins between these will be a pair of hinges and a pair of latches, respectively, that will allow opening the tube box to remove or rotate the tube. This whole assembly will then fit between the altitude bearing circles, which will in turn ride atop the rocker box on the furniture glides I have purchased.
Wood screws: $13.50 (box of 100)
Working time: about an hour
Unfortunately, though I got the hinges in nicely enough, I found at the end of my available hour of work time that the latches I had, draw catches of the sort one might find on a steamer trunk, had too much length in the fixed end to fit on the end of a 3/4" thick board that would be mounted directly on the face of the 12" diameter flat alt bearing. Something else was needed, and another trip to Eagle Hardware necessitated.
Working time: about an hour
After supper, I mounted the latches, and verified that they would in fact hold the box closed even when the stress applied was at right angles to their apparent latching direction. I also mounted the carrying handle on the top of the box, leaving only mounting of the alt bearings on the sides to complete this section.
Hole saw set: $14
Shopping time: about one quarter hour
Working time: about an hour
By the time I'd done this for both alt bearings, my time was out for the night, and it was time to read my e-mail and then go to bed.
Working time: about an hour.
With the tube box completed and mounted on the tube, I was able to concentrate wholly on the rocker box. First order of business was to locate the center of the 16" plywood disk that would form the base of the box. The same method as used on the alt bearing disks worked nicely, and I drilled the 3/8" hole needed for the alt bearing pivot bolt. I then located the correct location of the disk on the aluminum hexagonal cell ground board, and drilled the hole through that as well; last use of the 3/8" drill for this project was in enlarging the center holes of the two phonograph records ( you remember, the 12" diameter vinyl kind) to fit the bolt. A quick test fit verified everything would work and I had plenty of bolt length.
The same was not true of the 1/4" x 20 bolts that were to hold the pipe cap feet to the ground board -- after drilling the existing mounting hardpoints out to the required 1/4" diameter, and drilling the matching holes in the pipe caps, I found that the bolts I had, 3/4" long, were too short by about the thickess of the flat washer, lock washer, and nut that would go inside the cap. Another trip to Eagle Hardware, and I returned with the longer bolts needed (1 1/4" length), as well as the second triad of shorter thumbscrews needed to reseat the mirror cell on shorter bolts, allowing more retraction of the mirror to add focal relief at the focuser.
Later in the day, I finished installing the feet on the ground board, and commenced construction of the standing part of the rocker box. First came the saddles -- and the most fortuitous coincidences of the whole project started to surface. I found that the exact radius needed for the rocker saddles, that of the alt bearings plus the thickness of the bearing pads, was identical to that of the phonograph records: 6" (12" diameter, of course). That meant I could (and did) use the record platters as templates for marking the saddles before cutting them with my coping saw. The second great coincidence came when I realized that the needed width of the rocker box was only 1/8" wider than the combined dimension of one piece of shelf stock, plus two thicknesses on each side -- which was a construction layout I'd arrived at before finding the dimensional fit. Fortunately, I had a sheet of 1/8" plywood lying around from model construction -- and had a shim cut from it in just a few minutes. Much drilling, gluing, and screwing later, I had a base consisting of a vertical channel made from vertical grain lengths of shelf stock, with the horizontal grain saddles mounted at the tops of the sides with 4" overlap for strength. All joints were glued and screwed.
Tapping the furniture glides into the saddle pieces completed the basic contruction of the rocker box, and allowed me to assemble the scope for the first time.
Then it was time to knock off for the day.
Bolts and thumbscrews: $1.50
Shopping time: about one half hour
Working time: about four hours
Then, finally, came the time I've been working toward for almost seven months: First Light!
I took the telescope up to the back yard, dropped in a 25 mm eyepiece, and at 54x magnification, looked at Mars low in the western sky still light with dusk. Even at that low magnification, there was a clear disk, and after the tube currents settled a bit, I was able to confirm that the optics are acceptable -- I hope they're much better than that, but forty-five minutes of observing mostly spent finding things (I didn't have the Rigel finder aligned yet) doesn't give much more certainty than that. This scope is clearly more capable than my Meade 4400, and that scope will resolve Cassini's Division in Saturn's Rings and just show a disk of Ganymede at 182x. This scope, with good eyepieces and good seeing, should be capable of showing disks of all four Galilean satellites, resolving the six stars of the Trapezium inside M42, and even (assuming I can find them) allow me to see many Messier objects from within the city.
Should, of course, is different from does -- but only time will tell what it actually delivers.
It's said an amateur built telescope is never really finished -- there's always something going on, the builder is constantly tweaking this, replacing that, and upgrading the other thing -- but this one is at the point where I'm willing to call it finished. As Mark Twain once said, "Thus endeth this chronicle."
Working time: about an hour
Cost for tools: about $200.50
Cost for materials and parts: about $387.25
Total cost outlay to date: about $587.75
(note that some materials and most tools will last through more than one telescope project)
Time invested to date:
Shopping for parts and tools: about twenty-six and one half hours
Settling on design parameters: about an hour
Clearing and building workspace: about five hours
Making grinding tool: about two hours, twenty minutes
Resurfacing tool: about two hours fifteen minutes
Recycling grit: about forty-five minutes
Grinding: about fifteen hours, fifteen minutes
Making transport box: about thirty minutes
Trimming pad lap: about ten minutes
Making cleats: about three quarters hour
Making pinstick: about one quarter hour
Making pitch lap: about forty-five minutes
Building Foucault tester: eight hours, fifteen minutes
Polishing: about sixteen hours
Testing: about six hours
Making the Dobsonian mount: about ten hours
Making the optical tube: about twenty-two hours
Total to date: about one hundred nineteen and one quarter hours
Return to my home page
If you have comments or suggestions, email me at firstname.lastname@example.org