Making the mirror
Mirror blank arrived from ASM in mid April. I first ground its back flat using 16 inch diameter plaster / ceramic tile tool. Then, a 20 inch diameter plaster disk was cast using blank's convex side as a template.
After plaster dried for a week, hexagonal ceramic tiles were epoxied over it. They were glaze - that proved to be usefull to determine when tool achieved full contact with the blank during grinding as visible below. It took about 4 hours of grinding with #80 (mirror on top) to almost completely remove glaze from all tiles. Then, standard series of grits were used. The only difference is that BVC grinds like a charm so less than an hour ( three - four wets) were needed at each step! Unfortunately, when mirror was removed from tool after #600 - a lot of tiles separated from the tool :-( Of course it meant that mirror is ready for polishing, as I was not going to make new tool (which would required grinding with #80 again).
Polishing was started in mid June. Tool was made of 16 inch diameter plaster disk again cast in April against mirror blank surface (Saran Wrap was used in between). After it dried for a few weeks, pitch tool was made as follows:
blank's surface was coated with cerium oxide / dishwasher liquid mix
rubber mat with large hexagonal holes was also coated and placed over the surface of the blank
hot pitch was poured, making sure ity fills and overflows each cell
than plaster toll was pressed into pitch with a lot of preasure untill excess pitch oozed out
after pitch cooled down and hardened, tool was separated from the blank and its edges were trimmed
I made pitch on a soft side, so it had to be re-channeled (with large soldering iron) after every two hours of polishing!
BVC polishes quickly. Tool was extremely hard to push and its movement was irregular. After about 15 min. of work I could see what was tha problem. At that time only center and edge were partially (but quite well already) polished. There was a wide ring were tool did very little polishing at all. Most probably, before tiles fell of grinding tool, they were already flopping a bit and that created oblate spheroid figure. It seemed to me that I shoulod be able to polish it out anyway. Picture below shows that zone almost disappearing after another four hours of polishing:
I continued polishing while testing mirror with home-made Foucault tester until it was completely polished out and figure was close to sphere (it was actually a bit overcorected).
Then, mirror was parabolized. I started with 16 inch tool first, then switched to star-shaped 7 inch lap and finally used 6 inch star-shaped lap. It seems than the degree of corection was mainly dependent on the size of the tool as 16 ich did not go farther than about 50% correction while 7 inch got me to about 80% needed. I had to polish really hard with 6 inch tool to finally get closer. Pictures below show 7 inch tool as well as cold pressing with 6 inch tool:
After rough Foucault measurements on 10%, 70% and 90% zones indicated that the mirror may be close to parabola, I assembled the optical tube outside (altitide bearings were not ready at that time yet) and took a look at Polaris. Well, it did not quite focus :-( In star test, Ronchi test on a star and knife-edge test on a star it looked undercorrected. I took mirror out for 10 more minutes of figuring and when tested in the scope on a star again it got a lot closer - I could actually focus the star into tiny blob!
Same night the mirror was tested again in partially assembled telescope. While seeing was poor, star images were focused. Out of focus images slight turned down edge but Ronchi test (on the star) indicated this to be below 0.25 inch wide. While at it, I also took a peak at Andromeda Galaxy and the Moon. Even with bare glass, 20 inch collects a lot of light ... I do not think that Moon will be a safe target after mirror is aluminized :-)
For final check, a cardboard masks was made to block sectors of the extreme edge and mirror was star tested again. It seemed turned edge was not more than 0.25 inch wide. I call it done - the edge will be masked after mirror is coated.
Mirror was coated om July 24, 2002 with enhanced aluminum at Evaporated Metal Films Corp. in Ithaca, NY.
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© Jan Wisniewski