Parabolizing 2

2014 October 01. The question is whether to continue figuring the mirror or to be happy with what I have. A strategy: to do a star test. In September we attempted to get a tumble-down jig up to speed. Even though we got the pieces of the jig together, the streetlights around the community center reflect too brightly off the unfinished plywood of the jig, making the discerning of the star (Polaris) impossible.

So this evening I painted the internal sides of the jig flat black. And we did a quick and dirty aluminization of the mirror. We didn’t clean the surface of the mirror, we put it in the machine and alumnized it. Finger prints and measuring marks on the parabolized surface show through. That shouldn’t effect a preliminary star test. Unfortunately, Mother Nature wasn’t cooperating tonight; so we couldn’t try the jig out. Till next week. In the meantime, I’ll be working on the tube.

2014 12 30. A clear night, a Tuesday. I raced to Chevy Chase. We brought up and outside the star testing stage. Long story short: it didn’t work. It can’t collimate sufficiently, it’s a pain in the neck to aim.  Still, I did see the moon and Polaris in focus. Huzzah.

2015 01 28. A clear night, a Wednesday. The tube had progressed to a usable state, and I had picked up my mirror from the workshop since I was tired of waiting for a clear Tuesday. I schlepped the pieces out. I brought out the mirror an hour in advance to cool down. After dinner I put everything together. What I had not anticipated, but should have, is how time consuming a first collimation would be. Then there’s also the figuring out of the eyepieces are going to work, what their ticks in the new focuser are going to be, etc.

Anyway, I can report that the scope works more or less. It’s a little top heavy, which has an effect at about 40° and lower. I’d determined the c.o.g. with a 25# barbell weight. As for viewing, I did manage to focus on the moon, Jupiter, a range of stars, and the Orion nebula. The moon was blinding, but it’s features were crisp. I’m not sure I executed a successful star test. I think I have to work on collimating it for one. I also started with low stars so that I could keep my feet on the ground. But the atmospheric jiggle was apparent. The barlowed 12mm was just unwieldy. The unbarlowed 12mm seemed to give encouraging results. Then I started to get cold. I’ll keep trying.

On a general note: it was cool to have it out on the porch. Outdoors, it’s less the monster than in a basement, but it’s still a big.

De-Alumnizing: HCl treatment

De-Alumnizing: HCl treatment

2015 02 20. We began the process of properly alumnizing the mirror. This meant removing the “quick and dirty” alumnization from October. We used HCl (the green river). Then we washed it with Alconox powder and powdered USP calcium carbonate (separately) with water and cotton balls. A small problem: the water rinse kept showing the trace of a chord we had drawn across the surface of the mirror with a Sharpie (to do the wire test and then later without being cleaned off had been alumnized over). We had no acetone handy. We’ll use that next time to remove, we hope, the residue of the Sharpie. (Worst case scenario: the alumnized-over Sharpie actually etched the glass microscopically … requiring then a repolishing. Yuck.)

2015.02.24. I did several washes first in acetone, then in chloromethaline. None succeeded in eliminating “the Sharpie stain.” We’re working on the assumption that thanks to the aluminum vapor at circa 4000° in the vacuum that there was a pyrolesis of the marking ink. Now to jeweler’s rouge.

2015.03.04. Using jeweler’s rouge today, I went back to figuring. The foucault test reminded us that I have a little bit of a dimple in the middle, falling in the shadow of the secondary, and a bit of a TDE. I warm-pressed my tool. So: tool-on-top, center over center, 1″ overhang in each direction, six strokes, rotating the mirror 1/12 and the tool 1/6 clockwise. I went around the clock five times, each set; and then went to check it on the Foucault stage. I did this three times. For the third time, I put a paper star at the center before cold pressing for the sake of the dimple. In the final Foucault, both dimple and TDE seemed improved. I think it’s worth doing again before the knife test.

2015.03.17. We took St. Patrick’s Day measurements with the results: 1/7 wave and .95 Strehl. Over the course of the week I mulled it over and decided to declare victory. Alumnization next week.

parabolization, part 2: thirty minutes of tool-mirror contact-in-motion.


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