[ATM] My 12.5 inch project

["Francis J. O'Reilly" <foreilly@verizon.net>]

I just fed my cat. Unfortunately, the dog has acquired a taste for cat food. At least he hasn't acquired a taste for the cat.

It is a quarter past ten at night. I am sitting at my computer typing about my ruminations concerning my 12.5 inch approximately f/7 telescope mirror. I have discussed this mirror before on the list, approximately two weeks ago.

When I last addressed the issue, I had completed 7 1/2 hours polishing after fine grinding down to 3 micron AlO. There was no trace of grey on the mirror and I thought that I may have been complete, but I had some nagging doubts, I was concerned about a return based on looking at the glass with a laser. Looking at the glass in the sun, I noted a thin film, but I believe that to be due to my use of Cerium Oxide and my unique method of polishing which was taught to me by Bob Goff, which involves little more than the pressure of the weight of the glass during polishing.

Last Friday night I took the mirror down to Guy Brandenberg's telescope making workshop, where Guy,  looked at the mirror with a magnifying glass from Radio Shack and promptly showed me several pits that I decided needed to be addressed. 

I got home Saturday night and took the family out to dinner, Sunday we all went to Battery Park in NYC, Sunday night, I hot pressed the lap and resumed polishing the mirror with 30 minutes of tool on top. Previously I had polished for 3 1/2 hours with the tool on top and 4 hours with mirror on top. Since then, I have polished another five hours, One hour per day, half an hour in the morning before work and half an hour at night when I come home. I have expended a total time of thirteen hours to date. All polishing is equally split between tool on top and mirror on top.

I should note that when I went to Guy's mirror workshop, We put the mirror on a test stand and looked at some Ronchi images. (He has a real interesting test stand with a strap device to hold the mirror and a versatile testing apparatus complete with a round device that can change positions to allow knife edge readings and three different settings for different density Ronchi screens.) The mirror's figure was very smooth, all the way to the edge, no TDE at all, but to my dismay, It was not perfectly spherical, as I thought it should be, but rather was on it's way to being a parabola. I attribute this to the half hour extra time of mirror on top at the time of the test, but what do I know?

Back to the main issue,  At what point do I call it good and start figuring this piece of glass? I am pretty much committed to going another seven hours of polishing for a total of twenty hours, That should take care of most of my pits, actually there are very few left at this time. Probable less than one per square inch at the edge. I doubt that it will make much of a difference at that level, none in the center or for that matter none more than an inch from the edge.

I'd like to hear your input on the topic.

Best regards to all,

Francis J. O'Reilly
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