12.5″ mirror blank


Over the Labor Day weekend (2013) I took a spontaneous plunge and wound up with a mirror blank. I stumbled across it on eBay. The ad described a full-thickness, 12.5″-diameter, ’60s-era pyrex blank. It admitted stains, bubbles, and small chips on the edges, but nothing that would make it unsuitable for grinding into a telescope mirror. The starting bid was $100. I decided to take the risk, bid modestly with a low cap, and waited to see what would happen. I imagined over the course of two days of waiting for the auction to end that there would be a furious bidding war in the last thirty minutes. That war never broke out. Even with a couple other bidders, I won the mirror at about 25% the typical retail price today for a full thickness pyrex blank of 12.5″. 

The mirror arrived on Friday (2013 09 06), and my first inspection revealed no obvious flaws. The chips described in the ad will be irrelevant to the grinding. The bubbles at the surface are very small and on the back side. Submerged bubbles I think can be easily worked around.

By October I decided to start on the project. I would turn the mirror into an f/6. From hogging out to figuring (October 2013 to March 2015) required 36.75 hours of tool-and-mirror-in-contact time. That is, washing, warming, rinsing, resting, and measuring time isn’t in the number. 20.5 hours of rough/find grinding, 14.25 hours of polishing, 2 hours of figuring (to .95 Strehl). Hmmm, could I make a career and a living producing 50 mirrors a week?



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