At Delmarva, two years ago, I made two 8 inch f/5 mirrors. Well, actually, retouched a 1/4 wave under corrected parabola I had made 40 years before on one mirror. It's now perfect!
The other 8 inch I made from scratch. Rough ground the curve at home. By the time I got to Delmarva, it was ready for fine grinding. Did all the fine grinding by hand on a fixed grinding stand, then polished by hand, also on a fixed stand and did the laser test. I used polishing pads at first with about three hours of very aggressive polishing, then switched to about 6 hours of much slower pitch polishing by hand. All along I was doing the laser test and could see the glass getting "blacker", or so I thought , as did others who looked at it. At some point I called it fully polished.
I then spent a few hours polishing in a very nice parabola using Dick Parker's DPAC tester as witness. Jail bar straight lines, just like the other mirror I had retouched. Brought the two mirrors to Normand Fullum for aluminizing. After a few days Norm calls to tell me my mirrors were ready but, pausing and with a hint of frustration in his voice he tells me: you could have finished polishing before parabolizing! The mirror I had made from scratch had a definite haze about 1/4 inch wide at the edge. Aluminizing takes no prisoners! This was the second time in my ATM life I had made that mistake. I can tell you this: IT WON'T HAPPEN AGAIN!
The concept of "black" polish is a cute term but I'm not sure what it means. The entire surface is either fully polished or its not. The laser test is a good test but when the polish is nearing the end, the laser test can be subject to interpretation and, especially, bias (as in: "I want it to be fully polished so the laser beam I see near the edge, that must be dust...).
If you can, use other tests to confirm the polish is complete like:
- look at reflection from an incandescent or other bright light at a grazing angle and/or,
- use one of these small eyepiece type microscopes to examine the surface under a bright light
And I would add this: Once you think you've polished fully, polish a few hours more, just in case. It's very embarrassing to pay good money for an aluminizing job only to have to strip it off to finish the polishing job for real and have to go through parabolizing all over again.
Been there, done that. Twice .
Edited by Pierre Lemay, 15 December 2018 - 10:04 PM.