Re: [ATM] Polishing and a red laser

[Guy Brandenburg <gfbrandenburg@yahoo.com>]

In my opinion, for finding out whether it's adequately polished, a radio shack mini illuminated  microscope does a great job for roughly $10. If you can't see anything at 60x or 100x, then it's sufficiently polished. I don't trust the red laser test, because I think it's too subjective, especially if you don't have a really-and-truly-perfectly-polished reference mirror on hand to compare with.

That being said, surface smoothness or roughness is a bit harder to see. You CAN see it in a ronchi or foucault tester with the naked eye. According to a paper that a wonderful other member of this list sent me, and to whom I owe some rouge, different polishing compounds have different grain sizes and different chemical actions on the glass at different pH values, and the type of pitch probably makes a difference, too. I find that cerium oxide from most places does an adequate, but not great, job of getting the mirror to where you want; but if you want it really smooth, switch to rouge. I just use distilled water to make the slurry, so I assume the pH is roughly 7. I don't add anythiing else, and I haven't made tests of my own to find out what pH level is best for the materials we have.

However, it very may well be the case that other commercial sources of cerium oxide or zirconium oxide or whatever have smaller particle sizes than what we use, and it might not be necessary to switch to rouge in those cases. With what we have on hand, we find that rouge makes the surface much, much smoother at the end.

If the surfaces aluminized decently, then the mirrors you have were probably decently polished out; but microripple may or may not remain. Again, use one of those mini-microscopes and relax, knowing that you can really and truly tell whether pits remain or not.

I got a 16.5" polished out in something under 7 hours - was it 4 hours? by using a LOT of pressure and either cerium or zirconium oxide. Not plate glass, either. If I were not using lots of force, it would have taken a much longer time. Hoever, it had a horrible figure - terrible astigmatism, so it took a few hours to fix that, too.

Mitchell R <funnybone101@embarqmail.com> wrote: 
I looked at the surface with a red laser this evening. I was dismayed to
find that a substantial amount of the light was reflected by the first
surface rather than passing through. It seems to illuminate very small pits. I looked at other surfaces that I have made and found the same to be true with them, although they aluminized very well (I had subsequently removed the aluminum as it had become quite dirty from sitting around and I wanted to recoat them).

Does anyone have any experience with using a laser to ascertain the state of polish on a mirror substrate? I'd like to hear your thoughts.

Thank you.

Francis J. O'Reilly 

Well I've learned a few things about the laser test.
1. It will show blatantly on the surface if you're not fully polished out.
2. If you want to be really picky do it in a dim/dark room.
3. Even an extremely well polished surface with reflect a tiny amount but
that's no excuse for under done polishing. You can usually tell because the edge will be brighter. Even in the dark if the mirror is clean it is almost not there.

On my 10" I found after pushing for 10 hours *REALLY* hard (letting the mix dry up a bit so friction really, really picked up, good contact, etc) I was almost totally polished out. I called it done after 11, but there was the slightest improvement noticeable up until 12.5 hours. Then it was basically not visible.
Mitch 




Guy Brandenburg, Washington, DC
My home page on astronomy, mathematics, education:
http://home.earthlink.net/~gfbranden/GFB_Home_Page.html
or else 
http://tinyurl.com/r6fh2

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