Re: [ATM] Grinding questions

["John Stephen" <atmlist.net@johnstephen.org>]

> It is now time to start doing some TOT grinding to open the radius up.
> If you work at it a bit, you will be able to keep the ROC within 1/4" of
> the
> desired ROC without any real problem.  I'll note that it isn't til you get
> to a polished state that the real ROC will pop out.


I've been working TOT since the #80 grit (I'm on #220 now) because I
overshot my desired curve quite a bit initially.  I've put a lot of work
into flattening it back out again.  That reminds me -- how small a bevel is
still acceptable on the edge?  Mine's now down to about 1/16".  I know the
risk of chipping is higher, but do coating services also enforce minimums or
anything like that?

> As to the temp problem that you are seeing (thinking that you are seeing),
> if you really are seeing such, consider that piece of glass very suspect
> for
> stress as the ROC shouldn't be changing at all.

Now I'm really wondering if I should get a new blank and switch to Pyrex.  I
haven't tried Ken Hunter's suggestion to verify the spherometer's absolute
accuracy, but I've got a flat for zeroing it and I have pretty good
confidence in its ability to provide reproducable readings. Those readings
are definitely showing some pretty wide swings for 30 minutes or so after I
stop grinding.

> The larger grit size
> produces pits of substantial size and you may be seeing the legs of the
> spherometer falling into different holes as well as the nonsphericalicity
> of
> the surface at that grit size if you are still using a long stroke with a
> wide W.

I understand what you mean by the pits causing jitter in the readings -- I
now wash the mirror with soap instead of simply rinsing it because I've had
bizarre readings seemingly caused by stray particles of grit.  I have been
using a long W stroke with TOT for a while in order to flatten the curve,
but yesterday switched to a normal COC stroke.  I'm seeing a really nice
sphere when I stop grinding, but after it rests a while there are small
variations across the surface.  Perhaps that's another indication that I may
need some new glass <sigh>.

> Finally, measuring with a spherometer is indeed measutring the SAG of the
> surface.

Yeah, I kind of knew that :-)  I was feeling insecure about presenting my
"case" to the group and wanted it to be clear why I was expressing the
mirror in terms of focal length instead of the SAG of the diameter,
especially since I'm still on rough grinding.  I just worry too much...

Thanks to all for so much great advice,
- John


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