Re: [ATM] Fine Grind already Blanchard ground mirror blank back?

[Mike Lockwood <melockwo@uiuc.edu>]

Hi,

Michael Fremont wrote:
> I just received a 16" 1.625" thick pyrex blank from Newport Glass.  It
> is pregenerated to f4.5 and Blanchard ground on the back.
> 
> Having looked through the archive, I'm still unsure whether I should
> fine grind the back at 220 grit even though it's Blanchard ground; and
> am further unsure that I can even do it, since the front already has a
> curve.

If Blanchard ground, the back should be quite flat.  You should always 
check this, though.  When I have spent the money on quality blanks, 
I've never seen a bad Blanchard grind..... yet.

If the back is indeed flat right out of the box, then there's no real 
measurable benefit to grinding it, unless you desire a smoother 
surface finish in order to allow it to slide more easily on the mirror 
cell support points.  That's a good thing, in my opinion, but 
inconvenient if you have to make or buy a flat tool.

Check the back with a known good straightedge across several 
diameters.  You should get EXACTLY the same result across all 
diameters that you check, using the same side and portion of the 
straightedge.

If the back of the mirror is slightly concave or convex, you might 
consider grinding it, but it's not necessary if the back is a figure 
of revolution.  If it measures differently across different diameters, 
you should DEFINITELY grind it flat, because the back of the mirror is 
astigmatic, and that will tend to make the front astigmatic.

Once you start grinding the back and you get it flat, there really is 
little extra work involved in taking it down to 25u and really getting 
it smooth, so you might as well do it.

Some may argue that the subsurface damage from the generating of the 
back should be removed by grinding or by other means (Bob Goff used to 
dip pieces in HF acid to etch away any damage).  However, in 
well-annealed, reasonably thick glass where accuracy beyond 1/50th 
wave is not important, this is really not worth it, in my opinion.

	Mike Lockwood


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