Re: [ATM] Flat grinding the back of the primary mirror

[info@stathis-firstlight.de]

I think, I should tell more details about this:

> But, if the material was borofloat, one side was supposed to
> already be quite flat 

Yes, Borofloat is fairly flat, the unevenness is always below 0.1 mm, in
most cases less than 0.05 mm. It proved, that this is not flat enough at
least for sizes lager that 16”. My explanation: When working TOT, the
slightly higher spots press the glass and bent it a bit more then the rest
and thus the surface at this spots is polished a bit more than on the rest
of the area causing astigmatism. Rotating the mirror on the working table
(which is absolutely important when polishing TOT) does not help against
this problem, as the uneven spots rotate with the glass. 

> Grinding the 
> back flat and then retouching the front by polishing wouldn't make a 
> physical warp this size go away.

The amount of astig that others and I have observed, was not much (approx.
0.3 to 1 wave at the wavefront). After heaving fixed the back, polishing
for a while with a large lap (60-100% of mirror size) was sufficient to
get rid of the astig., no fine grinding was necessary. But indeed, in one
case a guy working a 16” had to go back to 15 my microgrit after he saw,
that the polish didn’t progress symmetrically.

> How do you grind back flat? Using A,B,C method or else?
> How flat is flat in this case?

I grind 3 or more blanks against each other with grit #180, until they
have full contact (saphire test) and smoothen them out with #320. To my
understanding they don’t need to be exactly flat as long as they are
symmetrical (figure of revolution). Slightly convex or concave is
harmless. I use this many blanks, because I sell them to others and have
sufficient of them. I think, that even grinding one blank on a thin well
supported window glass up to full grit contact will work fine (don’t
rotate the window in this case). 


> How the astigmatism
> of the front surface mirror disappeared by working its back?

As Mike clarified, after grinding the back flat, the astig is still
present of course. It disappears only AFTER polishing the FRONT with a
large tool.


Astig free wishes from cold and wet Germany 

Stathis Kafalis
www.stathis-firstlight.de

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