Re: [ATM] Working with Large Thin Mirrors

[Dale Eason <atmpob@yahoo.com>]

How do you know you have some astig?  What measurment
technique did you use?  Until it is polished out can
you really tell?

Dale Eason

--- Peter <peter@kitgear.com> wrote:

> Hi,
> 
> Aperture feaver has set in really bad so I've
> decided to get back to work on
> my 25" x 1" thick mirror. I'd like to ask for advice
> how is best to proceed
> to minimize astigmatism. I've polished just enough
> to see that I do have
> some astig which I'd like to remove before
> proceeding.
> 
> Here is the basic info so far:
> 
> - Mirror is 25" OD plate glass at f/4.8 and 1" thick
> at the edge
> - The tool is a sub-diameter ceramic 3/4 size (19")
> and 2.25" thick.
> - The mirror was until now supported on the bottom
> with thick carpet
> throughout all of the grinding and polishing
> - Polishing pads are being used
> - The back of the mirror has not yet been ground
> flat
> 
> Some questions I have before proceeding are:
> 
> 1)	Should I grind the back side? If yes then how
> would I do this for such a
> large mirror or could I make it a shallow sphere?
> 
> 2)	Is it best to go with a full size tool Vs
> sub-diameter?
> 
> 3)	Should I work it with the mirror on top to better
> support the glass?
> 
> 
> Please let me know what you think is best.
> 
> Thanks,
> 
> 
> Peter
> 
> 
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> 



		
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