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

["Shane LaPierre" <shane@ct-astronomer.com>]

I was reading Kestner's article on making large mirrors 
last night (I'm pretty sure that was the article - it is 
reprinted in Kriege's Dobsonian book)... anyway, there 
were strict cautions advising to ensure to grind the back 
of the mirror even if it appears flat / is blanchard 
ground.  It was to ensure a flat back to prevent 
astigmatism.  I don't know... I can't imaging a surface 
blanchard ground requiring additional grinding to make 
flat.  They have all really looked good to me.  FWIW, I 
did a slumped plate glass several years ago that had an 
extremely convex back (reverse f5 probably) and had no 
stig.  Still, I worked it on a rug.

Good luck... I'd like to hear your final answer too.  I'd 
also be interested in your sending me the amt they charged 
you for this work (that is if you are willing).  You can 
send it to my email directly if you don't want to post to 
the list.

Thanks,
Shane LaPierre

On Tue, 06 Mar 2007 14:49:48 -0500 hermit 
<hermit@outofoptions.org> wrote:

> 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.
>>
>>   
> You can, but not much point.  Depends on whether it 
>needs to be for your 
> cell design.
> 
> Ken Lowther
> _______________________________________________
> ATM mailing list http://www.atmlist.net/

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