Re: [ATM] Astigmatism

["Khail Rosenberg" <mikhail.rosenberg@rosenbergtelescopes.com>]

I second that... i have made many plate glass 16" ers... very cheap, and
good. most people are quite happy with it for an ameture telescope

On 1/23/08, Mitchell R <funnybone101@embarqmail.com> wrote:
>
>
>
> -----Original Message-----
> From: atm-bounces@atmlist.net [mailto:atm-bounces@atmlist.net] On Behalf
> Of
> Che Ers
> Sent: Wednesday, January 23, 2008 12:36 PM
> To: ATM Mailing List
> Subject: [ATM] Astigmatism
>
> Hi,
>
> Is a 7" or 8" X 0.75" blank at a greater risk of ground-in astigmatism
> than
> a 1:6 thickness blank? If yes, how? And how would one avoid it (except, of
> course, by getting a full-thickness blank : )? Focal ratio would be more
> than 7.
>
> Thanks,
>
> Zain
>
>      Technically yes it is. The thinner you go the more a blank flexes
> under
> grinding and polishing pressure. 6:1 is the old ratio which works fine but
> is not necessary. If you support the mirror/tool (which ever is on the
> bottom) with soft rubberized foam/newspaper, etc and don't used overly
> heavy
> pressure you should be fine. Rotate them often. A blank in the size you
> mentioned is barely considered "thin" by what a lot of people have pulled
> off. I've seen a few 16" mirrors out of .75" plate glass on the net, a 7"
> or
> 8" should be no problem. Good luck!
>
> Mitchell
>
>
>
>
>
>
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