Re: [ATM] Grinding questions

[Ken Hunter <atm_ken_hunter@yahoo.com>]

John,

If you are going to replace a mirror into an existing
telescope you have to match it fairly well in order to
avoid having to make new holes for the mirror mounts
etc. 

As you've surmised, the corrector is going to perform
best if the new mirror matches the original mirror.
You've also found out why most of us do not use plate
glass if we can help it. It does have a much larger
thermal expansion than other types of mirror
substrates. 

However, you can finish this glass into an acceptable
mirror with careful attention to letting it stabilize
before testing and knowing that it's performance will
be slightly degraded during operation due to those
temperature differences you will experience during a
nightly observing session.

Bet bet is to use (but verify) that spherometer. 
Spritz some water on the mirror and see if you can
find the ROC with a small light to get a cross check
on the spherometer. 

I'd think that you'd be best off if you can match the
focal length within 1/8 to 1/4 inch - 1/8 to 1/4 can
usually be taken up with the mirror mount adjustments.
Any more difference than that, and you won't be able
to focus through the regular full range of eyepieces
and may have to move the mirror mounts. 

Ken Hunter


--- John Stephen <atmlist.net@johnstephen.org> wrote:

> I'm grinding my first mirror, an 8" F4 on 3/4" plate
> glass, and have some questions.  


		
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