Re: [ATM] Grinding questions

[Bill <astronutski@yahoo.com>]

One thing I would add as far as the focal length is concerned is
don't forget to factor in the mirror thickness compared to the
original.  I don't know what's in there but would assume it's thicker
than 3/4".
-Bill

--- Ken Hunter <atm_ken_hunter@yahoo.com> wrote:

> 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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