Re: [ATM] Grinding questions

["John Stephen" <atmlist.net@johnstephen.org>]

I've now managed to get the FL to within about 1/8", though the thermal
expansion thing is really troublesome.  When I get done grinding and go to
measure my progress, the mirror goes through multiple changes -- sometimes a
perfectly smooth sphere at the correct FL,  but 15 minutes later it may have
small variations across the surface with the FL being up to 1/2" too long or
too short.  It's aggravating and I'm seriously considering ditching the
plate glass and getting a Pyrex blank instead.

I hadn't considered checking my spherometer by wetting the mirror.  Is there
a good way to make the water stick to the surface of the rough-ground mirror
better?  Maybe I should add a small amount of detergent or something.

Thanks,
- John


----- Original Message ----- 
From: "Ken Hunter" <atm_ken_hunter@yahoo.com>
To: "John Stephen" <atmlist.net@johnstephen.org>; "ATM List" 
<atm@atmlist.net>
Sent: Tuesday, October 05, 2004 1:50 PM
Subject: Re: [ATM] Grinding questions


> 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 


_______________________________________________
ATM mailing list http://www.atmlist.net/