Re: [ATM] undercorrecting?

[spoon@pageamerica.net]

I see I'm getting in pretty late on this thread. I correct larger Pyrex
mirrors (16 to 20inches) in slightly falling temps using autocollimation.
It is usually NOT a simple or smooth undercorrection and I've come to
think that perhaps anneal, strain and who knows what are affecting things.
When I check the mirrors in warming conditions the figure is often
horrible so I check the figure over a week or so to ensure a good null for
the conditions. It's only the occasional small mirror (~10 inch) that
responds a noticable amount. I've seen some 10" pyrex disks about as good
as low expansion glass. THicker mirrors are worse of course. I check with
the user before figuring - as Mel points out that the mirror wouldn't be
as good near morning with heating. I refigured a 20" that had been
returned to the maker and pronounced within specs that went 3/4 wave
overcorrected on the outer 3" during evening viewing. It looked pretty
good in the morning on the bench. But a 10" beat it up in the evening. I
guess it depends when you want to use the mirror in question. I typically
don't stay up all that late anymore and my most productive viewing is in
falling temps. Anyway I prefer low expansion substrates if I can find
them. I can't imagine worrying about the effect of coatings on cooling -
maybe someday I'll be able to make a mirror well enough that it would
matter but so far there's enough work to get the figures passable. I only
have experience testing mirrors in these different temperature deltas.
Don't have much knowledge of how forced cooling affects everything.

I guess if what I've described is myth, then I better erase the
photographic evidence and start looking for unicorns. :)

Best,
--Mike Spooner


> Yes, let's hope this myth is buried now.
>
> Jan
> http://home.wanadoo.nl/jhm.vangastel/Astronomy/
>
> ----- Original Message -----
> From: "Mike Lockwood" <melockwo@uiuc.edu>
> To: <atm@atmlist.net>
> Sent: Friday, August 04, 2006 6:41 PM
> Subject: Re: [ATM] undercorrecting?
>
>> Hopefully this thread of messages will debunk the ideas that coatings
>> significantly affect mirror cooling and correction, and that a mirror
>> should be undercorrected if it is to be used on nights of falling
>> temperatures.  We really want fully-corrected mirrors close to ambient
>> temperature.
>>
>> Mike Lockwood
>
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> ATM mailing list http://www.atmlist.net/
>

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