Re: [ATM] undercorrecting?

[Mike Lockwood <melockwo@uiuc.edu>]

Hi,

Nils Olof Carlin wrote:
> Now if the mirror were "black" in the relevant IR wavelength (I 
> believe of the order of 10 micron wavelength) at the side turned to
> the sky, it could cool by radiation loss. I suspect it is rather
> reflective here and this cooling wouldn't be very efficient, but I
> don't know. If the sky, seen from the mirror, takes only a minimum
> steric angle such as from a bottom of a tube (solid or shrouded),
> radiative cooling would be small and possibly negligible. If you
> ever get dew on your primary, it isn't, of course.

Nils, you've got it.

A friend pointed out to me that the IR emissivity of aluminum is quite
low at longer wavelengths, so the radiative cooling is very
inefficient.  (For those unfamiliar with the terms, that's different 
than CONVECTIVE cooling)!  Therefore convective heat transfer 
(transfer due to moving air) does indeed dominate.  This is done by 
ambient temperature air, as I mentioned in a previous message.

To get dew you just need a little bit of slightly warmer air laden 
with moisture, which is not hard to find.  (After all the observer, 
being warmer and largely composed of water, is often repsonsible for 
dewing the eyepiece and possibly the secondary!)

Mel Bartels wrote:
> Yes, the suspicion that people are making excuses for incomplete 
> mirror figuring by rationalizing that undercorrected incomplete
> mirrors are somehow ok.  I've made studies of figure change versus
> temperature drop at the eyepiece using the star test.  Believe me,
> you don't want an undercorrected mirror.  If nothing else, what do
> you do about the early morning hours when the air can begin to
> warm?

Agreed.  After fixing some undercorrected mirrors for friends they 
were definitely much happier with their optics!  (Of course 
overcorrection is bad too.)

Why did the original optician leave them undercorrected?  My guesses 
are laziness/rushing production, poor or inaccurate testing, and, much 
less frequently, on purpose.

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
http://bi-staff.beckman.uiuc.edu/~melockwo/index.html

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