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Re: [ATM] undercorrecting?
Aloha Arjan,
Congratulations on your splendidly informative paper!
Contributions like this certainly elevate our enterprize here, Thanks!
I do have an observation and comment regarding:
>Ah, that's easy. I found a number of web pages listing emissivity of various
>surfaces. Just Google 'emissivity' or go directly to
>http://www.electro-optical.com/bb_rad/emissivity/matlemisivty.htm
>You will see that polished Al is 0.04-0.06, and CVD Al with SiO overcoat is
>set at 0.04. For the various kinds of glass you can take 0.95. Polishing
>does not change that much.
As in your link above, you use values related to "Polished Al",
perhaps missing the Aluminum value in the Vapor Deposited Coatings
segment of your citation. Very few mirror makers polish aluminum and
use it as a reflector; rather, the bulk of our brethren use vapor
deposited aluminum over glassy substrates. The value in the above
link, gives a 0.02 emmisivity. I note that coating vendors routinely
measure reflectances at or above 0.99 at 10 micron wavelengths for
their aluminum films.
http://grus.berkeley.edu/~jrg/COATING/node3.html
Emissivity being related to reflectance, would be very close to
0.01 for pure aluminum films at that wavelength. Further, the
temperature of our mirrors in use (near 290 K.), yield a blackbody
peak very near that 10 micron wavelength. The bulk of the emitted
energy would be closeby.
In my mind, this 2x to 4x difference in the actual emission in no way
invalidates your fine contribution. It simply reduces the radiative
effect by that same factor. Convective thermal transfer dominates in
our situations, and these other, smaller, contributions should not be
ignored.
> > This has been a great topic by itself, and it also started a great
>> thread about how eyepieces affect the star test. Lots of good stuff
> > being investigated!
I agree strongly with Mike's accolade, and would be delighted to see
more of your work!
In appreciation, and with respect,
Stan Truitt
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