Path: utzoo!utgpu!news-server.csri.toronto.edu!mailrus!uflorida!rex!samsung!munnari.oz.au!uhccux!tholen
From: tholen@uhccux.uhcc.Hawaii.Edu (David Tholen)
Newsgroups: sci.space.shuttle
Subject: Re: telescope mirrors
Message-ID: <8884@uhccux.uhcc.Hawaii.Edu>
Date: 6 Aug 90 11:19:11 GMT
References: <9776.26b81e79@pbs.org> <32494@cup.portal.com>
Organization: University of Hawaii
Lines: 11

In article <32494@cup.portal.com>, mmm@cup.portal.com (Mark Robert Thorson) writes:
 
> BTW, I've heard that infrared mirrors use gold as a coating.

Some do, usually the secondary mirror.  It's not the reflectivity that makes
gold so desirable.  At infrared wavelengths, aluminim has perhaps 98%
reflectivity and gold has about 99%.  So why go through the expense of 
using gold if you only gain 1% more light?  Well, what doesn't get reflected
gets absorbed and then reradiated at thermal wavelengths.  So by going with
gold, you're cutting the emissivity in half -- a big gain if you're working
at thermal wavelengths!