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From: mmm@cup.portal.com (Mark Robert Thorson)
Newsgroups: sci.space.shuttle
Subject: Re: telescope mirrors
Message-ID: <32494@cup.portal.com>
Date: 6 Aug 90 05:43:48 GMT
References: <9776.26b81e79@pbs.org>
Organization: The Portal System (TM)
Lines: 21

I'm not an expert on infrared astronomy, but as a photography hobbyist I am
aware that photographers who use infrared film generally use the same
lenses as for visible light work.  Most lenses have a focusing ring, which
you turn to get the image in focus.  On a single lens reflex or rangefinder
camera, you focus by turning the ring and looking through the eyepiece
without looking at the little numbers engraved on the focusing ring.
These numbers tell you the distance the lens is focussed for.

When using infrared film, you focus the lens, then take your eye away from
the eyepiece and look at the focussing ring.  On the lens body next to the
focusing ring will be a mark indicating which engraved number on the
focussing ring correspond to the focus distance.  On a serious lens for
photography, there will be a second mark, usually indicated in red, for
infrared.  Once you've focussed the lens for the visible light distance,
you then turn the ring so that the same distance is lined up with the red mark.

I don't see how infrared optics would be any different from visible light
optics, except for the coating used on mirrors and the material used for lenses.
After all, parabolic reflectors are used for both optical Newtonian telescopes
and radar antennas.  BTW, I've heard that infrared mirrors use gold as a
coating.