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From: francis@hydracs.ua.oz.au (Francis Vaughan)
Newsgroups: sci.space.shuttle
Subject: Re: telescope mirrors
Message-ID: <1289@sirius.ucs.adelaide.edu.au>
Date: 6 Aug 90 07:54:54 GMT
References: <9776.26b81e79@pbs.org> <32494@cup.portal.com>
Sender: news@ucs.adelaide.edu.au
Reply-To: francis@cs.ua.oz.au
Organization: Adelaide Univerity, Computer Science
Lines: 61

In article <32494@cup.portal.com>, mmm@cup.portal.com (Mark Robert
Thorson) writes:
|> 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.


For the most part this is correct, but only when talking about conventional
refracting lenses. The different focusing points is due to the fact that
most materials refractive index is dependant upon wavelength. This property
is called dispersion. It is this property that causes both chromatic aberration
and rainbows. A refracting telescope will (if it is any good at all) have a
two element "achromat" lens that is made in two parts, each with a different
refractive index, and a design that attempts to correct the dispersion of 
visible light and bring all wavelengths to focus at the same point. (This is
of course impossible, but a reasonable compromise is made.) There do exist
special low dispersion glasses, Nikon's ED (Extra-low Dispersion) lens range
use them. They are noticably sharper than lenses using conventional glasses
because of an almost total lack of chromatic aberration. They have the
interesting
property that they have no special infra-red index mark. IR focusses at the
same point as visible wavelengths. They are also unbelivablely expensive.
(Like over $10k for some monsters.) 

So much for lens systems. For reflectors there is no such thing as dispersion.
A reflecting telescope has NO chromatic aberration and so long as the
reflecting
coatings reflect the light the image will as good in any wavelength
observed as 
another. (Well almost true, if the optics are REALLY diffraction limited
- the HST
was supposed to be, and we hope will eventually be so - the resolution drops
in proportion to the increase in wavelength.) The reason for coating mirrors
with gold and such is that reflectivity at visible wavelengths may have no
bearing on reflectivity at other wavelengths. Silver is transparent to UV (I
think, its been a while since I checked). 

Many of the amateur telescopes use special coatings on the mirrors and 
corrector plates (taking schmidt-cassegrains here), these would be worse
than useless if one wanted to do work in far wavelengths. Of course the
corrector plate being ordinary glass would absorb many interesting wavelengths
anyway. Schmidt-cassegrains do not suffer from chromatic aberration in the
way refractors do because the light is bent such a small amount when passing
through the corrector and the corrector is so thin that the aberration induced 
is undetectable.


						Francis Vaughan