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From: rclark@lpl.arizona.edu (Richard Clark x4971)
Newsgroups: sci.space.shuttle
Subject: Re: telescope mirrors
Message-ID: <602@organpipe.UUCP>
Date: 11 Aug 90 07:09:14 GMT
References: <9776.26b81e79@pbs.org>
Sender: rbc@organpipe.UUCP
Organization: Lunar & Planetary Laboratory, Tucson AZ.
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In Message-ID: <9776.26b81e79@pbs.org>

>Although I use three telescopes, a 3" Maksutov, an 8" Schmidt/Cass. and a 6"
>Newtonian, I may not fully understand mirror design.  I just use them. 
>According to an E-mail, design has no effect on spectral response - only coating
>does.  Do other telescope experts out there agree with this?  I was under the
>impression Infrared, for example, does not focus at the same point that visible
>light does and the thing that effects the focal point the most is the curvature
>of the mirror.  I don't see how coating alone is going to solve this problem of
>more than one focal point.  If the mirror is going to be used primarily for
>infrared work, wouldn't its shape be a bit different?  I do know widefield and
>planetary scopes, dedicated to those purposes vary significantly in the
>degree of curvature. 

With reflective optics the shape, spacing, and tilt of the surfaces are
the only factors controlling the quality of image formation. In the case
of IR the relatively long wavelength relaxes the tolerances to which the
surfaces must be produced for a given image quality. (Quarter wave, 1/10th
wave etc). Although this wave specification refers to the wavefront at the
image, not the actual mirror surface. The coating is not equally reflective
at all wavelengths so that will be a factor in the telescope's efficiency in
collecting light.

With transmissive optics (lenses, corrector plates) the refractive index
is a function of wavelength so these elements introduce the shift in focus
with wavelength. For these elements the same factors as above (surface
figure, element spacing, tilt) and the change in refractive across each
surface (glas-air, flint-crown etc) also effect the power of the surface.

Hence chromatic aberations can be introduced into catadioptric (hybrid
systems with both lenses and mirrors) systems although usually the
refractive element in these is a corrector plate designed to control
spherical aberation (such as introduced by the all spherical surfaces
in the maksutov) and has very little focusing power. So cat systems
essentially free from chromatic aberations. In other words, the focusing
is done by mirrors and the lenses do the correcting.

The achromatization of refractor systems is accomplished by using elements
of different refractive index and dispersion. Dispersion being the rate
at which index changes with wavelength. With the familiar refractor
doublet objective it is possible to have 2 wavelengths focus at the same
point. These wavelengths can be chosen to bracket the wavelength range
that the ccd, film, eye, etc is sensitive to. The other degrees of
freedom (4 surfaces, 2 thicknesses, and one separation) can be used to
control the non chromatic aberatic aberations like spherical, coma, or
distortion.

Hope the questions I answered are close to the ones you were asking.