Fundamental Astronomy

3.2 Optical Telescopes
Fig. 3.13. The coudè system of the Kitt Peak 2.1 m reflector. (Drawing National Optical Astronomy Observatories, Kitt Peak
National Observatory)
The reflector has its own aberration, coma. It affects
images displaced from the optical axis. Light rays do
not converge at one point, but form a figure like a comet.
Due to the coma, the classical reflector with a paraboloid
mirror has a very small correct field of view. The coma
limits the diameter of the useful field to 2–20 minutes of
arc, depending on the aperture ratio of the telescope. The
5 m Palomar telescope, for instance, has a useful field
of view of about 4 ′ , corresponding to about one-eighth
of the diameter of the Moon. In practice, the small field
of view can be enlarged by various correcting lenses.
If the primary mirror were spherical, there would
be no coma. However, this kind of mirror has its own
error, spherical aberration: light rays from the centre
and edges converge at different points. To remove
the spherical aberration, the Estonian astronomer Bernhard
Schmidt developed a thin correcting lens that is
placed in the way of the incoming light. Schmidt cameras
(Figs. 3.14 and 3.15) have a very wide (about 7 ◦ ),
nearly faultless field of view, and the correcting lens is
Fig. 3.14. The principle of the Schmidt camera. A correcting
glass at the centre of curvature of a concave spherical mirror
deviates parallel rays of light and compensates for the spherical
aberration of the spherical mirror. (In the figure, the form
of the correcting glass and the change of direction of the light
rays have been greatly exaggerated.) Since the correcting glass
lies at the centre of curvature, the image is practically independent
of the incoming angle of the light rays. Thus there is
no coma or astigmatism, and the images of stars are points on
a spherical surface at a distance of R/2, where R is the radius
of curvature of the spherical mirror. In photography, the plate
must be bent into the form of the focal surface, or the field
rectified with a corrector lens
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