Catadioptric telescopes

5.9 Catadioptric telescopes

This class of telescopes uses a combination of a mirror and a lens to produce the image. In general, spherical mirrors (which are cheap to make) are used to

produce the image together with the lens – normally called a corrector lens – to correct for the spherical aberration that would be caused by the spherical mirror.

5.9.1 The Schmidt camera

The Schmidt camera was invented in 1930 by Bernhard Schmidt. He wanted to design a new type of instrument which would have a very large fi eld of view yet

be free of aberrations such as coma and would have a short focal ratio so allowing fainter stars to be observed for a given exposure time. To eliminate chromatic aber- ration the new design would use a mirror as the primary, but to give a large fi eld of view and eliminate coma a spherical mirror would be needed. However, spherical mirrors suffer from spherical aberration. He realized that he could correct for spherical aberration if a corrector plate was placed at the radius of curvature of the spherical mirror. This has a varying thickness across its aperture to compen- sate for the fact that a spherical, rather than a parabolic, mirror is used.

Schmidt cameras have become one of the most useful tools of modern astronomy, ideally suited to photographing large star fi elds in the Milky Way, showing 10 000 stars on one negative (Figure 5.12)! Highly valuable sky surveys have been made using such cameras, one example being the 48 in. Samuel Oschin Schmidt Telescope at Mount Palomar which produced the Palomar Sky Survey, completed in 1958. The fi lm plates were 14 in. square and covered an area of sky

Observing the Universe

Figure 5.12 Schmidt plate showing the Orion Nebula region. Image: Asaigo Science Archive Database.

declination of minus ⫺27°. Plates were made with both blue and red sensitive emulsions and were sensitive to stars of ⫹22 magnitudes (about 1 million times fainter than the limit of human vision).

On a personal note, an asteroid numbered 15 727, now named after the author, was discovered in 1990 by the 2 m diameter Alfred-Jensch-Telescope at Tautenburg Observatory in Germany, the largest Schmidt camera in the world.

5.9.2 The Schmidt–Cassegrain telescope

The design of the Schmidt camera has led to a widely used variant of the Cassegrain telescope. In larger sizes, parabolic mirrors are expensive to make and the length of their telescope tubes make them rather unwieldy. Both of these issues are addressed in the Schmidt–Cassegrain design which uses a spherical mirror in a compact opti- cal tube assembly (Figure 5.13). The spherical aberration that would ruin the image quality of the spherical mirror is corrected by a ‘Schmidt’ corrector plate at the front

Introduction to Astronomy and Cosmology

Figure 5.13 Catadioptric telescopes.

through the primary as in the ‘Cassegrain’ design. There are thus no diffraction spikes (as caused by the spider in a Newtonian telescope) and, as the whole tube is enclosed, the mirror is kept dust free. The majority of Schmidt– Cassegrain telescopes have a focal ratio of f10 resulting in relatively long focal lengths. (A Schmidt–Cassegrain with a primary mirror of 200 mm will have a focal length of 2000 mm.) A consequence of the design is that quite a large ‘central obstruction’ is required; partly as a relatively large secondary mirror is required by this design, but also to help prevent light entering the telescope aperture passing directly into the eyepiece. As in a Newtonian telescope, the central obstruction also has an effect on the image quality; light being transferred from the central peak of the Airy disc into the surrounding rings – though the diameter of the central disc is actually reduced.

5.9.3 The Maksutov–Cassegrain telescope

The Maksutov is a catadioptric telescope design, developed by the Russian optical specialist Dmitri Maksutov in 1941, which employs a full diameter meniscus lens to correct the problems of off-axis aberrations. They are usually of a Cassegrain form, with the secondary mirror either silvered on the inner face of the correc- tor lens or supported by it as shown in Figure 5.13. The result is a very compact telescope that gives a relatively wide fi eld of view and excellent quality images. As large aperture corrector plates are diffi cult to manufacture, their professional application is limited.