Astronomy Principles and Practice Fourth Edition.pdf

The Schmidt telescope 339
Wavefronts
(a)
Focal plane
(b)
Focal plane
Single star
pattern
Star 2
Star 1
Figure 20.6. The production of speckle patterns for (a) a single and (b) a close double star.
20.4 The Schmidt telescope
Improvements in wide-field photography over astrographic refractor systems were contemplated by
considering the use of specially designed reflector cameras. The simplest system would consist of a
single spherical mirror with an aperture stop at its centre of curvature. Such a camera has no unique
axis and any point in the field would be imaged equally well—the focal surface would be curved, with
a radius given by the focal length of the mirror.
When such simple camera systems are designed to have smaller focal ratios than those of the
fastest astrographic refractors, the images suffered severely from spherical aberration. Many ways
were suggested whereby this aberration might be reduced but it was not until the 1930s that a design
was successfully manufactured by Schmidt in Hamburg. The system made by Schmidt was similar, in
principle, to an optical device for a good quality parallel light source suggested and patented by Kellner
some 20 years previously.
The Schmidt telescope is basically a two-element system, whereby the spherical aberration
produced by the spherical collecting mirror is compensated by using a thin aspherical correcting lens
or plate at the centre of curvature of the mirror. There is a range of shapes that the corrector lens can
take. The basic parts are depicted in figure 20.7 in which the departure of correcting lens from being a
parallel plate is greatly exaggerated. It is usual practice to make no effort to correct for the curvature of
the focal surface and specially produced photographic plates or film are required to match the curvature
of the surface of best focus of the camera. Since the introduction of the basic design, improvements
have been suggested and investigated which use more elements in the optical system but these will not
be discussed here. Schmidt cameras can have focal ratios of f/1·0 or smaller and can provide field