JODAS, © 2001-2006

Catadioptric

One Mirror
Schmidt Camera
Macsutov Camera
Houghton Camera
Two Mirror

See also:
Telescopes
Refractors
Reflectors

Schmidt Camera

Introduction
The Schmidt camera offers the unparalleled combination of a fast focal ratio and a large field angle of view. Compared to other astrocameras, the Schmidt camera offers an off-axis image sharpness ofer the entire field. Iths principal drawback is that the focal surface is curved and lies inside the instrument.

The Schmidt camera is based on the principle of symmetry and the ability of a corrector lens to suppress the spherical aberration of a spherical mirror. (see the One Mirror section). Bernhard Schmidt solved this problem by placing a thin aberration correcting lens at the center of the curvature of the spherical mirror.

Optical Schema: Ray Tracing Optical Schema: Telescope Specifications


In a spherical mirror, the focal distance of the central rays is greater than that of the rays farther away from the mirror's center – in short, it suffers from spherical aberration. Schmidt eliminated this by placing a specially shaped lens in front of the mirror.

The zone wherte the corrector is thinnest is called the neutral zone, because rays pass through it without deviation. In order to bring light rays entering at other zones to the same focus, corrector zones outside the neutral zone have negative power while zones inside the neutral zone have positive power.

Since a Schmidt corrector is a refracting element, it causeschromatic aberration, but the aberration is not severe and can be minimized by the location of the neutral zone. Higher order residual monochromatic aberrations, coma and astigmatism, also depend on the place of the neutral zone.

The profile of a Schmidt corrector satisfies the following equation:

Za= A1.h2+A2.h4+A3.h6

In which Za is the depth of the curve, h is the off-axis distance and A1, A2 and A3 are constants depending on the refractive index of the glass, the focal ratio, the aperture, and the position of the neutral zone. For focal ratios slower than f/3, the shape of the Schmidt corrector may be described with sufficient accuracy by:

Za= A1.h2+A2.h4

The most interesting corrector is the one with its neutral zone at 86.6% of the radius because its color aberration is smallest.

Design and Optimization
Schmidt and Wright cameras are aplanatic systems, that is, both spherical aberration and coma are corrected. The Schmidt camera, with its corrector at the center of curvature of a spherical mirror, is also anastigmatic. In a Wright Camera the corrector is closer to the mirror, so the mirror must be aspheric in order to correct coma.

The process of optimization consists of finding a particular combination of two or three parameters – the shape of the corrector, the position of the corrector, and (if there is an aspheric mirror surface) the degree of asphericity – for which the axial and off-axis aberrations are smallest. During optimization, the designer should keep certain ground rules in mind, rather than changing every parameter in sight. In Schmidt correctors, for example, the paraxial power depends on the position of the neutral zone. Aschmidt corrector with its neutral zone at 86.6% of the entrance pupil heigh, has the minimum color aberration possible. Other positions of the neutral zone may be choosen, of course, but they will result in greater color aberration.

Applet Tag: Note: Relation between index number and color:
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