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Schmidt-Cassegrain Design

Schmidt-Cassegrain Design

Above: Optical layout of a typical Schmidt-Cassegrain telescope

This section details the optical design and inherent aberrations of Schmidt-type catadioptric telescopes. For a more basic overview of these designs please see the Schmidt-Cassegrain page and Schmidt-Newtonian page. For a review of the optical design terms, see the Optical Aberrations and Optical Design sections.

Schmidt Camera Design

Since the advent of CCD cameras and the discontinuation of film use, the Schmidt camera is rarely seen any longer, but it is the basis for the other designs discussed here. The Schmidt camera uses a spherical mirror to focus light onto a curved film plane located inside the telescope tube. The curved focal plane eliminates the inherent field curvature. A full-aperture corrector lens is then placed at the center of curvature of the mirror. Placement at this location eliminates coma. The aspheric shape of the lens is used to correct the spherical aberration of the mirror. The overall system is very fast, usually f/1 to f/3.

Schmidt-Cassegrain Design

The Schmidt-Cassegrain telescope (SCT) is one of the most popular designs among amateur astronomers. It is a very versatile system, useful for all types of visual observation as well as photography. The SCT is based on the classical Cassegrain design. The classical Cassegrain uses difficult-to-make aspheric mirrors. In most commercial SCTs, spherical mirrors are used. A Schmidt corrector lens is then placed over theaperture to correct spherical aberration. This also has the benefit of eliminating the need for spider vanes to support the secondary mirror. A mechanical design feature of most commercial SCTs is a moveable primary mirror for focusing. This gives a larger focus range than using an external focuser would.

Schmidt-Cassegrain Aberrations

Spherical aberration is corrected by the Schmidt corrector lens. The main aberration present in commercial SCTs is coma. Like the Schmidt camera, placing the corrector plate near the center of curvature of the primary mirror would eliminate coma, but the tube length becomes impractical. For example, a standard 8" SCT would be 26" long instead of 16". Alternatively, one of the mirrors (most likely the secondary) could be aspherized to eliminate coma. This would increase the cost of the system, and for most purposes, the inherent coma is not problematic, so the aberration remains in favor of cost and ease of manufacture. However, for imaging with larger-format cameras, coma can be an issue. Some designs, such as Meade's RCX and LX200R series, use an aspheric secondary to remove coma but retain a compact system. Like most Cassegrain designs, field curvature is an issue. This can likewise be problematic for large-format imaging. Field flatteners are sometimes available to reduce this aberration. The Schmidt corrector does introduce some chromatic aberration, and in the interest of cost, the corrector lens is often designed for minimal glass use rather than optimal color correction. However, the amount of chromatic aberration is very small in either case and is not normally an issue. Astigmatism is negligible.

Schmidt-Newtonian Design

The idea behind the Schmidt-Newtonian is to take the basic Newtonian design and minimize coma. Fast Newtonians suffer from large amounts of coma. The Schmidt-Newt replaces the parabolic primary mirror on the standard Newtonian with an easier-to-make spherical version. A Schmidt corrector is then placed just ahead of the secondary mirror (supporting the secondary and eliminating the need for spider vanes). As in a Schmidt camera, placing the corrector at the center of curvature would eliminate coma entirely, but would result in an absurdly long optical tube. Typicalfocal ratios range from f/4 to f/5.

Schmidt-Newtonian Aberrations

The Schmidt corrector eliminates spherical aberration and minimizes (but does not eliminate) coma. Coma is about half of that in a comparable standard Newtonian, but about twice that of the similar Maksutov-Newtonian design. The corrector lens does introduce a small amount of chromatic aberration. There is some astigmatism, but the effect is lost in the coma. Field curvature exists although it is typically less than in a comparably-sized Cassegrain. As with most telescopes, distortion is negligible.

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