Schmidt camera

In some cases the detector is made curved; in others flat media is mechanically conformed to the shape of the focal plane through the use of retaining clips or bolts, or by the application of a vacuum.

A field flattener, in its simplest form a planoconvex lens in front of the film plate or detector, is sometimes used.

Since the corrector plate is at the center of curvature of the primary mirror in this design the tube length can be very long for a wide-field telescope.

It is now used in several other telescope designs, camera lenses and image projection systems that utilise a spherical primary mirror.

This method requires a high degree of skill and training on the part of the optical engineer creating the corrector.

[8][9] Schmidt himself worked out a second, more elegant, scheme for producing the complex figure needed for the correcting plate.

[10] A thin glass disk with a perfectly polished accurate flat surface on both sides was placed on a heavy rigid metal pan.

[8] The glass plate could also break if bent enough to generate a curve for telescopes of focal ratio f/2.5 or faster.

[11] Also, for fast focal ratios, the curve obtained is not sufficiently exact and requires additional hand correction.

The upper exposed surface is then polished flat creating a corrector with the correct shape once the vacuum is released.

[13] Because of its wide field of view, the Schmidt camera is typically used as a survey instrument, for research programs in which a large amount of sky must be covered.

The European Southern Observatory with a 1-meter Schmidt telescope at La Silla and the UK Science Research Council with a 1.2 meter Schmidt telescope at Siding Spring Observatory engaged in a collaborative sky survey to complement the first Palomar Sky Survey, but focusing on the southern hemisphere.

The spherical mirror used in this telescope was extremely accurate; if scaled up to the size of the Atlantic Ocean, bumps on its surface would be about 10 cm high.

In 1977 at Yerkes Observatory, a small Schmidt telescope was used to derive an accurate optical position for the planetary nebula NGC 7027 to allow comparison between photographs and radio maps of the object.

In the 1930s, Schmidt noted that the corrector plate could be replaced with a simple aperture at the mirror's center of curvature for a slow (numerically high f-ratio) camera.

In 1940, James Baker of Harvard University modified the Schmidt camera design to include a convex secondary mirror, which reflected light back toward the primary.

[24] A later paper by Baker[25] introduced the Paul-Baker design, a similar configuration but with a flat focal plane.