Schmidt Telescope

Most telescopes are either refractors or reflectors (click here to know more about telescopes). However, there are some examples of hybrid telescopes that use a large correcting plate at the entrance of the telescope to correct for an aberration (click here to know more about telescopes' aberrations).
The most widely used hybrid telescope is the Schmidt design, first developed by Bernard Voldemar Schmidt (1879-1935).

In this kind of telescope, the incoming light passes through a thin correcting plate, is reflected by a large short-focus spherical mirror, and focused on a curved photographic plate.
In this kind of telescope, the images are sharp over a large field of view. However, due to the shape of the mirror, there would be spherical aberration if the image wasn't fixed by the correcting plate that is placed at the entrance to the telescope. A primary use of this kind of telescope is to survey large areas of the sky. Because there is minimal coma, each image can record a large area in sharp focus. This reduces the number of images that are needed to cover the sky.

The most famous Schmidt telescope is the one at the Palomar Observatory, operated by the California Institute of Technology. The primary mirror is 1.83 m (= 72 inches) and the correcting plate is 1.22 m (= 48 inches). One of its first projects was to record the whole sky visible from southern California in both blue and red light. This was done in the 1950's when photographs were the only possible detector. Even today, however, only photographs are large enough to record the large area imaged by this Schmidt telescope (each survey needed 935 images to cover the sky visible from the observatory, and these were done in two colours). The images were then systematically searched for various things, such as clusters of galaxies and the gas ejected by dying stars.
The corrector plate of the Palomar Schmidt telescope has now been replaced with a better one, and the whole sky survey is being repeated using higher resolution, more sensitive photographs that are recording finer detail of fainter objects. In addition to revealing more of the objects found in the earlier survey, it will be possible to detect changes in form and changes in positions of objects during the nearly 50 years separating the two surveys.

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