施密特攝星儀
施密特攝星儀是一種設計用於廣視野但像差很小的天文照相機。其他相似的設計有賴特攝星儀和Lurie-Houghton望遠鏡。
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[编辑] 發明和設計
施密特攝星儀是伯恩哈德·施密特在1930年發明的 [1]。他的光學構造是以易於磨製的球面鏡做主鏡，和位於主鏡曲線前方的非球面鏡的修正透鏡，也就是熟知的修正板，底片或其他的檢測設備安置在攝星儀內部的焦點上。在設計上都允許快速的焦比和控制住彗形像差和球面像差。
施密特攝星儀的焦平面有很明顯的弧度(曲率)，因此使用的底片、乾版、或其他的檢測器都必須有相對應的弧度。在某些情況下，檢測器被製作出灣曲的弧度，在其他平面的媒介上則依據交平面的形狀使用螺栓或固定夾來調整，或是應用真空牽引。有時也會使用平場，-以他最簡單的形式，以一個平凸透鏡直接緊貼著底片。使用這種透鏡的稱為施密特-Väisälä攝星儀。
[编辑] 應用
The Schmidt camera is typically used as a survey instrument, for research programs in which a large amount of sky must be covered. These include astronomical surveys, comet and asteroid searches, and nova patrols.
In addition, Schmidt cameras and derivative designs are frequently used for tracking artificial earth satellites.
Starting in the early 1970s, Celestron marketed an 8-inch Schmidt Camera. The camera was focused in the factory and was made of materials with low expansion coefficients so it would never need to be focused in the field. Early models required the photographer to cut and develop individual frames of 35mm film as the film holder could only hold one frame of film. About 300 Celestron Schmidt Cameras were produced.
The Schmidt system was popular, used in reverse, for television projection systems. Large Schmidt projectors were used in theaters but systems as small as 8-inches were made for home use and other small venues.
Arguably the most famous and productive Schmidt camera is the Oschin Schmidt Telescope at Palomar Observatory. It was used for the National Geographic Society - Palomar Observatory Sky Survey (POSS), the POSS-II survey, the Palomar-Leiden (asteroid) Surveys, and other projects. The telescope used in the Lowell Observatory Near-Earth-Object Search (LONEOS) is also a Schmidt camera.
[编辑] 延伸的設計
[编辑] 無透鏡施密特式
Prior to Schmidt's design, the solution to spherical aberration was to place an aperture stop at the center of curvature of the mirror, stopping the aperture to f/10. This removes spherical aberration while preserving the wide field of the short focal-length mirror. However, it does so at the cost of light-gathering ability. Although this solution was well-known long before Bernhard Schmidt invented his corrector plate, the design is known as a "lensless Schmidt".
[编辑] 施密特-Väisälä式
Contemporary to Bernhard Schmidt's Schmidt camera design, but unpublished was also Prof. Yrjö Väisälä's identical design which he had mentioned in lecture notes in 1924 with a footnote: "problematic spherical focal surface". Once he saw Schmidt's publication, he promptly went ahead and solved the field flattening problem by placing a doubly-convex lens slightly in front of the film holder. This resulting system is known as: Schmidt-Väisälä camera or sometimes as Väisälä camera.
[编辑] 貝克-施密特式
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. The photographic plate was then installed near the primary, facing the sky. This variant is called the Baker-Schmidt camera.
[编辑] 貝克-Nunn式
The Baker-Nunn design, by Dr. Baker and Joseph Nunn, replaced the Baker-Schmidt camera's corrector plate with a small triplet corrector lens closer to the focus of the camera. A dozen Baker-Nunn cameras with 20-inch aperatures – each weighing 3.5 tons – were used by the Smithsonian Astrophysical Observatory to track artificial satellites from the late 1950s to mid 1970s.[1]
[编辑] 梅森-施密特式
The Mersenne-Schmidt camera consists of a concave paraboloidal primary mirror, a convex spherical secondary mirror, and a concave spherical tertiary mirror.
[编辑] 施密特-牛頓式
The addition of a flat secondary mirror at 45° to the optical axis of the Schmidt design creates a Schmidt-Newtonian telescope.
[编辑] 施密特-卡塞格林式
The addition of a convex secondary mirror to the Shhmidt design directing light through a hole in the primary mirror creates a Schmidt-Cassegrain telescope.
The last two designs are popular with telescope manufacturers because they are compact and use simple spherical optics.
