Schmidt camera

A Schmidt camera is an astronomical type of glasses built of kind to guarantee an important field of sight while limiting the aberrations. Similar glasses are the Chambre of Wright and Lurie-Houghton telescopes it.

Invention and construction

The Schmidt camera was invented in 1930 by Bernhard Schmidt. The originality of the optical device developped at the point by Bernhard Schmidt is the extent of its field, obtained thanks to the use of a spherical mirror diaphragm in its center of curve. The field thus obtained is generally about the thousandths of the entire surface of the sky, at least 100 times more important than the field of a traditional telescope. Its optical components are a principal mirror spherical easy to realize and an aspheric lens of correction, called correct blade of Schmidt, located at the center of curve of the principal mirror. The detector is placed on focal surface, in the center of the room.

The Schmidt cameras have focal surfaces curved, which constrained devices of observation to follow this curve. In certain cases, one builds curved detectors - sometimes, one maintains a film using screw where by making the vacuum with the lower part. Lastly, one uses sometimes an additional lens, planoconvexe, which makes it possible to use plane detectors: they are the rooms of Schmidt-Väisälä.

Applications

The Schmidt camera is generally used to draw up statistics and to follow research programs which require to cover a broad part of the sky. That includes the search of Comet S, Astéroïde S and the monitoring of the Nova E. Moreover, one can monitor with this kind of telescopes the terrestrial satellites.

As of the beginning of the year 1970, Celestron markets a Schmidt camera of 17 cm. The room was adapted in the factory and was made containing materials with low dilation coefficients to avoid the later deformations. The first models obliged the photographer to cut and develop individually sights of film 35 mm, since the device made it possible to place only one sight at the same time. Nearly 300 specimens were produced.

Another use of the Schmidt cameras was popular: turned over, it could be used as projector. Some were thus used in the Cinéma S, the more modest models being sometimes used by private individuals.

The most known specimen and most productive is the telescope Samuel Oschin, with the Observatoire of the Mount Palomar. It was used by the National Geographic for its projects POSS, POSS-II, Palomar-Leiden, and other projects. The telescope used with the LONEOS ( Lowell Observatory Near-Earth-Object Search ) is also a Schmidt camera. The greatest model of a Schmidt camera is lodged with the Observatoire Karl Schwarzschild. In France, the most powerful specimen is established with the Research center in geodynamics and astrometry (CERGA), with the Plateau of Calern.

Derived glasses

Schmidt without lens : Before the solution of Schmidt, it was current to cure the spherical aberrations while placing a diaphragm in the center of curve of the mirror, limiting the opening to f/10. That causes to limit the spherical aberrations by preserving the broad field of the mirror to short focal distance. However, that limits the quantity of light and thus the quality of the images obtained.

Room of Schmidt-Väisälä : The professor Yrjö Väisälä built a telescope similar to that of Schmidt, but did not publish its drawings. Väisälä mentioned it in its notes of conference in 1924, with at the foot of the page: “plane focal spherical problems”. When Väisälä had access to the publication of Schmidt, it analyzed it and solved quickly the problem of the field curved while placing a doubly convex lens.

Room of Baker-Schmidt : In 1940, James Baker of Harvard modified the Schmidt camera by adding to it a convex secondary mirror, which reflected the light in the first.

Room of Baker-Nunn : Joseph Nunn replaced the mirror of the room of Baker-Schmidt by triple lens of correction. A dozen rooms of Baker-Nunn of opening of 45 cm, weighing each one 3,5 tons, were used by the observatory Smithsonian, to follow the artificial satellites of the end of the year 1950 in the middle of the years 1970.

Room of Mersenne-Schmidt : In this device, the principal mirror is paraboloid, the second convex mirror, and a third mirror, concave, sends the beam on the film.

Room of Schmidt-Newton : The addition of a secondary Miroir plane to 45° of the optical axis of the Schmidt camera gives a device known under the name of Télescope of Schmidt-Newton.

Telescope of Schmidt-Cassegrain : Addition of a convex secondary mirror to the Schmidt camera which directs the beam through a hole in the primary education mirror.

References

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