Schmidt-Väisälä camera
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The Schmidt-Väisälä camera is an astronomical telescope Intended for wide-field (5 to 10 degrees of arc) photographic work. It was designed by Yrjö Väisälä.
[edit] Invention and design
Prof. Yrjö Väisälä originally designed an "astronomical camera" similar Bernhard Schmidt's Schmidt camera, but the design was unpublished. Väisälä did mention it in a lecture notes in 1924 with a footnote: "problematic spherical focal plane". Once Väisälä saw Schmidt's publication, he promptly went ahead and solved the field-flattening problem in Schmidt's design 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.
This field-flattening solution is not perfect, as images suffer from chromatic aberration with different color end up at slightly different places. However the approach is interesting thinking of modern electronic camera sensors which definitely cannot be forced into spherical surface shape, nor even manufactured as such.
Learning that he lost the inventor status did motivate Prof. Väisälä to publish his "less than perfect" designs.
Prof. Yrjö Väisälä not only designs the new optics, but also built several implementations of the design after Schmidt's publication. His two first efforts were experiments to learn how to make the correcting meniscus (its 4th degree polynom surface). The third one he built was the Väisälä 500/1031 camera mentioned below that has been used to find 807 minor planets and 7 comets.
[edit] The Väisälä 500/1031 camera
This telescope, which saw its first light in 1934 has corrector meniscus diameter of 500 mm, and primary mirror focal length of 1031 mm. The spherical primary mirror diameter is 600 mm, and it is on a german-type equatorial mount. It uses a 120 mm diameter doubly-convex field flatten lens 3 mm in front of the focal plane which gives it a 6.7 degree diameter field of view on 120x120 mm film plates giving a scale of 200 arc-seconds per millimeter.
The guide telescopes are L180/2300 and L80/1200
The telescope was used over some 20 years to find 807 minor planets and 7 comets. Indeed at the time its productivity was similarly overwhelming compared to the rest of the worlds minor planet hunters, and even rivals computerized searches have been in 1990's and 2000's. Its surveys, according to one story, may have been intended to detect earth-impacting asteroids.
For this rather massive photographic survey work, Prof. Väisälä developed also a protocol of taking two exposures on same plate some 2-3 hours apart and offsetting those images slightly. Any dot that did not have a "pair" was marked for follow-up photos. This method halved the film consumption compared to method of "blink comparing", where plates get single exposures, and are compared by rapidly showing first and second exposures to human operator.
The original location of the telescope is now the middle of large town next to a brightly lit harbor. The telescope was moved to another and still somewhat dark location in 1950's when first street light appeared in the horizon of the original location. The newer location is now property of and in care of Turun Ursa Astronomical Association, which is Finland's second oldest amateur astronomy society, and also founded by Prof. Väisälä.
This telescope is used rarely (in 2000's) to take sky photos on 4x5 inch flat plastic film plates, mostly its guider telescope is used to watch stars and planets - e.g. Pluto-hunt every spring, for which the guider is barely big enough to allow human eye to see it under good seeing conditions.
[edit] Bibliography
- [1] Doctoral dissertation of Liisi Oterma: Recherches portant sur des télescopes pourvoud d'une lame correctrice, 1955
- Y Väisälä: Anastigmatishes Spiegelteleskop, der Sternwarte der Universität Turku, Astr. Nachr. 254, 361, 1935.
- Y Väisälä: Über Spiegelteleskope mit grossem Gesichtsfeld, Astr. Nach. 259, 197, 1936
