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This is a variant of Schmidt cameras that are special astronomical telescopes intended for wide-field photographic work. In this sense, "wide" means 5 to 10 degrees of arc (or 10 to 20 times the apparent diameter of the moon.)

Contemporary to Bernhard Schmidt's Schmidt camera design, but unpublished was also Prof.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[1] in 1924 with a footnote: "problematic spherical focal plane". 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.

This field-flattening solution is not perfect, as images of different colour end up at slightly different places. However the approach is interesting thinking of modern electronic camera sensors which definitely can not be forced into spherical surface shape, nor even manufactured as such.

Learning that he lost the inventor status did motivate Prof. Väisälä to publish also "less than perfect" designs.

Prof. Yrjö Väisälä did not only design the new optics, but also built several implementations of it after Schmidt's publication.

His two first ones were for experimenting and learning about how to make the correcting meniscus (its 4th degree polynom surface), and his third was a big science tool used to find 807 minor planets and 7 comets.

[edit] The Väisälä 500/1031 camera

This telescope 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. To flatten the image field there is a diameter of 120 mm doubly-convex lens some 3 mm in front of the actual flattened focal plane which has 6.7 degree diameter field of view on 120x120 mm glass plate films giving scale of 200 arc-seconds per millimeter.

Guider telescopes are L180/2300 and L80/1200, minor auxiliaries compared to the main camera.

This telescope saw its first light in 1934, and 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, than computerized searches have been in 1990's and 2000's, and it was even financed with (as locally known and written down story goes) earth-impacting asteroid search in mind.

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-pairs that differed from background were moving, and deserved 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. (Blink-comparing was used to find e.g. Pluto.)

This telescope was originally in a place that is now in the middle of large town next to brightly lit harbour, and it was moved to another and still somewhat dark location in 1950es, when first street light appeared in the horizon of the original place. This another 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 films, 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.

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[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