Yrjö Väisälä
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Yrjö Väisälä (IPA: [ˈyrjø ˈʋæisælæ]) (September 6, 1891 - July 21, 1971) was a Finnish astronomer and physicist.
[A note on dates: the birth date is a Gregorian calendar date, because although Finland was part of Russia at the time and Russia used the Julian calendar until 1918, the Gregorian calendar continued in official use in Finland because Finland had been part of Sweden when Sweden had converted to the Gregorian calendar in 1753.]
He had brothers Kalle Väisälä (mathematician) and Vilho Väisälä (meteorologist), both successful in their own chosen fields of science.
He was also avid fan of Esperanto, and sailing.
His main contributions were in the field of optics, but he was also very active in geodetics, astronomy and optical metrology. He had even an affectionate nick-name of Wizard of Tuorla (Observatory / Optics laboratory), and there is a book of same (in Finnish) title describing his works.
[edit] Optician
He developed several methods for measuring the quality of optical elements, as well as a lot of practical methods of manufacturing said elements.
This allowed the construction of some of the earliest high-quality Schmidt cameras, in particular a "field-flattened" version known as Schmidt-Väisälä camera.
Contemporary to Bernhard Schmidt's design, but unpublished was also Prof.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 (back in the 1930s, astronomical films were glass plates.) The resulting system is known as the Schmidt-Väisälä camera or sometimes as the Väisälä camera. (This solution is not perfect, as images of different colour end up at slightly different places.)
Learning that he lost the inventor status also motivated Prof. Väisälä to publish also "less than perfect" designs...
Prof. Väisälä made a small test unit of 7 mirrors in a mosaic on stiff background steel frame, however it proved to be impossible to stabilize as "just adjust and forget" structure, and next time anybody tried it, was with active controls on Multiple Mirror Telescope.
[edit] Geodesy
In the 1920s and 1930s Finland was doing its first precission triangulation chain measurements, and to create long-distance vertices Prof. Väisälä proposed usage of flash-lights on 5-10 km altitude balloons, or on some big fireworks rockets.
The idea was to measure the exact position of the flash against background stars, and by precisely knowing one camera location, to derive an accurate location for another camera. This required better wide-field cameras than were available, and was discarded.
Later, Prof. Väisälä developed a method to multiply an optical length reference to precisely determine lengths of baselines used in triangulation chains. Several such baselines were created in Finland for second high-precission triangulation campaign in 1950s and 1960s.
Later GPS made these methods obsolete.
Prof. Väisälä developed also excellent tools to measure earth rotational axis position by building so called zenith telescopes, and in the 1960s Tuorla Observatory was in the top rank of North Pole position tracking measurements.
In the 1980s radioastronomy was able to replace earth rotation tracking by referring things against "non-moving background" of quasars.
For these Zenith Telescopes Prof. Väisälä made also one of the first experiments at doing liquid mirrors of mercury. (Such needs extremely smooth rotation speeds which have been able to achieve in the late 1990s.)
[edit] Astronomer
The big Schmidt-Väisälä telescope he built was used at the University of Turku for searches of asteroids and comets. His research group discovered 7 comets and 807 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-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.)
Notably, Yrjö Väisälä discovered the periodic comets 40P/Väisälä and 139P/Väisälä-Oterma (the latter was co-discovered with Liisi Oterma and at first classified as asteroid 1939 TN).
The University of Turku Astronomy department is known as VISPA: Väisälä Institute for Space Physics and Astronomy (http://www.astro.utu.fi/) in honour of its founder.
A crater on the Moon is named after him, and so is the asteroid 1573 Väisälä.
