Yrjö Väisälä

Yrjö Väisälä (born the September 6th 1891 with Kontiolahti - died the July 21st 1971 with Rymättylä) was a Astronome and a Physicien Finnish.

on the dates: the birth date is a date of [[Gregorian calendar], because although Finland belonged to the Russia at the time and that Russia used the Calendrier Julien until in 1918, the Gregorian calendar remained of official use in Finland because Finland belonged to the Sweden when the latter converts with the Gregorian calendar into 1753.]

His/her brothers Kalle Väisälä (mathematician) and Vilho Väisälä (meteorologist), were both successes in their own field of study.

He was also a strong partisan of the Esperanto and an amateur of veil.

Its major contributions related to the field of the Optique, but it was also very active in Géodésie, Astronomie and optical Métrologie. It had even an affectionate nickname: Magician of Tuorla (observatory and laboratory of optics), and there exists a book carrying the same title (in Finnish) which describes its work.

Optics

It developed several methods to measure the quality of the optical elements, as well as many practical methods to manufacture the known as elements.

This allowed the construction of some of oldest the Schmidt cameras of great quality, in particular a version " with field aplati" known under the name of Room of Schmidt-Väisälä.

Professor Yrjö Väisälä had developed a design identical to that of Bernhard Schmidt, without publishing it. He had only mentioned it in notes of course in 1924 with an annotation: " surface focal spherical problématique".

When he saw the article of Schmidt, he advanced quickly and " résolut" the problem of flatness of the field while placing a doubly convex lens slightly in front of the film holder (in the years 1930, the astronomical films were glass plates). The resulting system is called room of Schmidt-Väisälä or sometimes room of Väisälä (this solution is not perfect bus of the images of various colors are formed in slightly different places).

Learning that it had lost its statute of inventor, professor Väisälä was then motivated to also publish designs " not completely parfaites" …

Professor Väisälä manufactured a small assembly test made up of 7 mirrors laid out in mosaic on a rigid steel frame. However this structure proved to be impossible to stabilize by simple fixed adjustments, and the attempt at following use on the Multiple Mirror Telescope used an active control.

Geodesy

In the years 1920-1930, the Finland carried out its first precise chains of triangulation and to create tops at long distance, professor Väisälä proposed to use flashes installed on balloons with 5 or 10 km of altitude or on large rockets of fireworks.

The idea was to measure the exact position of the flash compared to basic stars, and knowing the position of the photographic room precisely, to deduce the precise position from it from another room. This required photographic rooms with large field of a nonrealizable quality at the time, and the idea was abandoned.

Later, professor Väisälä developed a method to multiply an optical reference length to precisely determine the length of the base lines used in the chains of triangulation. Several base lines of this type were created in Finland for the second triangulation campaign of utmost precision in the years 1950 to 1960.

The GPS made these methods obsolete.

Professor Väisälä developed also excellent tools to measure the position of the axis of rotation of the ground by building zenith telescopes, and in the years 1960, the observatory of Tuorla was among the first for the follow-up of the position of the north pole.

From the years 1980, radioastronomy made it possible to replace the follow-up of the position of the axis of rotation of the ground while using like " fixe" melts; the Quasar S.

For these zenith telescopes, professor Väisälä did also one of the first attempts at realization of liquid mercury mirrors (these mirrors require extremely stable number of revolutions which one was able to obtain only at the end of the years 1990).

Astronomy

Large the telescope of Schmidt-Väisälä which it had built was used at the university of Turku to seek Astéroïde S and Comet S. Its group of research discovered 7 comets and 807 asteroids.

For this work of photographic exploration rather important, professor Väisälä also developed a process consisting in carrying out two exposures on the same plate to 2 or 3 hours of interval and shifting these images slightly. Any pair of points which differed from the bottom moved, and required complementary photographs. This method divided by 2 the film consumption compared to the method of " comparison by clignotement" (in English " blink comparing"), where the plates are state only once and are compared by quickly presenting to an operator the first then the second exposure (the comparison by flickering for example was used to find Pluton).

Yrjö Väisälä discovered in particular periodic comets 40P/Väisälä and 139P/Väisälä-Oterma (the latter was Co-discovery with Liisi Oterma and first of all classified like the asteroid 1939 TN).

The department of astronomy of the university of Turku is called VISPA: Väisälä Institute for Space Physics and Astronomy (http://www.astro.utu.fi/) in the honor of its founder.

A crater on the the Moon bears its name, as well as the Astéroïde (1573) Väisälä.

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