Editing Yrjö Väisälä (section)

== 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 camera]]s, 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 Contemporary to [[Bernhard Schmidt]]'s design, but unpublished was also Prof.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".<ref name="Obituary-by-Öpik" />

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 ''(also see [[photographic plate]]s)''. 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.) 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]].