Yrjö Väisälä [ˈyrjø ˈʋæisælæ] (6 September 1891 – 21 July 1971) was a Finnish astronomer and physicist.

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 nickname of Wizard of Tuorla (Observatory/Optics laboratory), and there is a book with the same title in Finnish describing his works.

His brothers were mathematician Kalle Väisälä and meteorologist Vilho Väisälä.

Väisälä was also a fervent supporter of Esperanto, presiding the Internacia Scienca Asocio Esperantista ("International Association of Esperanto Scientists") in 1968.

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 Contemporary to Bernhard Schmidt's design, but unpublished was also Prof.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".