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In page William Stephen Finsen:

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Finsen invented an eyepiece interferometer which allowed measurement of very close double stars. His original device was used for many years at the astronomy department of the University of South Africa.[5] Finsen examined more than 8 000 stars, discovered 73 double stars and took 54 000 photographs of Mars. They were considered the best photographs of Mars prior to the first space probe pictures in 1965.[6][7] Finsen continued his observations of double stars after his official retirement from the observatory.[2]