MTO is the brand name given to the family of mirror lenses developed by [url=https://en.wikipedia.org/wiki/Dmitry_Dmitrievich_Maksutov]Dmitry Dmitrievich Maksutov[/url] and produced in the Krasnogorsk Mechanical Plant and the Lytkarinsky Optical Glass Plant in Russia.The name "MTO" is an abbreviation for "Maksutov Tele-lens" or "Meniscus Tele-Lens."At the World Exhibition in Brussels in 1958, a set of lenses, which included the MTO-500, was awarded the Grand Prix, the highest honor.In 1959 the lens was awarded the Diploma of II degree of the USSR VDNKh.Focal length: 500 mm (500.7 mm)Relative aperture: 1:8Angle of field of view: 5°Frame size: 24×36 mmFront vertex focal length: -2437.2 mmRear vertex focal length: 39.26 mmDistance from first to last surface: 121.12 mmWorking distance: 45.2 mmNumber of lenses/groups: 4/3Aperture: fixed, illumination is changed by applying a set of neutral light filters.Near focus limit: 4 mConnections:lens to camera: M39×1for screw-in attachments: SpM77×0.75for screw-on attachments: Ø80 mmmounting location for light filters: in frontTU resolution (center/edge): 35/22 lines/mmLight transmission coefficient: 0.50Overall dimensions:lens length with caps: 165 mmlargest barrel diameter: 100 mmCalculation: Dmitry D. MaksutovProduction: serialStart of production (KMZ): since 1957