One of the first problems investigated at the start of the 200-inch telescope project was that of a suitable site. Extensive tests of climatic and ‘seeing’ conditions at some dozens of sites in the semi-arid south-western part of the United States led to the selection of Palomar Mountain. This is a flat-topped mountain some 10 x 20 km. in extend located 150 km. south-east of Los Angeles. The altitude of the telescope is 1700 m. Temperature conditions are very stable with a diurnal range of about 6° C. and observing conditions during the past two years have proved to be at least as satisfactory as those at Mount Wilson.