In the mid 1960s the Smithsonian Astrophysical Observatory (SAO) decided to move their satellite tracking station from White Sands to a better observing site at higher altitude and to make this new site the location for an astronomical observatory.(1) Fred Whipple, the director of the SAO, began to investigate possible locations and settled on the Tucson area in southern Arizona as the most suitable. There he considered three possible sites, namely Kitt Peak, Mount Lemmon, and Mount Hopkins. He rejected Kitt Peak because of its relatively low altitude and potential light pollution. Mount Lemmon was the highest peak, and so should be the best for infrared observations. But Mount Lemmon was only 16 miles (25 km) from Tucson and was already suffering from light pollution. So Whipple chose Mount Hopkins in the Coronado National Forest, about 35 miles (55 km) from Tucson, as the site for the new SAO observatory.

The SAO began to build their new observatory in 1966 on a 7,600 to 7,800 ft (2,320 to 2,380 m) high ridge on Mount Hopkins, leaving the 8,590 ft (2,620 m) summit for the construction of a large optical telescope later.(2) The new observatory initially included a laser and f/1.0 Baker-Nunn camera for satellite range-finding and tracking, and a 10 m diameter optical reflector for gamma-ray astronomy. Then in 1969 the SAO built a 60 inch (1.5 m) telescope on the ridge, to be used for photoelectric spectrophotometry. It was named after Carlton W. Tillinghast, a Smithsonian administrator who died in 1969 at the age of 36.

Whilst the 60 inch was being constructed, Fred Whipple and colleagues investigated possible designs for the SAO's projected large optical telescope. At first they considered building a telescope with a fixed, large, spherical segmented primary mirror, similar to one proposed by Aden Meinel when he had been at Yerkes in 1953 as the optical equivalent of the Arecibo radio dish.(3) But they rejected this as the reflecting area would continuously change during an observation making the interpretation of infrared observations difficult.

At about the same time Frank Low of the University of Arizona's Lunar and Planetary Laboratory (LPL) had been developing observational techniques to detect faint objects in the infrared with a 1.5 m telescope.