Satellite Operations
Each of the three axes were stabilized and the optical axes of the three scientific instruments were coaligned. The entrance apertures of the scientific instruments were all located on one face of the central body. Once in orbit the flaps which cover the entrances to the ME and LEIT were swung open to act as thermal and stray-light shields for the telescopes and star trackers, respectively.
The orbit of Exosat was different from any previous X-ray astronomy satellite. To maximize the number of sources occulted by the Moon, a highly eccentric orbit (e ~ 0.93) with a 90.6 hr period and an inclination of 73° was chosen. The initial apogee was 191,000 km and perigee 350 km. To be outside the Earth's radiation belts, the scientific instruments were operated above ~50,000 km, giving up to ~76 hr per 90 hr orbit. There was no need for any onboard data storage as Exosat was visible from the ground station at Villafranca, Spain for practically the entire time the scientific instruments were operated.
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