National Aeronautics and Space Administration

HEAO-1 Software:
History of A2 Detectors

The HEAO-1 spacecraft was launched on 12 August, 1977 (day 244 of 1977). The first week of operation was devoted to turning the detectors on and to calibrating their performance. The detectors gradually achieved long-term stability. On day 232.04, the detectors' high voltage was changed to their normal state, where it remained for the rest of the mission. Stability was achieved for the HEDs at day 234.95; MED stability was achieved at 248.15.

Changes during the Mission

• From Day 233.01 to Day 233.09, events in Window 1B were not processed by any of the HEDs as a calibration.
• From Day 234.03 to Day 234.53, the bit to identify in which layer the X-ray occurred was dropped and the least significant bit of the pulse height was included for all detectors. After day 304.93, the same processing occurred for all detectors except HED 2.
• From Day 235.63 to 245.69, the MED did not process events in Window 2B because the PHA Window Layer 2 was inadvertently changed to not include Window 2B.
• From Day 432 the LED heaters were on part of the time to increase the temperature of the LEDs. However, there was a sharp increase in temperature of all the detectors.
• From Day 510 the heater in the gas system was on part of the time to increase the temperatures of the HEDs and the MED. After Day 510 the temperatures were less stable.
• From Day 645 until the end of the mission, HED 2 and HED 3 included only Layer 2 in the multiscalar rates.