AI intro:
The HDAC is a special channel of UVIS that allows the insertion of a hydrogen-deuterium absorption cell in the optical path of the FUV channel. The HDAC is used to measure the interplanetary medium (IPM) Lyman-α background and to separate it from planetary emissions. The HDAC was used extensively during the Jupiter flyby in 2000-2001, but only occasionally during the Saturn tour, mostly for calibration purposes. The HDAC was turned off in 2015 after a failure of its power supply.
The HDAC consists of a cell filled with molecular hydrogen and deuterium gas at low pressure, contained in a glass tube with MgF2 windows. When the cell is heated to about 250 °C, the gas dissociates into atomic hydrogen and deuterium, which strongly absorb light at Lyman-α (1216 Å).