What are the dead time and gain-sag issues?
There are essentially two sources of photometric non-linearity in the GALEX instrument: global dead time resulting from the finite time required for the electronics to assemble photon lists, and local sensitivity reduction resulting from the MCP-limited current supply to small regions around a bright sources (gain sag). (The microchannel-plate, or MCP, electron-multiplier array is the central component of the GALEX detectors.) Global dead time, defined here as the fraction of detected events lost due to the finite processing speed of the electronics, increases monotonically with global input count rate. It is easily measured using an on-board pulser, which electronically stimulates each detector anode with a steady, low rate stream of electronic pulses that are imaged off the field of view. Since the real rate of these pulses is accurately known, the measured rate is used by the pipeline to scale the effective exposure and thus correct the global dead time for all sources in the field
