What is linearity range of count rates? Has linearity been tested?
The GALEX detectors have a non-linear response at high count rates due to both local effects attributable to the intensifiers and global effects due to the electronics. The global effects are corrected in the pipeline, and amount to a correction as high as 40% for the highest allowable global rates (100,000 cps). The local linear range of count rates has also been tested on the ground for the two detectors. We found that for isolated stars, the FUV detector is linear to about 100 cps (m~14) and the NUV detector is linear to about 1000 cps (m~12.5). This difference is attributable to the proximity focus method of the NUV detector, which spreads the PSF out over a larger area on the intensifier surface, reducing the current density. We now have a wealth of flight data and are using it to refine the linearity calibration across each detector field of view.
