Why does GALEX observe only at night, whereas FUSE observes in all parts of the orbit?
GALEX can only observe when it is in the earth’s shadow, or eclipse, because on the day side of the orbit atmospherically scattered sunlight and airglow would swamp and might damage the detectors (especially the NUV detector). The GALEX field of view is 1.25 degrees in diameter. Even the small amount of residual atmosphere at the 700 km GALEX orbital altitude scatters significant flux into the telescope. FUSE, (the Far Ultraviolet Spectroscopic Explorer) also has to contend with atmospherically scattered sunlight and airglow, but its field of view covers about 100,000 times less sky, so much less of the scattered light enters the spectrograph. There are other details in the way the two instruments operate that make GALEX more susceptible to atmospherically scattered sunlight background. These include their wavelengths. FUSE operates at 905-1195 Angstroms. GALEX operates at 1350-2800 Angstroms, closer to the peak of the sun’s illumination. Also, during data reduction is not possible to