Why didn GALEX use big CCDs?
The microchannel-plate (MCP) detectors that GALEX uses have intrinsically low red leak so they reject longer-wavelength light that is outside the nominal bandpass. This is important in the ultraviolet since the sky is much brighter in the visible (redward) than in the UV. To avoid red leaks, CCDs require special filters that are difficult to make and prone to pinholes. In addition, CCDs require cooling, which greatly exacerbates the difficult contamination control necessary for ultraviolet instruments. Next, MCP detectors detect and time tag each photon. This permitted us to save in development cost by using looser satellite pointing requirements and reconstructing the image using software after the data is telemetered to the ground. Were data taken on a CCD detector with the same satellite pointing, the image would be blurred. Finally, the GALEX detector active area is 65 mm in diameter, ideal for this survey mission. CCDs are available in neither the requisite size nor shape, and CCD
