Why does the psf vary across the image, and from image to image?
The point spread function or psf is determined by the microchannel plate detector PSF, as well as the GALEX (Ritchey Chrétien) optical design and the as built tolerance errors. The detector psf is determined by the position digitization process, which is analog and subject to random noise. The psf varies across the image due primarily to gain variations (lower gain regions having a broader psf). Other effects that affect the wings of the psf include surface roughness of the optical surfaces, ghosts from multiple reflections in refractive optical elements, and grazing reflections from baffles or struts in the optical beam path. Most optical design aberrations cause the psf to vary radially over the field of view, but those associated with the dichroic beamsplitter cause variation along the satellite X axis, which can vary in sky coordinates, depending on the satellite orientation around the telescope optical axis. Thus, in general a given source in a repeated observation of the same par
