Unexpected image offsets in the GALEX GR4

The underlying GALEX observations used in SkyView are 3840×3840 pixel images where only  a circle with a radius of about 2900 pixels is actually exposed.  In earlier releases the exposed region seemed to be centered with the image square but that is no longer always the case for GR4.  The observation center can now be hundreds of pixels offset from the nominal image center.

SkyView tries to mask out the non-observed reqion when it combines GALEX observations, but when the mask is off-center this masking may go awry.  We’re downloading information from MAST giving the actual centers for the observations rather than the nominal image centers and we will update the survey description file for MAST appropriately within the next few days to accommodate this, but in the meantime GR4 images may show unexpected blank patches.

CLI users of SkyView can play with the image finder setting to try to mitigate this, but this is unlikely to help in most cases.

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