ufos?
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Ok, maybe not. But what are these black objects darting around?
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ElisabethB
Just optical artifacts, sorry ! ;D
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CTidwell3
Specifically, they are flat field defects. From Wikipedia:
Flat-field correction is a technique used to improve quality in digital imaging. The goal is to remove artifacts from 2-D images that are caused by variations in the pixel-to-pixel sensitivity of the detector and/or by distortions in the optical path. It is a standard calibration procedure in everything from pocket digital cameras to giant telescopes.
What happens is that sometimes the flat field that is generated to do this image correction has areas that are not correct (a defect) and cause too much signal to be subtracted from the raw image data. This can create these black spots.
However, because a flat field is a single data image used for correction, when you combine this with the practice of aiming the telescope at slightly different locations as each image is taken (for example in this case, aiming just slightly higher between images 1 and 2), these flat field defects will appear to move in a non-linear pattern like all artifacts do when the individual images are put together so that the stars align.
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