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Image artifacts - Background subtraction

The STEREO coronagraph images require a great deal of processing to bring out the coronal structure. An important part of this processing is the subtraction of instrumental scattered light. This is particularly important for the inner coronagraph COR1 because its front objective sits in direct sunlight, although all the coronagraphs are affected to some level. The instrumental background is derived from an examination of images over several weeks both before and after the observation. Thus, for the most recent images, only preliminary backgrounds can be applied. This sometimes causes too much to be subtracted from some regions, making these areas dark. To get around this problem, the coronagraph images are regenerated at intervals of 15 and 35 days after the observation, when better backgrounds are available.

COR1 image from STEREO Ahead showing dark regions caused by an imprecise background subtraction. These dark regions should be filled in when the image is reprocessed after 35 days.

Sometimes bright planets get mistakenly folded into the HI1 background calculations, creating dark "holes" in the data. For example, below is an image of the planet Venus as seen by HI1-B on September 15, 2009, compared with an image from a few days later. The computer is subtracting too much scattered light at the earlier location of Venus.

The planets Venus as seen by the STEREO Behind HI1 telescope on September 15, 2009		Hole seen 3 days later at previous location of Venus. Venus itself has moved slightly to the left.

More discussion is available about a related artifact caused by the effect of planets on the background subtraction.

Back to image artifacts page.