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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.

STEREO Ahead COR1 image with dark regions
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.

Venus as seen by STEREO Behind HI1   Effect of Venus on background subtraction
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.

COR1 field lens artifacts

There are also some permanent artifacts in the instrumental backgrounds of the COR1 telescopes. Since these artifacts are always there, in a perfect world they would be removed in the background subtraction process. However, they are sensitive to small moment-by-moment changes in the spacecraft pointing, and thus cannot be completely removed. The artifacts are demonstrated in the images below. (Only a few representative artifacts have been circled.) They are caused by small defects in the field lens of each COR1 telescope, though some have also appeared since launch due to the migration of individual dust particles onto the surface of the field lens. Although they can appear anywhere within the image, they are most visible near the edge of the occulter. These artifacts appear as bright rings with a dark center, reflecting the shape of the input aperture with the occulter in the center. Generally, only one side or edge of the ring is visible, giving the artifact a "fingernail" appearance.

Field lens artifacts in COR1-A   Field lens artifacts in COR1-A
The red circles highlight a few of the image artifacts caused by defects in the COR1-A field lens. The blue circle highlights dark areas where image saturation is occurring. Click on the image for a full-resolution version.   The red circles highlight a few of the image artifacts caused by defects in the COR1-B field lens. Click on the image for a full-resolution version.

The blue circle in the COR1-A image demonstrates a different artifact related to the instrumental background. The background in the COR1-A telescope has been slowing increasing over the last several years in the region just below the occulter due to aging of the instrument. There are now some small areas where the images are saturated. These appear as dark areas in the background-subtracted images. Occasional adjustments are made to the COR1-A exposure time to reduce the area of image saturation.

Back to image artifacts page.

Last Revised: Monday, 05-Aug-2013 16:37:38 EDT
Responsible NASA Official: [email address: therese.a.kucera<at>nasa<dot>gov]
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