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Image artifacts - Background subtraction - Effect of Mercury

Here we discuss an artifact that did not appear on the STEREO website, but instead appeared on our sister website for the SECCHI instrument suite. On December 1, 2011, a bright rectangular shaped artifact appeared to the right of Mercury, as shown in the image below

HI1 image from STEREO Ahead showing a bright artifact to the right of Mercury.

This artifact was caused by a combination of the way the instrumental background is removed from the image, together with the time at which the image was processed. To understand how this came about, one should first take a close-up look at how the planet Mercury appears in the raw data. One such image is shown below. Since Mercury is so bright, it causes vertical bright and dark bars running across the entire height of the image. Also visible are several background stars. However, what is important for the following discussion is the horizontal line as pointed out by the arrow. This line is caused by diffraction in the telescope optics.

Close-up of Mercury as it appears in one of the raw images taken on December 1, 2011. The arrow points at a horizontal bar caused by diffraction in the telescope optics.

The background image for a given day is derived by forming the median of all the images taken that day. Because the stars are drifting from left to right during the day, they do not appear in the daily median image. Similarly, the planet Mercury is moving leftward and slightly upward during the day, and the vertical bars do not appear either. However, Mercury is not moving fast enough to allow the horizontal bars to the left and right of Mercury to be completely removed, and these show up as two horizontal tracks in the background image, as shown below. When this estimated background is removed from the data, too much brightness is removed in the region around Mercury, showing up as a dark area in the image. When the image is later reprocessed with more up-to-date backgrounds, this artifact disappears.

Close-up of the region effected by the presence of Mercury in the daily median background image for December 1, 2011.

The image below shows the same area in the median background image for the previous day, November 30.

Close-up of the region effected by the presence of Mercury in the daily median background image for November 30, 2011.

When the background is removed from an image, what is really removed is the interpolation of the two closest background images. Normally, this interpolation is done using backgrounds before and after the image in question. However, if the background for the next day doesn't exist yet, then the interpolation is done using the previous two backgrounds. The image below shows the result of interpolating the backgrounds from November 30 and December 1 to the time of the image at the top of the page. The region which was bright on November 30, and at normal levels on December 1, is now too dark in the interpolated image. When this background is removed from the data, the dark region in the background appears as a bright region in the image.

Interpolation of previous two backgrounds to Dec. 1, 23:29 UT.

The processing of the images on the STEREO website is similar, except that the backgrounds removed have been further processed using the data from several weeks. This additional processing avoids the artifact seen in the image at the top of the page. More of the background coronal streamers are visible in the HI1 images on the STEREO website. On the other hand, the processing on the SECCHI website, by suppressing the streamers, shows the coronal mass ejections in greater contrast.

Back to background subtraction page.