Before & After - a Success Story

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Before	After
Centered 304Å image, 1024x1024	Centered 304Å image, 1024x1024
Centered 195Å image, Off-center 195Å image, Off-center 304Å image	Centered 195Å image, Off-center 195Å image, Off-center 304Å image

Caption: If you happened to look at our real-time images and movies some time near the end of last week (Thursday 8 February 2001), you may have noticed that the Sun was jumping around in the images taken by the Extreme ultraviolet Imaging Telescope (EIT). It was in fact the SOHO spacecraft that was offpointing - as part of a plan to improve the EIT images.

Most instruments have a varying sensitivity in different locations on their detector. This is usually calibrated against a known source, and the effect is corrected by dividing the observed images by the so-called "flatfield" image, which is simply a representation of the varying response across the detector. This is, however, not an easy thing to do once you put an instrument in space, so care is taken to construct the flatfield before launch.

As time went by, however, EIT's detector response to light was degraded, and more so in those regions that were exposed to more light than others. Since the Sun is not a uniform source of light, it meant that the response dropped off more in some regions than in others. The result was a gradually increasing "error" in the image, causing a distorted appearance of the Sun. In other words, the flatfield changed on board, and the changes were not known with enough precision to cancel them out.

The manoeuvre performed on 8 February, however, changed this. If you put the image of the Sun in several different positions on the detector, it is possible to infer the spatial variations in the flatfield (assuming, of course, that the Sun doesn't change much in appearance between exposures). As can be seen above, the new flatfield that was constructed makes a lot of difference. It is most apparent in the offpointed images, where the flaws are not disguised by the general appearance of the Sun in the images.

Instrument/observatories: Extreme ultraviolet Imaging Telescope (EIT)
Taken: 8 February 2001
Picture credit: SOHO/EIT (ESA & NASA)