Strictly speaking, defect mapping and correction are not part of the calibration process, but because defect correction is sometimes performed immediately after calibration, we include it in this chapter. CCDs are not perfect: they have single photosites, clusters of photosites, and even whole columns that either do not respond to light, or that respond in a markedly different way than normal photosites. These defects pass through calibration because the photosites do not behave normally, and appear in calibrated images as dark or bright pixels, clusters of pixels, or columns. They appear in every image frame regardless of exposure.
Defect mapping consists of identifying the defective pixels, generating a list of the defects, or creating a defect map—a special image in which the pixel values classify the defects by type. For example, a value of 1,000 might mean the problem is a single pixel defect; a pixel value of 2,000, that the pixel is in a defect cluster; and a value of 3,000, that the pixel is part of a defective column of pixels.
To correct the defective pixels, image processing software scans the defect map and applies a corrective action to the image based on the pixel values of the defect map. Upon detecting the code for a single defective pixel, the software might determine the median value of pixels surrounding the defect, and replace its value with the new one. For a cluster, the software could determine the median of the surrounding region and replace each pixel in the cluster with the new value; and for a defective column, the software could repair each pixel in the column with the median of pixels from adjacent columns.
Strictly speaking, defect correction does not actually "correct" or "repair" defective pixels—it simply discards the original "defective" pixel value and finds a value characteristic of the neighborhood to replace it. Because of this, defect-corrected images should not be used with those intended for astrometry or photometry. However, they certainly look much nicer than images with pixel, cluster, and column defects.
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