Another form of defect that can occur is a column defect. This is usually a pixel that can trap or drain charge away resulting in a dark spot. As the image is read from the CCD the charge must be pushed from pixel to pixel in the vertical direction. As each pixel spends time in the bad pixel it is in turned drained of charge. Thus each pixel below the trap becomes dark.
Most software contains features to correct this type of defect by averaging out the pixels on either side. Since the pattern of column defects always remains the same in any specific CCD a defect map can be created that can be used to correct this issue in all images made with the camera.
How many of these problems exist in any given CCD is reflected in the grade of CCD, all CCD's have some defects of some degree. The manufacturer's data sheet will define how many defects can be seen in each grade. In astronomical CCD cameras a few of these defects can be tolerated and will cause little to no issues in actual use.
