Most CCD's will have a few pixels that do not respond to light properly. Usually the effect is due to some defect in the CCD stucture that occurs during manufacturing of the CCD. The result is a pixel that is not as sensitive to light as the surrounding pixels and appears dark in a flat field. A single pixel with this issue is a point defect, several pixels together is called a cluster defect, and a section of a column is a column defect.
Most point defects are only a few percent less sensitive than the average pixel and as a result can usually be corrected with a flat field. A few pixels might be ten to twenty percent less sensitive and flat fielding becomes less effective. These pixels might need to be manualy corrected during processing. Most software contains features to perform this. Since the pattern of point defects always remains the same in any specific CCD a defect map can be created that can be used to correct these pixels in all images made with the camera.
How many of these pixels exist in any given CCD is usually reflected in the grade of CCD, all CCD have some defects of some degree. The manufacturer's data sheet will define how many defects can be seen in each grade. The data sheet will also define how much less sensitive the pixel must be to be considered a defect, usually 10-20% below mean. In astronomical CCD cameras these defects can be tolerated and will cause little to no issues in actual use. Lower grade CCD's cost less, high grade CCD's with few defects can be many times the cost.
