As stated earlier, the CCD has become the detector of choice in most astronomical applications. Its many advantages were listed at the end of the previous chapter, and some of its practical applications will be described in Chapters 10, 11 and 13. In this chapter we shall describe the necessary steps involved in eliminating noise and other sources of error so that CCD images can be used for accurate analysis.
Noise in the data
A raw CCD image contains information about your science targets. It also contains noise. Noise is usually categorized as random or systematic. Random noise causes a measured quantity to deviate from the “true” or “expected” value according to simple statistical relations, such as the normal distribution (see Appendix 1). Random noise cannot be eliminated; it can only be measured (characterized) so that its contribution to the signals from the science objects can be understood. A major advantage of a CCD over some other types of detectors is that its noise can be characterized accurately.
Systematic noise is caused by one or more processes that are not characterized by statistical distributions describing random events. Systematic noise can result from known sources or unknown ones. Known sources can be corrected; this is part of the calibration process. We will discuss below several types of known systematic noise in CCD data and how to correct for them. The presence of unknown systematic noise is always the greatest fear of the observational astronomer.
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