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Limitations, Extra Observations, and the First Stages of Analysis

The complex fabrication process for CCDs produces non-uniformities in pixel response on small and large scales, some surprising optical effects, and defects which stamp each CCD as an individual. For the most part, these features are readily removed by a few auxiliary observations and operational procedures, yielding clean end-product CCD images characterized very closely by four parameters: pixel size, quantum efficiency, saturation level and readout noise.

Fig 5a is a raw CCD frame, the direct readout array from a 300-second exposure with a thin-chip CCD camera at the prime focus of the 3.9 m Anglo-Australian Telescope. Contrast has been enhanced to show the image defects to worst possible advantage. Fig 5b is the same frame at the same level of contrast enhancement, after the first stages of data- analysis in which the auxiliary observations have been applied to rectify the imperfections. These first stages clean the image from its raw (Fig 5a) to its laundered (Fig 5b) form, and a summary of these initial steps provides the basis for a description of the shortcomings of image 5a:

  1. Subtract bias frame.
  2. Subtract dark-count frame (if necessary).
  3. Divide by (normalized) flat-field frame.
  4. Patch cosmic-ray events and bad columns (if essential).

Fri Jun 10 17:31:56 BST 1994