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You would have obtained during the night (read again Chapter 4) several frames with no illumination to determine the modulations of the ``zero'' level, or dark current, across the chip. This zero level also changes during the night, so we find convenient to use the underscan columns for each frame (1-11 in the GEC CCDs) to determine it.

We proceed as follows:

  1. Average all the bias frames and smooth them slightly (e.g. top-hat 3 pixels).

  2. Check for -- and eliminate -- hot spots.

  3. Trim the four edges to get rid of rubbish that will otherwise ruin your mean values, but taking care to leave some of the underscan columns (columns 7-10 is usually enough; 1-4 after trimming). The dark level for each frame is to be obtained from them.

  4. Subtract the mean value of the whole frame to obtain a new frame with the modulations and a mean value of zero. This we will call, in what follows, the bias frame.

Wed Apr 6 22:56:39 BST 1994