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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:
- Average all the bias frames and smooth them slightly (e.g. top-hat 3
- Check for -- and eliminate -- hot spots.
- 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.
- 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.