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Figure 4.4 shows the CCD format, and some associated information. The size of the image area (and in fact the ``underscan'') are fixed when the chip is manufactured. The size of ``overscan'' in both directions could be varied if required.
The CCD is mounted with its long axis (horizontal) in the dispersion direction. Row 1 is read out first and appears at the top of the LEXIDATA screen - this is blue on the 235 camera and red on the 500 camera. (Note that the CCD does not maintain the same dispersion direction on both cameras, unlike the IPCS).
It is possible to read out a reduced window of the CCD frame, although the full-frame is recommended for all work at present.
For the GEC CCDs columns 1-11 are ``underscan'' and not subject to illumination; they can be used to give a zero reference level for the data frame. Columns 12-396 are true image elements. Columns 397-400 are ``overscan'', as are rows 579-580; these elements contain residual signal and give a measure of any poor charge transfer efficiency in both directions - this can be ignored.