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With Arcon it is possible to read out a CCD using more than one amplifier in
parallel, leading to a substantial reduction in readout time. Once properly
reduced, such data is indistinguishable from that obtained when reading out
through only a single amplifier. However, the raw images are rather different
and consequently must be processed somewhat differently. Firstly, each readout
will typically have a slightly different, zero level, gain, and readout noise,
and may differ slightly in its departures from perfect linearity. As a result
both zero frames and uniformly illuminated exposures will show a characteristic
chequer board pattern, the sections of the data read through each amplifier
having different levels. Secondly, there will be a separate overscan strip,
used to monitor the zero level, for each readout. The location of these overscan
strips in the raw frame depends on which amplifiers are used. When all four
amplifiers are used the four overscan regions form a vertical stripe down the
centre of the raw image. A CCD read out through two amplifiers can have its
two overscan strips, running side by side down the center of the picture; or
they may be one above the other, on the same side of the picture; or the strip
for the upper and lower halves of the CCD can be at opposited sides of the
image (in which case there will also be a horizontal displacement of the data
in the two sections).
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