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The bias or zero-level for each pixel is preset, and it must be subtracted in the reduction process.
Experience with both the GEC and RCA chips shows that the large-scale and pixel-to-pixel structure of this bias frame is stable on time-scales of weeks. The RCA chip, however, shows changes in the zero-level of the bias frame on time- scales > 60 minutes. Hence for either chip, a single noise- free determination of the bias frame for each observing run is adequate; the changes in zero level are monitored by the chip overscan, and hence the zero levels are obtained automatically for each exposure
Bias frames are obtained by simply reading out the cleared and unexposed chip via a zero-second exposure :
Adam:> RUN 0 ``ZERO BIAS" 4cm (see II.3.2)
Many of these added together produce the required master frame. Take at least six per night at regular intervals.