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Design

CCDs require electronics to generate electrode biasses, clocking waveforms, low-noise amplification of weak signals, A/D conversion, buffering, and digital storage. They also require temperature-controlled environments mountable at different telescope foci for direct imaging, and at spectrograph foci for use as spectroscopic detectors. The details of how operational systems meet these demands differ, but the major elements are similar.

The cryostat or dewar provides the cooled environment for the CCD. A liquid-nitrogen cryostat designed at RGO can be used in either downward or upward-looking modes, for example in direct imaging at prime or Cassegrain foci. It has a hold time of some 12 hours so that it only has to be filled once a night. The CCD itself is mounted behind an anti-reflection coated quartz window, on a copper block with which it makes good thermal contact and which has a temperature sensor attached. Resistive heating controlled by a feedback loop maintains the CCD at the optimum operating temperature of 150 K to within 0.05 K. A preamplifier is mounted on the cryostat.

The main functions of the electronics needed to drive the CCDs are: analogue signal sampling and CCD control, digitisation of data, telemetry of system voltages, control interfaces and data links to the instrument computer. The systems may be reprogrammed in order to use a wide variety of different chip types or operating modes. Windowed readout of the chip can be used to minimise readout time or data storage. On-chip binning (available for most chips) can be used to increase signal to noise at the expense of spatial resolution.