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CCDs and their controllers

Most observations on the WHT will be carried out using a CCD detector. Several different detectors are available, with different formats, sensitivity and read-out noise. The properties of these detectors are described in detail in La Palma Technical Notes 55 and 79.

Each CCD is mounted in a cryostat (often referred to as the CCD camera-head); this is filled with liquid nitrogen in order to cool the CCD to its operating temperature (typically 150 kelvin).

The electronics required to operate the CCD comprises a CCD controller, (mounted on the telescope), and a preamplifier (mounted on the cryostat). Briefly, the CCD controller carries out the following functions:

• Operation of the CCD shutter
• Monitoring of CCD telemetry (operating temperature, voltages)
• Generation of the clock voltages required to read out the CCD
• Amplification and analogue to digital conversion of the signal from the CCD preamplifier

The CCD controller used on the WHT is a new design, developed specifically for this telescope, and is described in some detail in Bregman & Doorduin 1986 (proc SPIE vol 627, p616), Bregman & Waltham 1986 (ESO/OHP workshop proc vol 25, p127), Jorden 1990 (proc SPIE vol 1235, p790).

Each controller was originally intended to be capable of operating up to 16 CCD devices in two separate camera heads simultaneously. However, due to various practical constraints, the WHT normally operates with one controller per detector.

As for all the instrument controllers, the CCD controller receives commands from the system computer over the utility network. The data produced by the CCD controller is transmitted over a dedicated fibre-optic data-link to the detector memory system.