Schematic view of the CCD systems

The CCD systems in use at the WHT are described by P.R. Jorden et al. in ING Tech Notes 92 (Nov. 1993) and 93 (Jan. 1994). In brief the CCD systems consist of the following components, ordered approximately by the data path from detector to user.

• CCD chips contained in Oxford Instruments cryostats (the cryostats are painted bright red). New chips and types of chips are frequently brought into use, but at the time of writing the main chips in use at the WHT are GEC P8603, EEV P88300 and Tek 1024. The properties of these, and the next generation of CCD chips (the EEV P88500) are tabulated below. New types of chips will be announced in Gemini as they become available. The Oxford Instruments cryostats contain liquid nitrogen, and the CCDs are mounted on a heated copper block. By controlling the current into the heating elements, and by sensing the chip temperature with a platinum resistor, it is possible to maintain the CCD temperature at a preset value to an accuracy of about 0.1 C. The value of this preset temperature depends upon the CCD, and typical values for the CCD types are listed in Table 3 below.
• A preamplifier box, usually painted blue, which is mounted on the side of the cryostat.
• The compact CCD controller, designed by NFRA Dwingeloo, and described in detail by J.D. Bregman and A. Doorduin in Proc. S.P.I.E., volume 627, page 616 (1986), and by J.D. Bregman and N.R. Waltham in Proceedings of the ESO-OHP workshop on The Optimisation of the use of CCD Detectors in Astronomy, p127 (1986), and which is mounted in one of the electronics racks on the Cassegrain cable wrap. The compact controller contains the following components:-
  • A low heat dissipation power supply.
  • A CPU card containing an MC68008 microprocessor running a PROM based FORTH system.
  • A Temperature, shutter, and preflash control card for each CCD head controlled by the controller.
  • Cards which provide and sequence the clock and bias voltages for each CCD.
  • An analogue to digital converter card containing a 16 bit analogue to digital converter.
  • A Correlated Double Sampler (CDS) board.
  • A fibre optic transmitter and multiplex control card.
  • A floppy disc controller for FORTH software development.
  • A real time board with opto-isolated RS232 ports for communication with the Utility Network and with terminals.
• A fibre optic cable run, with a number of connectors, passing through the cassegrain cable wrap. The fibre optic cables usually have a bright yellow (Cassegrain) or orange (prime) sheath.
• The Fibre Optic Multiplexer (FOX) unit, containing the CCD Interface sequencer and one CCD interface card for each controller, in the upper rack of the Detector Memory System cabinet in the control room. The CCD interface card includes a fibre optic receiver, which decodes the serial data from the optical fibre, and a 512 by 24 bit FIFO buffer into which the data is placed in parallel form. The Interface Sequencer card is responsible for reading data from the CCD interface cards into the DICI card (see below). There can be up to 16 individual CCD interface cards in the rack, although there are not usually more than three. Each pixel, when read from the CCD interface card FIFO, has 24 bits, of which 16 are data bits, and 7 are address bits, which ensure that the data arrive in the correct buffer in the Detector Memory System. The CCD interface card has three lights on the front. These are:-
  • Data Available, a green light which indicates that data are available in the FIFO, and the CCD controller is sending data.
  • FIFO Full, a yellow light which indicates that at some point the FIFO has been filled, and CCD data have probably been lost.
  • G.O.A.T, a red light which indicates that CCD Interface card cannot detect the carrier for the optical signal.

  If either the yellow or red lights are showing on the relevant CCD interface card then there is a problem.

• The Dual IPCS CCD Interface (DICI) card in the DMS rack.
• The Detector Memory System. This is described in brief in the next chapter.