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In common with all other `common user' instruments on the INT, FOS is controlled from the instrument computer, a Perkin Elmer 3210, via the ADAM environment. To obtain data, the chip is cleared of all previously accumulated charges, and then exposed for the required time. When the exposure is completed, the pixels are clocked out one at a time, the charge from each pixel being deposited on the gate of the output transistor, and read out as a voltage. This is amplified, and digitised to 16 bits,whence it is transmitted to the instrument computer via Camac. It is then stored and displayed. The process of reading out the chip takes approximately 30 seconds.
FOS is currently using an extended blue response GEC P8603 type chip, coated for the Observatory by ESO. This type of chip is characterised by very low internal noise, which, coupled with a small contribution from the pre-amplifier and ADC, gives a total noise of around 7 electrons rms. The overall chip characteristics are excellent, and no preflash facility is included. Details are given in appendix C. As the CCD is operated at low temperature, the thermal noise is negligible, so the charge stored in the potential well of each pixel is proportional to the number of absorbed photons.
The new chip was installed in May 1988. The chip in use prior to this was an uncoated GEC P8603 which was ineffective in the second order. The quantum efficiency of the new blue coated chip as measured at Herstmonceux is shown in Figure 1.4.