The spectrograph has an exposure metering facility which uses a very light sensitive (peak DQE 70% at 7000Å) actively quenched avalanche photodiode photon counting module, from EG&G Canada. A thin glass slide in front of the fibre optic exit diverts 4% of the beam to a lens, which focusses the 50 m diameter fibre optic core onto the 200 m diameter APD detector. The output from the APD can be monitored using the program expose, on the 74" instrument PC. It will quite likely be necessary to reload expose at some time during a complete night. This can be done simply by typing "expose".
It is advisable to be sure that the APD is off before switching the flat field lamp on, or opening the door. The most reliable way of doing this is to check the indicator lamps at the entrance to the coude room. It should not be necessary to switch off the APD for arc exposures.