Servo systems for Cerenkov light receivers

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Abstract
The paper describes a variety of servo systems which have been developed for use with Cerenkov light receivers. Particular attention is devoted to the problem encountered in the high-energy γ-ray astronomy application, namely that the total background light within the field of view should be maintained constant during individual drift-scans or on–off comparison runs.Both filament lamps and midget fluorescent indicating lamps have been used as the active elements in the servo loops, and the systems have been operated either (a) from the phototube current or (b) from the counting rate at a selected pulse-height discrimination level. The characteristics of the various systems are discussed, in relation to the control factor, dynamic range, and stability. One of the systems developed was fast enough to respond to stellar atmospheric scintillations in the frequency range d-c. to 1 kHz.A servo system of a different type is also described, in which the sensitivity rather than the background light can be stabilized; in this way it was found possible to accommodate large variations of background light over long periods.
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