Instrument Overview
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The dual beam photometer is designed to provide object and sky
photoelectric photometry through a series of 24 filters.  The light
passes through a field lens and then through a 24-position filter
wheel the positions of which are selectable by means of a stepper
motor.  The next element in the optical path is a chopping mirror
which alternates at a rate of 24 Hz between the object and a nearby
patch of sky.  The resulting beam then enters the detector.  A
choice among three different available detectors was made on the
basis of the required sensitivity and wavelength range for the given
object.  The three available detectors include a high quantum
efficiency GaInAs photomultiplier, an S-20 photomultiplier, and an
S-1 photomultiplier.