DUAL BEAM PHOTOMETER
====================

  Instrument Overview                                                         
  ===================                                                         
  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.

References
==========

McCord, T.B., A Double Beam Astronomical Photometer,
        Applied Optics, vol. 7, pp. 475-478, 1968.