People's Photometer

First light:

Last commissioning: August 1996.

Designed and built by:

Description: During the 80s the most often scheduled of the f/15 instruments for the Kapteyn Telescope was the People's Photometer. It was designed in the 70s by R. Bingham at the RGO and five were built for various observatories throughout the world. These instruments are conventional two-channel photoelectric photometers, most frequently used in a star-sky mode where the separation of the two apertures is 172 arc sec. Each channel has an independent filter slide to take six filters; typically Kron Cousins ubvri or Strömgren uvby and Hß. When the transparency is variable it is possible to use only one aperture and put a neutral beam-splitter immediately behind it which sends the light into the two different photomultipliers. By selecting different filters for the two channels one may measure colours in non-photometric conditions even when it is impossible to measure magnitudes; the best example is the Hß index. It is possible to replace the neutral beam-splitter with a Foster prism which forms a polarizing beam-splitter, and at the same time insert a rotating waveplate in the beam above the aperture. This combination modulates the light from a polarized source by an amount proportional to its polarization. A half-wave plate produces a modulation at four times the frequency of the plate rotation which is solely dependent on the linear polarization of the source. A quarter-wave plate is also sensitive to the linear polarization but the modulation has only half the amplitude; however there is an additional modulation at twice the plate frequency proportional to the circular polarization of the source.

Clearly the People's Photometer allows the astronomer to choose between a variety of configurations to study the particular problem he has in mind. 

More information: Peoples Photometer Quick Guide

More photos of this instrument: http://www.ing.iac.es/PR/archive/jkt/instruments.html