First light: August 1996.
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 astronomers to choose between a variety of configurations to
study the particular problem they have in mind.
Peoples Photometer Quick Guide