Polarimetry at the 0.9m

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Two polarimetry runs were carried out at the 0.9m in February and July
2006, by Hugo Schwarz and collaborators.  Here is a brief outline of
the capability, which may be offered to the user community in the
future.

INSTRUMENT

The CCD camera used at the 0.9m remains unchanged.  The only change to
the instrument configuration is to swap out the filter slide with a
new filter slide-sized A1 plate.  The new plate has a sandwich of a
halfwave plate and rotator, filter, and calcite block, the latter of
which is permanently mounted.  To do polarimetry, one simply slides
the usual two filter slides on the 0.9m out and slides in the single
polarimetry unit.  A patch panel that already existed is used to
connect the little box (downstairs) that controls the rotating
halfwave plate to the instrument.

The calcite block produces two orthogonally polarized beams on the
chip.  This means that cirrus passing during an observation
(consisting of 8 images at 8 positions of the half wave plate) does
not affect the result, except by taking away photons.  Standard star
observations, during which the counts varied by more than a factor of
three due to clouds, yield polarization values virtually identical to
known values.  Thus, increasing the exposure time to compensate is all
that is needed.

OBSERVATIONS

In good conditions, ??? magnitude stars yield SNR = ??? in ???
minutes.

CONTACTS

Should polarimetry at the 0.9m be offered to the community, the
following people will serve as contacts:

Hugo Schwarz (scientific PI of polarimetry on the 0.9m)
Mario Magalhaes (owner of polarimetry module)
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