Calcite with dust
Sometimes a "dust cloud" is easily visible from a FAPOL-image
(see the green circle on the left).
As a test a zero polarization star (BD +33 2642) was observed
with 16-retarder plate angles through R-filter
in different positions on the calcite. These positions are labelled as a1-a5
for the first pointing and as b1 & b2 for the second pointing .
The green circle indicates the "right hand beam".
The data were reduced with the standard procedures
(bias and flat field correction, for the master flat the optical elements
were just R-filter).
Normalized Stokes parameters:
The labels a1 to a5 and b1 & b2 indicate the
positions of the star (see the image above).
- Close to the centre of the FoV the results from two pointings
are almost equal (positions a1 and b1)
- close to the edge of the FoV there appears to be more scatter than
from the centre (positions a2, a3, b2)
- All the measurements are are consistent with (nearly) zero degree of
- The "dust cloud" seen on the FAPOL-image makes no or only little
effect to the measured polarization (positions a2 and b2)