Calcite with dust

(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).

Summary:

• 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 polarization
• The "dust cloud" seen on the FAPOL-image makes no or only little effect to the measured polarization (positions a2 and b2)