Polarimetry

An important improvements with the sequencer is the ability to change filters and focus inside scripts. This is fundamental for a proper use of the Polaroids. Now one can rotate between the 4 angles (0, 45, 90, and 135 deg) per dither position, thereby making NOTCam polarimetry with a relatively frequent cycling possible. This is crucial in order to minimize transparency changes and make the measurements more reliable.

Some first polarimetry tests were made with the Polaroids of a highly polarized and a zero-polarization standard. A reduction script ``pol.cl'' was developed to do the image reduction, place apertures on the stars to do aperture photometry and give preliminary polarization results. As noticed earlier the image quality of the Polaroid filters is not very good. They may warp in the cold vacuum. Some scattered light is sometimes seen in the background (perhaps reflections). Preliminary results of these data are inconclusive. More accurate analysis of the existing data and more measurements are needed. In order to assess the instrumental polarization and its stability we need to measure the same zero-pol standards several times and at different orientations of the rotator. The high polarization star gives reasonable values. Unfortunately, we have not had a photometric night with NOTCam recently.