Polarimetry

It was decided to simplify and streamline the FAPOL polarimetry mode. In the new mode, FAPOL will always be used with the internal focus set to 1070 both for imaging and spectroscopy. This should improve the ease with which FAPOL setup and observations can be done. The telescope focus offsets for each element used in FAPOL was measured such that the instrument set-up and use of polarimetry can be automated through the use of observing scripts.

Since 2008 we have had the calcites mounted vertically in order to speed up readout when doing fast spectro-polarimetry. This made sense with the old detector controller as the difference was 40s versus 13s readout time. With the new controller it is in the worst case 8s versus 5s. Therefore, it was decided to change the orientation of the calcites permanently back to horizontal (i.e. horizontal polarimetry slitlets), thereby avoiding the need to rotate the grisms which would mean a non-standard set-up (which can be an issue for ToO and monitoring programs) and requires a more extensive set-up (different slits for normally mounted grisms and rotated grisms) which sometimes causes problems.

The section in the ALFOSC Cookbook about imaging polarimetry will be updated, and a section about spectro-polarimetry will be added.