Polarimetry

We have looked at the suggestion from an astronomer from the Instituto de Astrofísica de Andalucía' of using a Wedged double Wollaston (WeDoWo) to do polarimetry with ALFOSC. Some tests were made with a WeDoWo normally being normally used in an ALFOSC clone called AFOSC at the Asiago observatory in Italy.

The WeDoWo mode was compared with the already existing FAPOL device, and we have also considered a possible upgrade of the existing ALFOSC/FAPOL device which consists of replacing the old calcite plates with a simple Wollaston prism to be put in the ALFOSC parallel beam. In practice, this means the filter wheel since having it in the grism wheel would be unpractical for spectro-polarimetry.

Our basic conclusion is that in general the suggested WeDoWo mode for ALFOSC is not the best polarimetry option, taking into account both accuracy, sensitivity and flexibility (spectro-polarimetry, circular polarimetry, etc.). We believe that an upgrade of the ALFOSC polarimetric mode in the sense exchanging the two old calcite plates (which have low transmittance) in order to gain higher sensitivity, larger FOV and better image quality is in principle the best option. However, in some specific cases of targets with relatively high time-variability in polarization the WeDoWo mode might be better and one could consider having both modes available.