The SPIE paper: Investigating the pupil dependence of the polarimetric sensitivity on SALT

Our initial goal was to achieve high-precision spectropolarimetry, given the 10 m primary mirror. Extensive observations of this nature have revealed a variable pseudo-sinusoidal wavelength position angle calibration error, with an amplitude of up to 0.5 degrees. This error varies predictably with the position in the SALT "track". During an observation, SALT fixes the primary mirror and tracks the primary focus instrument, causing variation in the pupil illumination. This effect likely arises from varying illumination of the pupil on the waveplate, which is not in the collimated beam.

We developed a new calibration technique that explicitly models the pupil dependence of the polarimetric sensitivity. This involves:

• Calibrating a model of the pupil illumination vs track position and FOV.
• Calibrating a model of the polarimetric response over the pupil as a function of the FOV position

The paper can be download at https://doi.org/10.1117/12.3017792

Or download a copy of the paper pdf here => SPIE_paper.pdf