Data Reduction Frequently Asked Questions

What do I need to enter for the RETARDER_WAVEPLATE_CHROMATISM table when running the pmos_science_recipe?

The RETARDER_WAVEPLATE_CHROMATISM table contains the color dependency of the linear polarization vector. It is produced by running the fors_pmos_calib recipe on a set of zero-polarization and high-polarization pmos standard star observations on a polarization standard which is recognized by the pipeline. A list of the FORS2 PMOS standard stars which are recognized by the pipeline can be found at

After running the fors_pmos_calib recipe, the resulting RETARDER_WAVEPLATE_CHROMATISM file should be specified as an input file for the fors_pmos_science recipe to process the science data.

If no full set standard star observations are available, a static version of the RETARDER_WAVEPLATE_CHROMATISM file can be used. This is available as the file fors-calib-4.9.23/cal/fors2_eps_theta_2009-01-25.fits, included in the FORS2 static calibration tables at

Alternatively, the fors_pmos_science recipe can be run without an RETARDER_WAVEPLATE_CHROMATISM input file on the science data by unchecking the chromatism option. However, in this case no correction will be applied for the color dependency of the linear polarization vector (c.f. FORS pipeline manual, Fig. 9.8).