ZIMPOL spectropolarimetric measurement of Ca I 4227 Å at µ=0.3

This dataset contains full Stokes spectropolarimetric profiles obtained in a quiet Sun region at µ=0.3 in the solar North on 2025-12-10. The Ca I 4427 Å line was observed with the ZIMPOL polarimeter in PEM configuration at the Gregory Coudé Telescope at IRSOL in Locarno together with the slit spectrograph, where the slit was oriented perpendicular to the solar limb. The slow modulation technique was used to obtain high zero level accuracy.

ZIMPOL camera integration time was 2 seconds per each of the accumlated 192 images per linear polarization state, the measurement lasted a total of 16 min.

The data reduction included: dark, polarimetric calibration, rotating Q <–> U by 3 degrees to compensate for instrumental crosstalk, dust flat and residual intensity fringe removal, de-smiling, wavelength calibration with the second solar spectrum, µ scale calibration (since the slit is covering the limb, µ could be determined to high accuracy). A few pixels covering a µ range of 0.07 were averaged for a better signal-to-noise ratio.

 The data set contains a single .fits file, containing the 4 Stokes profiles and the wavelength calibration. It can be read as follows with python:

from astropy.io import fits

with fits.open(‘251210_profile_mu0.3_slit_perpendicular.fits’) as hdul:
    stokes = hdul[0].data
    wavelength = hdul[1].data