Calibration of a ground-based solar coronal polarimeter

The Mk4 K-coronameter records polarization brightness images of the solar corona from the Mauna Loa Solar Observatory, Hawaii, USA. Calibration is required to quantitatively measure coronal polarization brightness, which in turn is used to infer coronal electron density. Matrix techniques are used to map the instrument polarization response. Brightness scaling depends upon precise knowledge of properties of an opal glass attenuator and calibration polarizer, sky transmission, and telescope pointing. In addition, account must be made for polarization at the objective lens and from the terrestrial atmosphere. Calibration parameters are stable to a few percent over a day, but when coupled with uncertainties in calibration optics values, sky transmission, and pointing, the average measurement uncertainty is ±15% ±6×10-9 pB/BSun.