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Please check the quality report for your data set to note any failures in PolCal fitting outputs. We have seen data sets where certain variables (transmission of polarization calibration optics) are far away from metrology expectations. For example, in some cases the variable representing the polarizer transmission is fitted to be near 100% transmission when the optic is known to be 91.5% transmission +/-0.3%. This fitting error can create cross-talk of all types through the correlation between fitted variables. The settings for polarization calibration are currently being investigated, and we expect improvements in the demodulation matrix accuracy as the algorithms are tuned.
Current ViSP Polarization Coordinates
In December 2022, Tetsu Anan (tanan@nso.edu), performed an analysis of the current polarization coordinates of the calibrated data from the ViSP instrument. Tetsu compared his analysis to both the Stokes Polarimeter (SP) data from the Hinode Solar Optical Telescope (SOT) and to the Helioseismic and Magnetic Imager (HMI) instrument on the Solar Dynamics Observatory (SDO). The polarization coordinates of Hinode/SOT/SP are detailed in Ichimoto et al. (2008), and is show below together with the derived polarization coordinates of ViSP.
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Hinode/SOT/SP is consistent with SDO/HMI and previous instruments such as the Michelson Doppler Imager (MDI) on the Solar and Heliospheric Observatory (SOHO).
Conclusion
The signs of Stokes U and V in ViSP are opposite to Hinode/SOT/SP and SDO/HMI. This will be rectified in a future reprocessing to enable more easy comparison to data from other instruments.
References
Ichimoto, K., Lites, B., Elmore, D. et al. Polarization Calibration of the Solar Optical Telescope onboard Hinode . Sol Phys 249, 233–261 (2008). https://doi.org/10.1007/s11207-008-9169-9
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VBI Data Set Caveats
The following issues have been found / are being worked on with VBI datasets.
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