Visible Spectropolarimeter (ViSP) Processing Information

Camera non-linearity and blemish correction

The camera software is capable of onboard image corrections. Applicable corrections may include linearization and interpolation of bad pixels.

Noise Estimation*

Calculate the noise estimate on a per image basis.

Estimates pixel-value uncertainties using raw detector data. Uncertainty is a combination of Poisson counting statistics and detector read noise. This procedure disregards polarimetric information.

*This function is not currently implemented by the Data Center.

Dark Correction

Correct data with the Dark Calibration parameters.

Test that dark data is a consistent set. Compute the correction parameters for electronic offset, thermal current, and other sources of non-solar light, to be applied in the processing of data. Verify that a dark calibration meets a minimum quality threshold.

Gain Correction

Correct data with the Gain Calibration parameters.

Test that gain data is a consistent set. Normalize the gain calibration to unity. Compute the correction parameters for gain variations over the field of view, to be applied in the processing of data. Verify that a gain calibration meets a minimum quality threshold.

Geometric Correction

Computes the correction parameters for gain variations over the field of view, to be applied in the processing of data. It also computes the Geometric Calibration, which is used to align the two ViSP beams.

Determine the spectral tilt from the slit hairlines, and determine the offset from cross-correlation. De-distort and extract both beams from a single frame. This will also de-mirror the second beam. The end result is that both beams will correspond to the same input field location. Fit the skew and curvature of the spectrum in the gain calibration data. Rotate and shift the data to remove spectral tilt, skew, and curvature. Remove the average spectrum from the data. Verify that a solar gain calibration meets a minimum quality threshold.

Instrument Polarimetric Correction (Polarimetric Observations)

Demodulate measured intensities into a Stokes vector. Each beam is demodulated separately.

Compute demodulation matrices (i.e., Instrument Polarization Calibration) based on a separate set of ViSP PolCal observations. Test that PolCal data is a consistent set. Extract calibration curves from macro-pixels that will be used in the polarimetric calibration. Take a single demodulation matrix produced by the PA&C Pipeline (for a single beam) and up-sample it to fill the full ViSP frame. Combine demodulation matrices from the two ViSP beams into a single object.

Average Polarimetric Beams (Polarimetric Observations)

Combine the two beams of the polarimeter into a single measurement of the Stokes vector.

Calculate the average and standard deviation of a set of images. In polarimetric mode, for each position of the ViSP slit, there is a set of N distinct images corresponding to the M states of the modulation cycle.

Telescope Polarimetric Correction (Polarimetric Observations)

Correct the Stokes vector for telescope polarization.

Use ViSP metadata to generate an inverse Mueller matrix for DKIST mirrors 1 – 6.

The following step is a long-term QA related process that is not currently implemented. It does not affect individual science observations.

Compute the I → Q and I → U cross talk in a continuum region of the spectrum. Extract Stokes data over a specific wavelength range for a single slit position.

Average Beams (Non-Polarimetric Observations)

Average the two beams, ignoring any polarimetric information.

Note that beam 2 is NOT reflected in this function because it is assumed to have already been corrected by Geometric Correction.

Generate Science Data

This calibration will result in version 4.0 FITS files grouped into datasets with similar observing characteristics such as primary wavelength. All the metadata in the calibrated frames will be SPEC-0214-compliant in step with calibrated data from other instruments.