Spectroflat: a novel calibration approach for spectropolarimetric data

J. HOELKEN; H.P. DOERR; A. FELLER; IGLESIAS, F.A.

Niigata

Simposio; The 6th NAOJ Symposium Hinode-16/IRIS-13; 2023

NAOJ Research Coordination Committee, NINS

Flat fielding spectro-polarimetric data with one spatial and one spectral dimension isinherently difficult as the imprint of the spectral lines needs to be separated from otherwavelength dependent effects, such as fringes or prefilter profiles, and wavelengthindependent effects, such as dust and sensor response. Current approaches are often basedon moving the grating, depend on optical models and/or on (non-timely) lab calibrationdata.The approach we present here allows to obtain timely flat field calibration data fordiffraction-grating-based, long-slit spectrographs and temporally-modulated polarimeters,during nominal flat fielding procedures in the observational configuration of theinstrument.From the raw modulated flats we derive separated flat fields for sensor and slit dustfeatures. Then, we remove the spectrographic curvature effect (smile) to ensureorthogonality of the spectral and spatial dimensions. We identify distortions by trackingthe position of multiple spectral lines on each individual modulation state. From the desmiled flat frames, we derive an instrument gain table free of spectral line imprints.Optionally, wavelength calibration and continuum correction can be included in thisprocess.We have created a generic python library that can be plugged in into existing pipelines orused as a standalone calibration tool. First results show a significant performancecorrecting for sensor artifacts and suppressing fixed-pattern noise in demodulated data.Finally, we discuss the applicability to different wavelength regimes for ground-(SST/TSP, GST/FISS-SP), stratosphere- (SUNRISE-III/SUSI) and potential nextgeneration space-based (SOLAR-C) instruments with a spectral range spanning multiplesolar lines.