spectroflat 2.0.0

Flat field and smile correction spectro-polarimetric solar images.

spectroflat

spectroflat is a Python based library to flat field spatially resolved spectro-polarimetric data. It allows for flat-field calibration data to be to obtained for diffraction-grating-based, long-slit solar spectrographs combined with temporally modulated polarimetry from high-resolution solar telescopes. This approach is based on nominal flat-fielding procedures performed during the instrument’s science operations.

The Python library can be plugged into existing Python-based data reduction pipelines or used as a standalone calibration tool. Our results demonstrate a suppression of fringes, sensor artifacts, and fixed-pattern imprints in demodulated data by one order of magnitude. For intensity images, the photon noise level can be closely attained after calibration. The data calibrated with the spectroflat method offer robust and precise inversion results and allow for spectral image reconstruction.

When using this library to reduce your data, please cite: Hoelken et al. "Spectroflat: A generic spectrum and flat-field calibration library for spectro-polarimetric data" (DOI: 10.1051/0004-6361/202348877, NASA ADS)

Theoretical Background

Generally this is intended to be an extension of the "Precise reduction of solar spectra obtained with large CCD arrays" method, presented by Wöhl et al. (2002), to spectro-polarimetric instruments covering a large spectral field of view with many lines.

Original algorithm

The spectroflat algorithm 1.X.X is described and evaluated in Hölken et al. (2023) "Spectroflat: A generic calibration library for spectro-polarimetric data". [see above]

Changes in 2.X.X:

The smile extraction can now be done iteratively (default=2, use Config.iterations to adjust). Usually, the residual offset correction is below 0.1 px in the second iteration.
Consequently, we removed the dust flat iteration, as now the first extraction yields correct results.

• The property gain_table of the Analyzer class is now deprecated and replaced by illumination_pattern.
• The property pre_flat of the Analyzer now exclusively holds the actual pre flat from the preparation step. The dust flat is now stored in the dust_flatproperty.

Input data

The input data must be calibrated for the camera zero-input response (dark current and bias level) and relevant non-linearity effects. If each modulation state is to be corrected with its own set of calibration data an average of all frames from the flat field recording belonging to each modulation state has to be provided.

Input data shall be provided as a numpy Array with dimensions modulation state, spatial location along the spectrograph slit and wavelength position.

Extracted Data

After the successful execution of the Analyzer the following results are available.

Pre-Flat

analyzer.pre_flat

The pre-flat is generated in the preparation step from fitting a polynomial along every column of the input data. It is a preliminary version of the dust flat (see below).

Dust-Flat

analyzer.dust_flat

The dust-flat is a combination of the sensor and slit flat that contains most of the "hard" flat field features. Most prominently the following is corrected:

• Sensor features (e.g. column-to-column response patterns, dust on the sensor itself)
• Slit features (e.g. dust on the slit resulting in line features in the spectral direction)
• Fixed optical fringes and illumination impurities.

The dust flat might be split in Sensor Flat and Slit Flat by the mean profile method applied along the spectrograph slit dimension.

spectroflat provides a utility function separate the sensor- and slit-flat parts in the dust flat: spectroflat.utils.ffing.split_sensor_slit_flat. It takes the dust-flat and the corrected rotation. The rotation value can either be taken from the header of the offset map or the configuration.

Smile offset map

analyzer.offset_map

spectroflat characterizes the smile distortion by tracking the change of every spectral absorption or emission line with respect to the reference profile generated from the central rows. The map list the offset each pixel has, compared to that reference, with sub-pixel precision.

Illumination pattern

analyzer.illumination_pattern

Typically, the illumination patterns are in the 10−6 range and are not used.
This result is kept for compatibility with the approach of Wöhl et al. (2002).

PDF Report

A report summary with relevant plots to inspect the quality of the extracted products.

Technical Documentation

NOTE This library expects the spacial domain on the vertical-axis and the spectral domain on the horizontal axis. spectroflat does not include any file reading/writing routines and expects numpy arrays as input.

Please refer to the API Documentation for implementation details. Especially the entries on available Configuration values are of general interest.

The example.py script provides an usage example.

Contact

This code is developed and maintained at the Max Planck Institute for Solar System Research (MPS) Göttingen.

Maintainer

• Johannes Hoelken ([e-mail])

Contributions

• Alex Feller
• Francisco Iglesias

License

BSD clause-2, see LICENSE