Calibration Data Pipeline
Calibration Data Pipeline (CDP)
The Calibration Data Pipeline (CDP) produces the spectroscopic calibration reference files necessary for the spectroscopic Science Data Pipeline (SDP) to process the Level-2 grism/prism data into the Level-4 spectroscopy data products. The spectroscopic calibration reference files produced by CDP are the optical model (which includes direct-to-dispersed mapping, trace & wavelength solution), pixel-level ("small-scale") flat-field, relative flux calibration, absolute flux calibration, and spectral PSF. These products are produced through a combination of ground-based measurements and in-flight measurements (of calibration fields).
Calibration Reference Files for Spectroscopic Science Data Pipeline
The table below lists the full set of calibration files needed to run the SDP along with the corresponding pipeline stage where it is used.
Table of Spectroscopic Calibration Reference Files
| Reference | Product Description | Product Name | G2DP Pipeline Step |
|---|---|---|---|
| OPTMODEL | Optical model (includes direct-to-dispersed mapping, trace & wavelength solution) | wfi_calibration_optical_model* | CoordMapping, WCSUpdate |
| SFLAT | Pixel-level ("small-scale") flat-field | wfi_calibration_smallscale_flat | FlatField |
| RELFLUX | Relative flux calibration | wfi_calibration_relflux_cal | FluxCal_Relative |
| ABSFLUX | Absolute flux calibration | wfi_calibration_absflux_cal | FluxCal_Absolute |
| SPECPSF | Spectral PSF | wfi_calibration_spectral_psf* | Decontamination, 1DExtract |
Each of these products are available separately for grism and prism.
Description of Spectroscopic Calibration Reference Files
Optical Model
This reference product represents the full optical model transformations for the grism/prism optical element, consolidating three main calibrations:
Direct-to-dispersed mapping: This provides the mapping from the source/reference on-sky position to the location of its spectral trace (at a selected, reference wavelength) across the focal plane. Ground-based (TVAC) measurements provide a model for this mapping for both grism and prism. This initial TVAC-based mapping will be refined in-flight with periodic observations of a spectroscopic touchstone field (a stellar field) and further monitored with periodic observations of the same. The calibration will be obtained by measuring the blue edge (the edge of the grism and prism transmission curve) of multiple stellar spectra across the Roman FoV.
Spectral trace curvature: This parameterizes a model of curvature (or clocking) of the spectral trace a function of wavelength and position on the focal plane. Ground-based (TVAC) measurements provide a model for this calibration for both grism and prism. This initial TVAC-based calibration will be refined in-flight with observations of a spectroscopic touchstone field (a stellar field) measuring the trace on multiple spectra across the Roman FoV, and further monitored with periodic observations of the same.
Dispersion solution: This parameterizes a model of the wavelength solution along the spectral traces across the focal plane. Ground-based (TVAC) measurements provide a model for the dispersion for both grism and prism. This initial TVAC-based dispersion solution as well as the wavelength zero-point will be refined in-flight with observations of (emission line) planetary nebulae at multiple positions on each of the detectors. The dispersion solution will be monitored with periodic measurements of the same, where as the wavelength zero-point will be monitored with observations of stellar spectra (absorption features; e.g., Ryan et al. 2019).
Note: Due to the SDP functionality, this dispersion solution is parameterized as the inverse of the dispersion solution (position along the trace as a function of input wavelength).
Pixel-level ("small-scale") flat-field
This calibration product represents a pixel-scale, spectral flat-field for each detector that is representative of the Zody background spectrum. Ground-based measurements from detector characterization provide the best model for the small-scale flat, at a 10nm resolution over the full wavelength range; but this type of a measurement is not possible in-flight. Instead, in-flight calibration will utilize the photometric flat-fields taken with the RCS to monitor and calibrate for any changes affecting the data.
Relative Flux Calibration
This calibration product provides the corrections for wavelength-dependent changes in the disperser throughput (and other optics) across the focal plane (also known as the "large-scale flat"). Ground-based throughput measurements provide an initial model for this calibration. The in-flight calibration will be obtained with periodic observations of a spectroscopic touchstone field (a stellar field). The calibration will obtained by observing a group of stars on each detector and comparing their spectra with each other.
Absolute Flux Calibration
This product provides the calibration to convert the flux from detector units (DN/s) to science units (cgs). Ground-based throughput measurements provide an initial model for this calibration. The in-flight calibration will be obtained with periodic observations of spectro-photometric standards. This calibration will be obtained by observing spectro-photometric standards at two positions for each detector, and validated with a second spectro-photometric standard.
Spectral PSF
This product provides the PSF FWHM for the grism/prism spectral traces in the spatial dimension. Ground-based (TVAC) measurements provide an initial model for this calibration. This initial TVAC-based calibration will be refined in-flight with observations of a spectroscopic touchstone field (a stellar field). This calibration will be obtained by measuring the the spatial FWHM of stellar spectra across the Roman FoV.
References
Ryan, R. E. Jr., et al. 2019, WFIRST Wavelength Calibration: A Strategy with M67, Technical Report WFIRST-STScI-TR1901