Relative Flux Scaling Product¶
Metadata¶
Data product name |
DpdSirRelativeFluxScaling |
Data product custodian |
SIR |
Name of the Schema file |
|
Processing function using the data product |
SIR |
Proposed for inclusion in EAS/SAS¶
This product is proposed for inclusion in the SAS: no
This is an internal Data Product.
Data product elements¶
Header |
object of type sys:genericHeader |
Data |
object of type sir:relativeFluxScaling |
QualityFlags |
object of type dqc:sqfPlaceHolder |
Parameters |
object of type ppr:genericKeyValueParameters |
Detailed description of the data product¶
The sirRelativeFluxScaling solution corrects for the large-scale (~120 pixel) spatial transmission variations of the NISP grism. The transmission variations are measured for each grism configuration as a function of position over the focal plane at multiple wavelengths, using repeated observations of bright stars in the self-calibration field. The data product is provided to the Apply module to correct the flux of individual grism spectra. A validation module examines corrected spectra to ensure that the fluxes of the same object observed at different positions are consistent.
The operation of the SIR relative flux calibration module is similar to the NIR self-calibration module, which uses bright stars (a tunable parameter set to default H < 18 AB) observed at different positions on the focal plane in the deep fields in order to derive a non-parametric large-scale flat response. However, the OU-SIR implementation is more complex because of the grism dispersion. The SIR solution is measured using each individual spectrum divided into 10 wavelength bins both to ensure consistent expected flux and to account and monitor for potential weak wavelength dependence of the response.
Relative variations in transmission across the focal plane result from both detector (quantum efficiency, gain) and optical (grism throughput, vignetting) effects. The SIR relative flux module is agnostic as to whether the source of the spatial transmission variations results from the detector or the optics. It is important to distinguish spatial transmission variations (e.g., due to vignetting or detector quantum efficiency variations) from spectral transmission variation due to grism throughput. The latter are corrected for using spectrophotometric standard stars for which the intrinsic spectral energy distributions of the sources are known, and is dealt with separately in the absolute calibration module. The SIR relative flux correction module produces spectra that are corrected for spatial transmission variations only. Once the flat field solution is applied, the same flux should be measured regardless of where a source falls on the detector.
For the grisms, there is a direct mapping between wavelength originating from a source at position x,y in the instrument field-of-view (FoV) to detector coordinates in the focal plane assemble (FPA). Therefore, the flat solution can be computed and expressed equally well in either frame. For practical reasons, we store the large-scale flat field solution in FPA coordinates rather than FOV coordinates. The resulting flat field F(i,j,lambda,G) is applied directly to each wavelength element of each spectrum depending on where it falls on the FPA, and what grism/rotation combination was used.
The fits file solution contains:
No |
Name |
Ver |
Type |
Cards |
Dimensions |
Format |
|---|---|---|---|---|---|---|
1 |
RGS000.SCI |
1 |
ImageHDU |
26 |
(68, 68, 10) |
float64 |
2 |
RGS000.DQ |
1 |
ImageHDU |
26 |
(68, 68, 10) |
float64 |
3 |
RGS180+4.SCI |
1 |
ImageHDU |
26 |
(68, 68, 10) |
float64 |
4 |
RGS180+4.DQ |
1 |
ImageHDU |
26 |
(68, 68, 10) |
float64 |
5 |
RGS000-4.SCI |
1 |
ImageHDU |
26 |
(68, 68, 10) |
float64 |
6 |
RGS000-4.DQ |
1 |
ImageHDU |
26 |
(68, 68, 10) |
float64 |
7 |
RGS180.SCI |
1 |
ImageHDU |
26 |
(68, 68, 10) |
float64 |
8 |
RGS180.DQ |
1 |
ImageHDU |
26 |
(68, 68, 10) |
float64 |
9 |
RGS270.SCI |
1 |
ImageHDU |
26 |
(68, 68, 10) |
float64 |
10 |
RGS270.DQ |
1 |
ImageHDU |
26 |
(68, 68, 10) |
float64 |
11 |
BGS000.SCI |
1 |
ImageHDU |
26 |
(68, 68, 10) |
float64 |
12 |
BGS000.DQ |
1 |
ImageHDU |
26 |
(68, 68, 10) |
float64 |
Figure below shows the solution over the red grisms with vignetting in the two tilted ones.
Validation¶
Reduced dispersion in the spectra of white dwarfs observed on different positions on the focal plane before and after correction by the sirRelativeFluxScaling solution validates this product.
