Bandpasses¶
The simulator can render scenes in a number of different bandpasses. The choice of bandpass affects the point spread function used, the sky backgrounds, the fluxes of sources, and the reference files requested.
At present, romanisim simply passes the choice of bandpass to other packages—to webbpsf for PSF modeling, to galsim.roman for sky background estimation, to CRDS for reference file selection, or to the catalog for the selection of appropriate fluxes. However, because catalog fluxes are specified in “maggies” (i.e., in linear fluxes on the AB scale), the simulator needs to know how to convert between a maggie and the number of photons Roman receives from a source. Accordingly, the simulator knows about the AB zero points of the Roman filters, as derived from https://roman.gsfc.nasa.gov/science/WFI_technical.html .
One technical note: it is unclear what aperture is used for the bandpasses provided by Goddard. The Roman PSF formally extends to infinity and some light is received by the detector but is so far from the center of the PSF that it is not useful for flux, position, or shape measurements. Often for the purposes of computing effective area curves only light landing within a fixed aperture is counted. Presently the simulator assumes that an infinite aperture is used. This can result in an approximately 10% different flux scale than more reasonable aperture selections.
romanisim.bandpass Module¶
Roman bandpass routines
The primary purpose of this module is to provide the number of counts per second expected for sources observed by Roman given a source with the nominal flat AB spectrum of 3631 Jy. The ultimate source of this information is https://roman.gsfc.nasa.gov/science/WFI_technical.html .
Functions¶
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Compute the AB zero point fluxes for each filter. |
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Compute the count rate in a given filter, for a specified SED. |
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Get the zero point flux for a particular bandpass. |
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Read an effective area file from Roman. |