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 stpsf 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., such that \(\mathrm{flux} = 10^{-\mathrm{AB mag} / 2.5}\)), the simulator needs to know how to convert between a maggie and the number of electrons 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 . The conversion between electrons received and fluxes in AB “maggies” is determined by integrating a flat 3631 Jy AB reference spectrum over the Roman filters to determine the number of electrons per second corresponding to a zeroth magnitude AB source.

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 electrons 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

compute_abflux(sca[, effarea])	Compute the AB zero point fluxes for each filter.
compute_count_rate(flux, bandpass, sca[, ...])	Compute the count rate in a given filter, for a specified SED.
etomjysr(bandpass, sca)	Compute factor converting e/s/pix to MJy/sr.
get_abflux(bandpass, sca)	Get the zero point flux for a particular bandpass.
read_gsfc_effarea(sca[, filename])	Read an effective area file from Roman.