Pandeia is the software used to calculate exposure times and signal-to-noise ratios for the Wide Field Instrument (WFI) on the Nancy Grace Roman Space Telescope, and it is developed by the Roman Science Operations Center at STScI.

Functionality

Pandeia  is designed to perform high-fidelity modeling of small regions of a single Roman WFI detector. These simulations support detailed calculations of signal-to-noise ratios (SNR), allowing users to optimize their observing strategy. Each simulation typically includes one to a few astronomical sources. The default scene size is 5 arcseconds in width (about 45 pixels on WFI), but can be expanded up to about 25 arcseconds.

Components of the Simulation

A  Pandeia simulation is composed of two components: the astronomical scene and the observing setup. The scene is composed of one or more astronomical sources specified by their spatial (point/extended) and spectral properties. The observing setup consists of instrumental parameters, including the WFI imaging filter, and the specification of a multi-accumulation (MA) table, which describes the WFI readout pattern. Below, we describe the options for both of these simulation components.

Astronomical Scene 

Pandeia is designed to identify the optimal observing setup for a given science case, and/or to determine the SNR that can be achieved for a given astronomical scene. As discussed above, Pandeia is optimized to work with relatively small fields of view (under about 25 arcseconds in size).

A variety of astrophysical sources are available as inputs. The default source is a point source with a flux of 0.001 mJy (23.9 AB magnitudes) at all wavelengths. Additional example sources are available and are described below. The Pandeia Tutorials article provides additional examples including how to modify the input source spectral properties.

Extended Sources

Extended sources are modeled by the following shapes:

• Flat: a circular step function. Requires major ( major ) and minor ( minor ) axes scale lengths (arcseconds)
• Sérsic surface-brightness profile has two options:
  • sersic: follows Sérsic's R^1/n model. See Graham & Driver (2005) Eq(1) for its mathematical form. Requires  major ,  minor , and a sérsic index ( sersic_index ) 
  • sersic_scale: an e-folding profile, normalized to 1 at the scale radius. See Graham & Driver (2005) Eq(14) for its mathematical form. Requires  major ,  minor , and sersic_index . 
• Gaussian2d: a special case of sersic_scale profile where sersic_index is set to 0.5. Requires major and minor as inputs. 
• Power: Requires a core radius ( r_core , in arcsec) and a power index ( power_index ). 

Pre-Configured Spectral Energy Distributions

A variety of source spectra are available for use as illustrated by the Table of SED Inputs. It is also possible to add emission and absorption lines to a spectrum as well as input custom spectra. In the Pandeia Tutorials article, an example shows how to modify the SED from the default flat spectrum to an A0V star from the Phoenix models.

SED Normalization 

The flux of a source can be normalized by providing a flux density (e.g., mJy or AB mag) at a specific wavelength or a magnitude in a specific bandpass (e.g., SDSS z). For a full list of parameters for the configuration dictionaries see the latest Pandeia JWST documentation.

Observing Setup

The observing setup involves loading the default configuration for the WFI and then setting the filter (or filters) of interest. 

Loading the Default Roman Exposure

For observing with Roman WFI imaging, the code below will provide the appropriate Pandeia settings. This example will create an observing setup using the "im_135_8" MA table without truncation, corresponding to a science exposure of 139 seconds with 8 resultants. The default MA table for the spectroscopic mode is "sp_300_16" (300 science exposure time with 16 resultants). The detailed interpretation of this exposure configuration can be accessed from the Appendix: WFI MultiAccum Tables in the Roman APT User's Guide.

# Setup default Roman Imaging mode
from pandeia.engine.calc_utils import build_default_calc
calc = build_default_calc('roman', 'wfi', 'imaging')

Setting the Filter

WFI imaging options for  FILTER are: f062, f087, f106, f129, f146, f158, f184, and f213 (this is case-sensitive; lowercase only). A description of the optical elements for the WFI are given in the WFI Optical Elements article in the The Wide Field Instrument. 

The FILTER is a global variable that can be easily defined as follows: 

# Set the global variable for the filter name (change to any valid filter)
FILTER = 'f129'

Outputs from Pandeia

The output of a  Pandeia calculation provides two-dimensional signal-to-noise maps as well as extracted flux and signal-to-noise products that, for Roman, are based on aperture photometry. The Pandeia Tutorials article provides an example of computing the signal-to-noise ratio using default instrument configurations, as well as several other examples that explore how to use the inputs and outputs from Pandeia to optimize the WFI observational designs.

Additional Documentation

While Pandeia was originally developed as the Exposure Time Calculator (ETC) for JWST (see Pontoppidan et al. 2016 for more information on the general approach and design), most features can be applied to Roman calculations. Please note the Caveats of using Pandeia for Roman to understand current limitations. Users may also find it helpful to review the JWST documentation for an overview of the features and functionalities of the Pandeia engine and a detailed description of its outputs.

References

1. Graham & Driver (2005), "A Concise Reference to (Projected) Sérsic R1/n Quantities, Including Concentration, Profile Slopes, Petrosian Indices, and Kron Magnitudes", PASA 22 118
2. Brown et al. 2014, "An Atlas of Galaxy Spectral Energy Distributions from the Ultraviolet to the Mid-infrared", ApJS 212 18
3. Pontoppidan et al. 2016, "Pandeia: a multi-mission exposure time calculator for JWST and WFIRST", SPIE.9910E.16
4. This page uses data from the Cycle 9 instrument reference information, which were generated and delivered by the Roman team at Goddard Space Flight Center (GSFC).
5. The JWST Documentation (JDox) website.

For additional questions not answered in this article, please contact the Roman Help Desk.