STScI Logo

Wide Field Infrared Survey Telescope
Source Simulation and Exposure Time Calculation

High-redshift galaxy with superimposed SN viewed in six filters.
Simulated high-redshift galaxy with a superimposed supernova, viewed in the six filters of the WFIRST Wide Field Instrument. Axes are labeled in arc seconds.

Introduction to Pandeia for WFIRST

Pandeia is the exposure time calculator (ETC) system that is being developed for the James Webb Space Telescope (JWST). It's based on a Python engine that calculates three-dimensional data cubes, based on user-specified spatial and spectral properties of one or more sources. The cubes are projected to a detector plane given an instrument configuration and two-dimensional pixel-by-pixel signal and noise properties are extracted. This allows for appropriate handling of realistic point spread functions (as calculated by WebbPSF), MULTIACCUM detector readouts, correlated detector readnoise, dithering, and multiple photometric and spectrscopic extraction strategies. Pandeia includes support for a wide variety of observing modes and has a highly modular, data-driven design that allows for easy extensibility to other instruments and telescopes.

This extensibility has been applied to create an implementation of the WFIRST Imager (WFI), as it is currently prototyped.

Functionality

Pandeia is designed to perform high-fidelity modeling of a small, representative portion of a detector image and to allow for extensive parameter space investigations. This typically small "postage-stamp" field of view is referred to as the "scene" and is composed of one or more astronomical sources. The size of the scene is dynamic and can in principle be arbitrarily large. However, because of the detail involved in the calculations Pandeia performs, calculations for very large scenes are not currently supported. STIPS is the more appropriate tool for simulating full fields of view.

Pandeia supports both point and extended sources. Extended sources are modeled by a Sersic surface brightness profile with major/minor axis scale lengths, a position angle, and a Sersic index (e.g. n=1 for exponential disk, n=0.5 for gaussian, and n=4 for de Vaucoulers). A wide variety of source spectra are available for use including power-law, blackbody, stars (via a grid of Phoenix models), and integrated spectra of a range extragalactic objects (via Brown et al. (2014)). It is also possible to add emission and absorption lines to a spectrum as well as input custom spectra. Normalization of source flux is accomplished 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).

The output of a Pandeia calculation provides 2D signal-to-noise maps as well as extracted flux and signal-to-noise products that are defined by a flux extraction strategy. For WFIRST imaging, this consists of aperture photometry with a circular source aperture and annular background estimation region. WFIRST IFU will support strategies that use dithers to facilitate background subtraction.

Using Pandeia-WFIRST

Pandeia is currently under rapid development in the lead-up to the initial limited release for JWST in 2016. As such, it must be considered a work in progress. Several planned features are not yet implemented.

The code for Pandeia is not yet openly available and the only way to access it currently is via a Jupyter notebook interface on the WFIRST JupyterHub server at https://wfirst-tools.stsci.edu. Please refer to our WFIRST Tools Server page for details on how to request an account and how to access/use the system. Since the software is presently in beta release, access is restricted to people connected to the WFIRST Project and Science Teams.

Example notebooks are provided on the JupyterHub server that include simple GUIs for configuring calculations and displaying basic results. They also contain more detailed examples and documentation on how to customize the calculation inputs, analyze the outputs, and set up scripts for doing batch analysis.

Pandeia-WFIRST Jupyter notebook-based graphical interface. Screenshot of the Jupyter notebook interface for Pandeia-WFIRST. This illustrates its layout, how the results are presented in the form of interactive plots, and its use of realistic input spectra.

Pandeia-WFIRST calculation results including a point and extended source. An example of a Pandeia-WFIRST calculation that has been modified to add a second source. This illustrates Pandeia's ability to model multiple sources of various sizes, shapes, and positions and thus include the effects of contaminating flux in the final signal-to-noise calculations.

Documentation

Much of the Pandeia documentation has yet to be written. Some basic API documentation will be provided along with the notebooks. A paper describing the general approach, features, and design of Pandeia is in preparation. For now users are encouraged to cite the Pandeia paper Pontoppidan et al. (2016, in prep.) which will be presented at the SPIE Astronomical Telescopes and Instrumentation conference in 2016.

Assumptions and Caveats

Pandeia functionality in general and its WFIRST functionality in particular is still in continued development. The present implementation is made available to the community as a beta version.

The WFIRST Mission is not yet in development and observatory designs continue to evolve. Hence, Pandeia simulations may not accurately reflect the actual future observatory.

At this stage the model for the WFI makes several assumptions and approximations:

Future Improvements

We are planning on many additions and improvements for future Pandeia versions. These include the following:

Credits, Acknowledgements, and Feedback

Pandeia is developed by the ETC team at STScI which is lead by Victoria G. Laidler. The Pandeia ETC engine and WFIRST extension are developed by Klaus Pontoppidan and Timothy Pickering.

Users are encouraged to address questions, suggestions, and bug reports to help@stsci.edu with "Pandeia-WFIRST question" in the subject line. The message will then be directed to the appropriate members of the Pandeia-WFIRST team at STScI.