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Wide Field Infrared Survey Telescope
Complex Scene Simulation

Simulated globular cluster using STIPS.
Pseudocolor image of the central region of a globular cluster viewed in the Z087, J129, and F184 filters of the WFIRST Wide Field Imager, as simulated using STIPS. Click for a full view of the cluster on one of the 18 4kx4k detectors.

Introduction to STIPS for WFIRST

The STIPS (Space Telescope Image Product Simulator) software produces simulated imaging data for complex wide-area astronomical scenes, based on user inputs, instrument models and library catalogs for a range of stellar and/or galactic populations. It was originally developed for the JWST mission, but now has been extended to include WFIRST functionality as well. The current WFIRST version produces images covering a single detector in the Wide Field Imager (WFI) field of view; a future enhancement will be able to generate the full WFI focal plane simultaneously. STIPS includes the most current information about the telescope sensitivity, spectral elements, and detector properties; it uses the PSF model generated by WebbPSF for WFIRST, and it calls the appropriate Pandeia modules to compute instrumental throughput and count rates.

STIPS is based on a Python module and a web interface that provides a straightforward way of creating observation simulations. In its current implementation, it runs server-side and allows users to submit simulations and view/retrieve the results. To gain access to the STIPS interface please request access as described in the Using STIPS section below.

Functionality

At the heart of STIPS is a multi-faceted scene generation that allows varying degrees of user control. STIPS can generate a randomized set of stars or (idealized) galaxies based on very concise user specifications, or it can generate point or extended sources based on a detailed input catalog. In addition, an existing image can be used as background.

Source specification

Each of the sources populating a STIPS simulated scene can be a point source or an extended source. Point sources are characterized by their position and spectral energy distribution. Extended sources are in addition characterized by major axis, minor axis, orientation, and light profile, assumed to follow a Sersic parametrization. For either point or extended sources, the spectral energy distribution is converted into a count rate at the detector depending on the filter(s) chosen and the current instrument model; the user can choose to specify directly the count rate in each filter instead.

Randomized source catalog

For a quick implementation that will satisfy many user needs, the distribution of point or extended sources can be specified in terms of a stellar or galaxy population based on a number of global parameters. The parameters for point sources are designed to mimic a stellar population; those for extended sources are more suited to a galaxy population.

While the source distributions are random, they are also repeatable. For any STIPS simulation the user is able to specify a seed for the random number generator, and two scenes produced with both the same input parameters and the same random number seed will be identical.

Stellar populations

Stellar populations are built on the basis of the Padova isochrones , specifically the version CMD 2.7. The basic parameters required are the number of stars, their metallicity and age range, and the characteristics of the IMF. In addition, the spatial distribution of the stars can be specified via its shape and radius, and a distance range can be specified as well. If desired, the mass distribution can be modified to allow more massive stars to be closer to the center, in order to mimic the effect of mass segregation. Mass segregation is done by sorting the sources by their distance from the center, and then placing high-mass stars (stars with masses greater than 75% of the maximum mass) at shorter distances with a slightly higher probability than would be produced by random distribution. Multiple populations can be included. The module will generate a catalog of stars, which can be retrieved as part of the simulation products. Stellar spectral energy distributions are then generated using the Phoenix models, available at https://phoenix.ens-lyon.fr/simulator/index.faces .

Galaxy populations

Galaxy populations are built on the basis of a user-specified range of sizes, brightnesses, and redshifts. Spectral energy distributions are drawn at random from a predefined set of Bruzual-Charlot models, and are assigned randomly either an exponential or a de Vaucouleurs profile with randomly-generated shape and orientation. The spatial distribution can be specified in the same way as for stellar populations. As for stellar populations, the catalog of galaxies can be retrieved together with the simulation results.

User-provided input catalogs

If the populations-based description is not sufficient, the user can have full control on the definition of the astronomical scene by providing one or more input catalogs specifying the properties of each individual source, including position, brightness, and, for extended sources, size, elongation, orientation, and light profile (in the form of a Sersic index). When an input catalog is provided, the apparent brightness of each source needs to be specified in each filter; a future upgrade will allow spectral energy distributions to be specified instead.

Background Image

The user can specify a background image to be added to the scene. The wcs of the image may be derived from its FITS header, or the image may be treated as centered at the scene center. The image's flux units and pixel scale must be specified, and the image will be remapped to the WFIRST detector pixel scale prior to being added to the scene.

Observations

The user specifies the desired exposures by filter, exposure time, and number of coadds. Exposures can be oversampled if desired. A background level based on a model of zodiacal light is included. Currently, STIPS uses a constant background value for each instrument/filter combination representing an average background flux per pixel for that instrument/filter. In the future, STIPS will be modified to use the Pandeia background model.

Instrument and error model

STIPS includes a full description of the instrumental PSF as obtained from the WFIRST version of WebbPSF .

Images produced by STIPS by default include noise and residual errors estimated on the basis of a simple instrument model. This model includes reasonable estimates of possible post-pipeline residuals from flat-fielding, dark-surrent subtraction, and imperfect cosmic ray correction. Error terms are calculated as follows:

Products

  1. Source Catalogs for each included population
  2. Image data in FITS format
  3. Quick-look images in JPEG format
  4. PSF images
  5. As-observed catalogues for each observation of the included populations.

STIPS produces quick-look JPEG images, FITS images of each individual detector and offset, mosaic images combining detectors into instruments, and combining detector dither patterns. In addition, it provides copies of generated catalogues, both as-created and as-observed. The as-observed catalogues are generated during the observation, and provide the detector co-ordinates and count rates of every source appearing in the exposure. Because they are generated before PSF convolution, and before residual noise is introduced, the as-observed catalogue can be used to test the efficiency and accuracy of a source-detection program (e.g. daophot) on the output data.

STIPS mosaics are generated by the Montage program, and are built without any co-ordinate distortion. They are not intended as accurate representations of drizzled images, but as a quicklook to see the rough result of (e.g.) a combined dither pattern.

Using STIPS

The STIPS central module will soon be added to the STScI github server, as will the web server interface. In the future, a standalone desktop interface may also be created. For now, STIPS is only available only through an STScI website. Since STIPS is presently in beta release, access is restricted to people connected to the WFIRST Project and Science Teams. Access is offered upon request.

Users are authenticated using their Mikulski Archive for Space Telescope (MAST) account. If you do not yet have such an account, please register here. Then send a message to the STScI helpdesk at help@stsci.edu with your name, institutional affiliation, the MAST account name, and a description of your connection to the WFIRST Project or Science Teams. Put "Request for STIPS access" in the subject line to ensure your request is routed to the appropriate person. You will receive instructions from the helpdesk on the next steps. Once your account is authorized you can visit the STIPS simulation website to run STIPS simulations.

Block diagram of the STIPS User Interface
Block diagram of the STIPS User Interface

Documentation

A detailed description of the format of input source catalogs is available. Much of the remaining STIPS documentation has yet to be written. Some documentation can be accessed as follows:

Assumptions and Caveats

STIPS 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, STIPS-WFIRST simulations may not accurately reflect the actual future observatory.

STIPS makes several assumptions:

Future Improvements

Enhancements currently planned in the functionality and fidelity of STIPS include:

If you have suggestions on additional features, please send us suggestions at help@stsci.edu .

Credits, Acknowledgements, and Feedback

STIPS is developed by Brian York, with guidance and contributions from Klaus Pontoppidan, Jason Kalirai, Pey Lian Lim, and Stefano Casertano. STIPS makes use of open source astronomical software developed by Justin Bird, Ahmed Fasih, and Adam Ginsburg.

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