NASA's James Webb Space Telescope

Space Telescope Image Product Simulator (STIPS)

The Space Telescope Image Product Simulator (STIPS) is a tool designed to help the astronomical community prepare for JWST observations. 

NOTE: In order to access STIPS JWST, you will need an STScI SSO Account.

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.
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.

Introduction to STIPS for JWST

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 catalogues 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 JWST version produces images covering the MIRI detector, either one or both NIRCam Long detectors, and either one, four, or all eight NIRCam Short detectors. STIPS includes the most current information about the telescope sensitivity, spectral elements, and detector properties; it uses the PSF model generated by WebbPSF for JWST, and it calls the appropriate Pandeia/JWST ETC 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 JWST Server, simply visit the linked webpage.

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 catalogue. 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 Catalogue

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 centre, in order to mimic the effect of mass segregation. Mass segregation is done by sorting the sources by their distance from the centre, 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 catalogue of stars, which can be retrieved as part of the simulation products. Stellar spectral energy distributions are then generated using the Phoenix models. (Note: This is the best link for STIPS implementation, but the page doesn't consistently load correctly.)

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 catalogue of galaxies can be retrieved together with the simulation results.

User-provided Input Catalogues

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 catalogues 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 catalogue 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 JWST detector pixel scale prior to being added to the scene.

Observations

The user specifies the desired exposures by detector, number of detectors, filter, background level, and exposure time. Exposures can be oversampled if desired. A background level based on a model of zodiacal light is included. Currently, STIPS uses the JWST Background Tool (JBT) to determine background count rates, and offers a choice between no background or the mean, median, maximum, and minimum observable background for the input observing co-ordinates.

Instrument and Error Model

STIPS includes a full description of the instrumental PSF as obtained from 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:

  • Poisson Noise: For each pixel, create a random number following a Gaussian distribution with mean 0 and standard deviation 1, then multiply by the square root of the absolute value of the flux in that pixel.
  • Readnoise: STIPS currently calculates readnoise by dividing the exposure into groups of no more than one thousand seconds, and generating the per-pixel readnoise as SQRT(number_of_groups) * k * time_per_group**a with k and a being per-instrument constants. This value is then multiplied by a set of random values the same size as the detector. STIPS currently does not implement correlated read noise.
  • Flatfield: For each instrument, a fixed-pattern flatfield residual file (with a mean of zero) was created, and this file is multiplied by the data.
  • Dark: As with flatfield, but using a separate file, multiplying the residual file by the exposure time, and adding the resulting dark residual to the image rather than multiplying it.
  • Cosmic Rays: The probability of a cosmic ray hit (per pixel) is calculated based on pixel size and exposure time. For each hit, a cosmic ray size and energy are calculated based on the distribution of cosmic rays, and the cosmic ray is added to the appropriate pixel(s). The pipeline process is assumed to remove 99% of the impact of cosmic rays.

Products

  1. Source Catalogues 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 is available as a private STScI github, as is the web server interface. In the future, a standalone desktop interface may also be created.

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

Documentation

A detailed description of the format of STIPS input source catalogues is available. In addition, STIPS includes assorted documentation on the web server.

Assumptions and Caveats

STIPS is still in continued development. The present implementation is made available to the community as a beta version.

Exact data on JWST's sensitivity is not yet available, and as such STIPS may not reflect on-orbit performance.

STIPS throughputs are derived from and tested against Pandeia. The same source, specified in both STIPS and Pandeia, will differ by less than 15%.

STIPS makes several assumptions:

  • STIPS relies in part on calculations with WebbPSF and Pandeia/JWST ETC. Therefore, the same assumptions and caveats apply as listed for those tools.
  • Field-dependence of the PSF is not currently included in STIPS.
  • The Spectral Energy Distribution (SED) for stars and galaxies are obtained from the Phoenix models and a subset of Bruzual-Charlot models, as described above, and are subject to the same limitations as those models. SEDs for galaxies are assumed constant over their images.
  • Models for possible post-pipeline residuals from flat-fielding, dark-current subtraction, and imperfect cosmic ray correction are approximate guesses that are not unreasonable for the HST WFC3/IR instrument, but which may be different for the JWST instruments.

Future Improvements

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

  • Field-dependent PSF
  • A more realistic treatment of readout modes and noise estimates
  • Saturation and sub-pixel effects
  • More customizable user input (e.g., per-source SED specification)

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 the JWST Help Desk.