Caveats of using STIPS for Roman

This page contains a description of known limitations and issues when using the Space Telescope Imaging Product Simulator (STIPS) software for the Nancy Grace Roman Space Telescope with relevant advice for users. 


This documentation is written for STIPS version 2.1.0 (released on November 15, 2023).

Limitations

The limitations of STIPS are given below. These are fundamental to the scope of the software design and are not expected to change with future development.

Instrumental Effects and Output Images

STIPS creates Level 2 (L2) data products to produce fast simulations of the Roman Wide Field Instrument (WFI) outputs. Generating Level 1 (L1) data products, as required for a full forward-modeled simulation, would slow down the computation significantly and it is out of scope for STIPS . A forthcoming Roman Science Operation Center (SOC) simulation tool, Roman I-Sim (see Github for more information), is intended to create full forward-modeled simulations,  including Level 1 (L1) data products.

Bright Sources

The point spread function (PSF) model radius in STIPS is forced to be 22 WFI pixels, meaning that the PSF is not modelled beyond this radius limit (note that the PSF is sampled at four times the resolution of the detector). Brighter sources may have long diffraction spikes that place significant signal beyond the default radius. Users requiring simulations beyond the default limit may use two keywords: Input Parameter psf_bright_limit or Input Parameter psf_xbright_limit (i.e., extra-bright limit) to indicate magnitude thresholds for simulating the PSF to larger PSF radii (44 WFI pixels and 88 WFI pixels, respectively). A summary of the PSF radius specifications is given in Table 1. An image cut out from the M13 star cluster simulation shown in the STIPS landing page article is displayed in Figure 1; this simulation required use of the three PSF radii available in STIPS . 

Because the point source flux is computed for every pixel, the computation for bright targets will take about four times as long as standard sources, and the extra-bright sources will take about 16 times as long. Given the long computation times, users are cautioned to only use these options if a high level of detail is required.


PSF KeywordDescription

Simulated PSF  Radius

WFI Pixels

Arcsec

default  

default PSF radius applied to all sources in the input catalog regardless of their magnitude222.42

psf_bright_limit

stars brighter than the magnitude limit specified by this keyword will be computed to a large radius444.84

psf_xbright_limit

stars brighter than the magnitude limit specified by this keyword will be computed to a extra-large radius889.68

Table 1. PSF Radius used in  STIPS settings. The Radius is given in both pixels on the WFI detector and converted to arcsec assuming a pixel scale of 0.11 arcsec per pixel.


Figure 1. An example of using the default for faint stars and   psf_bright_limit and psf_xbright_limit for brighter stars from the the M13 star cluster simulation shown in the STIPS landing page article. Inspection of individual sources within this cutout reveals the three PSF radii used in the simulation for stars of differing brightness. Many scientific use cases for observations of a star cluster like M13 may require understanding the impacts of the large dynamic range over the stellar sequences in this cluster, which STIPS is able to simulate per the descriptions in Table 1.

Issues

A list of STIPS known development issues is maintained on the readthedocs documentation. These are items that may be resolved in future releases.

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

Latest Update

 

Updates for STIPS v2.1
Publication

 

Initial publication of the article.