This article contains a list of current limitations and known issues of Roman I-Sim version 0.7.0 (released on December 6, 2024). For more information on individual Roman I-Sim methods and functions, please consult the readthedocs documentation.

Current Limitations 

Detector effects

• Some detector effects like 1/f noise or snowball artifacts have not been implemented yet. 
• Reference pixels in Level-1 data, i.e., the 4-pixel wide borders around each detector and the virtual pixels read by the 33rd amplifier, which are not exposed to the sky and are only used for calibration purposes, are currently not simulated.
• Guide windows, i.e., the small regions in each detector where pixels are read faster for guiding purposes, are currently not simulated.

Bright Sources

• The size of the PSF model is limited to 44 by 44 WFI pixels (about 4.85 by 4.85 arcsecs). This causes the wings of stars to be truncated outside of that region. For bright sources, the truncation is evident (see the left panel of the figure below). If a non-zero roll angle is specified during the simulation, e.g. via the "–-roll" command line argument if using the executable version of Roman I-Sim (romanisim-make-image), the truncation happens twice, first in R.A., Dec. space and then in pixel space (see the right-hand side of the figure below).  The same truncation also happens to galaxies, but it is only evident in bright galaxies with sharp cores.

CRDS

• Not all reference files in CRDS are currently used by Roman I-Sim even when the "--usecrds" parameter is set. Examples are, e.g., the photom and IPC reference files.
• If CRDS is not enforced, e.g., if the command-line argument "--usecrds" is not present when running the executable version of Roman I-Sim , the simulator outputs images in which East erroneously points to the right instead of to the left. This behavior is due to the default WCS models used in the GalSim software.

Other Limitations

• When generating and saving input source catalogs, we recommend using formats that are automatically identified when read in with astropy.Table.read(), as the enhanced character separated values (.ecsv) format or as FITS tables. 
• Astropy.table is currently used by Roman I-Sim to load external input catalogs. As a result, reasonably-sized Roman full-frame catalogs, containing upwards of a few million sources, are read at a very slow speed (even hours) while using up to several 100s of GB of RAM if ASCII formats like .csv or .ecsv are used. We strongly recommend users make use of catalogs in binary FITS formats when many million sources are present in the catalog, or trim ASCII catalogs to only contain those sources that are expected to fall within the simulated Sensor Chip Assembly (SCA).
• Roman I-Sim does not save uncalibrated Level-1 and calibrated Level-2 images in the same simulation run. Users interested in both uncalibrated and calibrated output products need to run the simulation twice (making sure to use the same seed for random-number generation).
• Users planning to create full-frame WFI images with the command-line executable romanisim-make-image need to set the --sca parameter to the value -1, so that all 18 SCAs are generated in serial, or run the executable 18 individual times, and then manually collage the 18 outputs together.

Known Issues

There are no known issues in the current version of  Roman I-Sim .

Acknowledgements

Roman I-Sim is written and maintained by the Roman Science Operations Center (SOC) at STScI.

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

References

• Rowe, B. T. P., Jarvis, M., Mandelbaum, R., et al.  2015, "GALSIM, The modular galaxy image simulation toolkit", Astronomy and Computing, 10, 121. doi:10.1016/j.ascom.2015.02.002
• Roman I-Sim on readthedocs: https://romanisim.readthedocs.io/en/latest/
• Roman Calibration Reference Data System (CRDS): https://roman-crds.stsci.edu/
• Astropy table documentation: https://docs.astropy.org/en/stable/table/