Contamination & Visibility Calculator

This tool is designed for the slitless observation modes of all JWST instruments. For slitless observations, the spectrum of a target star may be contaminated by partially overlapping spectra of nearby stars. For a given target, this contamination depends on the position angle (PA) at which the observations are taken. This tool simulates NIRISS SOSS observations of a given target, and produces an estimate of the level of contamination as a function of the PA of the observation, thus making it useful to plan observations at the optimal PA. The tool also computes the JWST accessibility windows of the target, along with the corresponding accessible PAs for the chosen instrument/observation mode.

NIRCam and MIRI options: Users can calculate contamination levels for NIRCam Grism Time Series (F322W2, F444W) and MIRI Low Resolution Spectroscopy mode by calling this tool locally on a terminal. Please keep in mind that the calculation for the NIRCam and MIRI modes can take anywhere from 1-3 hours due to the complexity of their algorithms. For instructions on how to generate the contamination plots for NIRCam and MIRI, please refer to our Contam Visibility JWST-Docs page.

Potential caveats:

• The field stars used for this analysis are retrieved from the Gaia EDR3 point source catalogue. Contamination from stars missing from this catalog are thus not modelled; this may be important for faint targets.
• Distortion has been observed to create a trace offset as large as ~4 to 5 pixels in both the X and Y direction in the science frame which may contribute to uncertainty in the contamination plots.
• Traces on the MIRI detector may fold over below 4.5 microns. Our model traces assume no spectral foldover. Depending on the observation, this may lead to inaccurate results.

If there are any questions regarding these caveats please send us a ticket through the JWST help desk and we will get back to you shortly.