HCI NIRCam Limiting Contrast

Treatment of limiting contrast (Climit) is based on current information about telescope aberrations and the expected performance of JWST NIRCam.

See also: NIRCam Coronagraphic ImagingNIRCam Bright Source LimitsNIRCam Coronagraphic Occulting Masks and Lyot StopsNIRCam Filters for Coronagraphy

Caution on Limiting Contrast

The information contained in this article is presented as a general guide based on pre-launch and post-launch experience. The ultimate contrast will depend on many factors including, but not limited to, post-observation processing.

Limiting contrast, Climit(s)is the companion-to-host flux ratio of the minimum detectable companion. It is the detection limit and the best that can be done.

The article HCI Contrast Considerations provides a general treatment of "contrast" (C), including Climit(s).

Climit(s) is a function of essentially everything related to high-contrast imaging (HCI): myriad eclectic technical factors and procedures, end-to-end. This treatment of Climit(s) for NIRCam is based oBeichman et al. (2010). Figure 4 in NIRCam Coronagraphic Imaging (top left "pre-flight") shows  Climit(s) for the 5 NIRCam coronagraphs, adapted from Figure 6 in Beichman et al. (2010). For the round occulting masks, the assumed filters are F210M, F335M, and F430M. For the bar-shaped masks, the filters are F210M and F430M. The "technical factors" behind the curves in Figure 1 (below) include:

  • Images taken at roll angles differing by 10° and differenced in post-processing
  • Random positional errors of 10 mas and and wavefront errors of 10 nm, introduced between rolls
  • An adopted false alarm probability of 6 × 10–7 (5-sigma) (Normally distributed errors with zero mean are assumed after image differencing.) 
  • Estimates of JWST aberrations available at the time of publishing the Beichman et al. (2010) article were adopted

The interpretation of these results is as follows: if the user's operating point (sCflux) lies above a color curve, that source is estimated to be detectable.

The was the best information on limiting coronagraphic performance for NIRCam at the time. We now have a better understanding of wavefront errors and other technical factors, or when users become interested in different combinations of technical factors. Figure 1 and 2 below show on-sky examples of measured  Climit(s) for two of the main round occulting masks, and how they compare to the best simulations available today.

Figure 1 shows an example of measured in-flight contrast using the MASK335R/F356W setup, adapted from Carter et al. (2023), and computed using different strategies.

Figure 2 shows an example of the contrast achieved for the ERS program PID = 1386 as a function of the apparent separation in arcseconds with about 20 minutes on-source versus the estimated functions Climit(s) for NIRCam MASK335R occulter coupled with the F356W filter.

The of Climit(s) estimates using the Exposure Time Calculator (ETC) are adequate to prepare proposals.

Figure 1. NIRCam limiting contrast example (in-flight measurements versus simulation with the PanCAKE package, an extension to the ETC) using MASK335R/F356W.


An example of measured in-flight contrast using the MASK335R/F356W setup, adapted from Carter et al. (2023), and computed using different strategies.
See Figure 4 in NIRCam Coronagraphic Imaging for a comparison of the now obsolete pre-flight and in-flight contrast curves from Beichman et al. (2010).
Figure 2 (below) shows an example of the contrast achieved for the GTO program PID = 1193 as a function of the apparent separation in arcseconds with about 14 minutes on-source versus the estimated functions Climit(s) for NIRCam MASK430R occulter coupled with the F356W and F444W filters. It is worth noting that while MASK430R has a worse inner-working angle than that of MASK335R, the contrast beyond 1 arcsecond is significantly deeper. As for Figure 1, on-sky contrasts agree very well with simulations.
Figure 2. NIRCam limiting contrast example (in-flight measurements versus simulation with the pyNRC package, an extension to the ETC) using MASK430R.


An example of contrast curves (left) and detection limits in Jupiter masses (right) for the Fomalhaut system as observed in GTO Program PID = 1193. The top panels (a, b) are simulations performed with the pyNRC package using the MASK430R/F356W and MASK430R/F444W setups. The comparison with measured in-flight contrasts (c, d), adapted from Figure 7 in Ygouf et al. (2024), is very good for both occulter/filter combinations, both in contrast and mass sensitivities. The contrast curves in (c) represent the deepest contrasts achieved to date with NIRCam Coronagraphy, the wiggles in the flat part of the curves (background and detector limited regime beyond 3 arcseconds) are due to the presences of many background objects. The detection limits in (d) are only displayed for the F444W, the most sensitive filter for planet detection (red curves in b). At these wavelengths, small wavefront errors of 0, 2, or 5 nm between science rolls and/or the reference star do not make significant differences except within 1 arcsecond from the star.

References

Beichman, C. A., et al. 2010, PASP, 122:162
Imaging Young Giant Planets from Ground and Space

Carter, A., et al. 2023, ApJ Letters, 951, 1 
The JWST Early Release Science Program for Direct Observations of Exoplanetary Systems I: High Contrast Imaging of the Exoplanet HIP 65426 b from 2-16  μm

Girard, J. H., et al. 2022, Proceedings of the SPIE, 121803Q   
JWST/NIRCam Coronagraphy: commissioning and first on-sky results

Leisenring, J. / pyNRC
pyNRC: a JWST NIRCam ETC and Simulator

Perrin, M., et al. 2018, SPIE, 1069809
Updated Optical Modeling of JWST Coronagraph Performance, Stability, and Strategies

Ygouf, M. et al., 2024, AJ, 167, 1 26 
Searching for Planets Orbiting Fomalhaut with JWST/NIRCam




Notable updates
    • Updated Figure 1
    • Added Figure 3 based on Ygouf et al. (2024)
    • Added new references


  • Added a NIRCam in-flight contrast example (Fig. 2)
Originally published