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 Imaging, NIRCam Bright Source Limits, NIRCam Coronagraphic Occulting Masks and Lyot Stops, NIRCam 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 on Beichman 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 (s, Cflux) 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.
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