HCI NIRCam Limiting Contrast

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

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

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 1 shows the best available treatment of Climit(s) for the 5 NIRCam coronagraphs. This graphic is 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 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.) 
  • Currently available—and outdated— estimates of JWST aberrations are adopted

Figure 1 shows the estimated functions Climit(s) for the 5 NIRCam coronagraphs as a function of the apparent separation (s) in arcseconds.

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, but only under the same technical and procedural assumptions of Beichman et al. (2010).

This is the best information on limiting coronagraphic performance for NIRCam at the current time. In the future, with a better understanding of wavefront errors and other technical factors, or when users become interested in different combinations of technical factors, improved calculations of Climit(s) will be made available.

Meanwhile, users may be able to extrapolate estimates of Climit(s) using the Exposure Time Calculator (ETC) and other proposal tools.

Figure 1. NIRCam limiting contrast example

Example of estimating the imiting performance of NIRCam's 5 coronagraphs. Adapted from Beichman et al. (2010), Figure 6.


References

Beichman, C. A., et al. 2010, PASP, 122:162
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