HCI NIRISS Limiting Contrast
Treatment of limiting contrast (Climit) is based on current information about telescope aberrations and the expected performance of JWST NIRISS AMI.
See also: NIRISS Aperture Masking Interferometry, NIRISS Bright Limits, NIRISS Non-Redundant Mask
The contrast limit, Climit(s), is the highest detectable host to companion flux ratio.
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. Figure 1 shows the estimated limiting contrast ratio for a 3-sigma detection of a faint companion near a bright host star as a function of their apparent separation (s) in mas, as measured from in-flight data during JWST commissioning (Sivaramakrishnan et al. 2022). If the user's operating point (s, Cflux) lies above a color curve, that source is detectable. Users may be able to extrapolate estimates of Climit(s) using the Exposure Time Calculator (ETC) and other proposal tools.
Figure 1. NIRISS AMI limiting contrast example
These contrast curves were generated from single-companion fit models (based on Ray et al. 2023). The two cases presented are (1) HIP 65426 calibrated with the reference star HIP 65247 (shown with a solid dark purple line) and, (2) HIP 65426 calibrated with itself (by splitting the observation into two sets, shown with a dashed light purple line).
The calibrated closure phases and squared visibilities were compared against those of a grid of single companion models with varying separations, contrasts, and position angles. For each companion separation, the average $\chi^2$ value across all sampled position angles ($\chi^2_{sep}$) was computed. The 5σ contrast was then defined as the contrast where $\Delta\chi^2 = \chi^2_{null} - \chi^2_{sep} = 25$, with $\chi^2_{null}$ representing the $\chi^2$ value for the null (no companion) model†. This is the contrast curve shown in the figure for the two different cases.
†A $\Delta\chi^2$ value of 25 was chosen because the contrast is the only parameter affecting the difference between the null model and the 5σ detectable model at each separation. This threshold corresponds to a 5σ significance for a model with 1 degree of freedom, reflecting the difference in the number of parameters between the companion and null models.
References
Ray, S., Sallum, S., Hinkley, S., et al., 2023, arXiv:2310.11508
The JWST Early Release Science Program for Direct Observations of Exoplanetary Systems III: Aperture Masking Interferometric Observations of the star HIP 65426 at 3.8 µm
Sivaramakrishnan, A., Tuthill, P., Lloyd, J. P., et al., 2023, PASP, 135, 015003
The Near Infrared Imager and Slitless Spectrograph for the James Webb Space Telescope – IV. Aperture Masking Interferometry.