Parent article: JWST High-Contrast Imaging
Main articles: NIRISS Aperture Masking Interferometry, NIRISS Bright Limits
See also: NIRISS Non-Redundant Mask
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.
See the article Contrast Considerations for JWST High-Contrast Imaging for a general treatment of "contrast" (C ), including Climit(s) in particular.
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 NIRISS/AMI is based on Greenbaum et al. (2015).
Limiting contrast, Climit(s), for NIRISS/AMI
Figure 1 shows the best available treatment of Climit(s) for the NIRISS AMI. This graphic is adapted from Figure 6 in Greenbaum et al. (2015).
The "technical factors" or assumptions behind the curves in Figure 1 include:
- Theoretical binary target
- Good flat field measurements with sub-pixel accuracy by precise positioning
- No variation of sensitivity within a pixel (intra-pixel sensitivity, or IPS)
Negligible phase-closure errors due to static piston in the pupil
- Companion-to-host flux ratio Cflux = 10–2 (solid curves) and Cflux = 10–3 (dashed curves)
- False-alarm probability (unknown)
- Currently available estimates of JWST aberrations (outdated)
- Filters F277M and F430M
Under those assumptions, Figure 1 shows the estimated limiting contrast ratio for 5-sigma detection of a faint companion near a bright host as a function of their apparent separation s in mas.
If the user's operating point (s, Cflux) lies above a color curve, that source is detectable under the technical and procedural assumptions of Greenbaum et al. (2015).
This is the best information on limiting performance for NIRISS/AMI at the current time. In the future, with a better understandings 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 extrapolated estimates of Climit(s) using the Exposure Time Calculator (ETC) and other proposal tools.
Figure 1. NIRISS AMI limiting contrast example
Example of estimating HCI limiting performance for NIRISS/AMI. Adapted from Figure 16 in Greenbaum et al. (2015).