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Main articles: NIRISS Aperture Masking Interferometry, NIRISS AMI Recommended Strategies
See also: NIRISS AMI Template APT Guide, NIRISS AMI-Specific Treatment of Limiting Contrast

The "NIRISS/AMI Architecture of directly-imaged extrasolar planetary systems" GTO program (PI Rameau) will target HR 8799 which has 4 known planetary companions, between 15 and 70 AU, discovered using ground based observations. The goal of this program is to search for additional planets at distances under 15 AU that are suspected to exist based on disturbances in the circumstellar disk.

The aperture masking interferometry (AMI) mode on NIRISS enables high contrast imaging, reaching separations of ~75–500 mas for a brightness ratio as small as 10-4. This contrast-separation regime is inaccessible to JWST's near-infrared coronagraphs at similar wavelengths (approximately 3-5 microns). The AMI mode is therefore ideally suited to detect exoplanets that are at separations less than 0.1" to 0.4" from their host stars. At these separations, the observations are sensitive to young companions as small as a few Jupiter masses. AMI is thus efficient for probing young planetary systems within 5–20 AU of their host stars. The spectral energy distribution of a young planet peaks in the thermal infrared regime between 3–5 µm, which is the operating range of AMI.

Data analysis with the non-redundant mask (NRM) requires observations of the target and a point spread function (PSF) reference star. The PSF reference star is used to calibrate out the instrumental contributions to closure phases (CP; the sum of three phases around a closed triangle of baselines) and squared visibility amplitudes (SqV). For more demanding observations, with contrast ratios smaller than 10-2 or so, the PSF reference star should be observed with the same dither pattern to minimize the impact of detector effects. (Note: dithering is generally discouraged for AMI mode, though this option is available to observers.) In such cases, the target and reference observations should be scheduled close in time so that the telescope is in a similar state, thermal or otherwise, for all the observations. The science target(s) and PSF reference star(s) should be observed using the same telescope optical configuration, so no wavefront correction should occur between any of the observations.

For this science use case, HD 217783, an F0V star of magnitude WISE W2 = 5.948 (Vega), serves as the PSF reference star.  


Estimating the total number of photons needed to detect desired contrast.

Main articles: Contrast Considerations for JWST High-Contrast ImagingNIRISS AMI-Specific Treatment of Limiting Contrast

For the purpose of this calculation, we assume that the flux ratio for HR 8799 and the planetary companion we wish to detect is ~10-4. According to Ireland (2013), the number of photons necessary to detect this constrast is:

1.5 x Nhole/ (contrast ratio)2, where Nhole refers to the number of apertures (holes) in a mask.

Since there are 7 apertures in the AMI NRM, this translates to:

73.5 / (contrast ratio)2

Considering the fact that NRM has not been used in space before, we use a slightly more conservative value of:

100 / (contrast ratio)2 = 100 / (0.0001)2 = 1010

Therefore, the goal of our calculation is to detect 1010 photons from the target with NRM and the F480M filter in the ETC simulations.

The Step-by-Step ETC Guide walks the user through navigating the JWST Exposure Time Calculator (ETC) to determine exposure parameters appropriate for the science goals for this program, providing a conservative average SNR estimate.

The Astronomer Proposal Tool (APT) is used to submit JWST proposals. The Step-by-Step APT Guide provides instructions for filling out the APT observation templates. The exposure parameters determined by the ETC are specified in the APT observation template.

Continue the tutorial:


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JWST technical documents

Ireland 2013, MNRAS, 433, 2