NIRISS AMI Observations of Extrasolar Planets Around a Host Star

Example Science Program #23

This example science program provides a walk-through of  a JWST observing program using NIRISS Aperture Masking Interferometry (AMI), focusing on overarching science goals from the GTO program "NIRISS/AMI Architecture of directly-imaged extrasolar planetary systems" (PI: Rameau) for context. This article discusses how to navigate the Exposure Time Calculator to determine exposure times required to meet the science goals, and how to set up the observation templates in the Astronomer Proposal Tool GUI. 

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Main articles: NIRISS Aperture Masking Interferometry, NIRISS AMI Recommended StrategiesNIRISS AMI-Specific Treatment of Limiting Contrast
See also: Step-by-Step ETC Guide for NIRISS AMI Observations of Extrasolar Planets Around a Host Star
Step-by-Step APT Guide for NIRISS AMI Observations of Extrasolar Planets Around a Host Star

The "NIRISS/AMI Architecture of directly-imaged extrasolar planetary systems" GTO program (PI Rameau) will target HD 218396 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.

PSF Calibration Strategy

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 instrumental contributions to the interferometric observables of closure phases (CP) and visibility amplitudes. Closure phases are the sum of the fringe phases from three holes. A closure phase must theoretically be zero for a point source (see Lawson 2000 and Monnier 2003 to learn more about these observables). 

For more demanding observations (e.g., contrast ratios smaller than 1/100), 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 218172 serves as the PSF reference star.  When picking a PSF reference star, it is important to choose a star that is single (i.e., not in a binary system). In general, we recommend observing more than one PSF reference star if the reference star has not been previously observed interferometrically to mitigate against using an undiscovered multiple-star system as a point source referenceUse the JWST General Target Visibility Tool to determine the target visibility windows. You can also look at the available position angles of a known companion if you are concerned about the placement of the faint companion on the detector.

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 HD 218396 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 for NIRISS AMI Observations of Extrasolar Planets Around a Host Star walks the user through navigating the JWST Exposure Time Calculator (ETC) to determine exposure parameters appropriate for the science goals for this program.

The Astronomer Proposal Tool (APT) is used to submit JWST proposals. The Step-by-Step APT Guide for NIRISS AMI Observations of Extrasolar Planets Around a Host Star provides instructions for filling out the APT observation templates. The exposure parameters determined by the ETC are specified in the APT observation template.


Ireland 2013, MNRAS, 433, 2

Phase errors in diffraction-limited imaging: contrast limits for sparse aperture maskin

Lawson P. R. 2000
Principles of Long Baseline Stellar Interferometry
Course notes from the 1999 Michelson Summer School, held August 15-19, 1999. Edited by Peter R. Lawson. Published by NASA, Jet Propulsion Laboratory, California

Monnier J. D. 2003, Reports on Progress in Physics, 66, 789 
Optical interferometry in astronomy



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