NIRISS SOSS Time-Series Observations of WASP-39

The goal of this example program is to observe an exoplanet transit of WASP-39b, and is based on the ERS Program "The Transiting Exoplanet Community Early Release Science Program". This program was designed to study a set of exoplanets to explore their atmospheric composition, energy budget, and dynamics. 

Example Science Program #31

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See also: Step-by-Step ETC Guide for NIRISS SOSS Time-Series Observations of WASP-39, Step-by-Step APT Guide for NIRISS SOSS Time-Series Observations of WASP-39

Steps for creating observations

Step 1 - Determine the required wavelength coverage: near-infrared or mid-infrared

See also: 
NIRCam Grism Time SeriesNIRISS Single Object Slitless SpectroscopyNIRSpec Bright Object Time-Series Spectroscopy
MIRI Low Resolution Spectroscopy

Five molecules of interest in exoplanet atmospheres (water, carbon monoxide, hydrogen cyanide, methane, and ammonia) are expected to show significant spectral features at near-infrared wavelengths—depending on atmospheric pressure and temperature. The signal-to-noise ratios (SNR) of the host stars is greatest at lower wavelengths, enabling better precision in the measurement of exoplanet atmospheres. We thus omit MIRI low resolution spectroscopy from consideration, as well as NIRCam grism time series since that only provides coverage between 2.4–5.0 μm, at longer wavelengths than NIRISS single object slitless spectroscopy (SOSS) and NIRSpec bright object time-series spectroscopy

Step 2- Select an instrument observing mode

See also: NIRISS SOSS Recommended Strategies 

NIRISS SOSS simultaneously covers the wavelength range from 0.6–2.8 μm. NIRSpec bright object time-series spectroscopy would require 3 visits to achieve the same wavelength coverage via their high and medium-resolution modes and suffers from saturation when using the PRISM mode. The SOSS slitless configuration avoids pointing-related flux variations, and combined with the cross-dispersed properties, minimizes flat field and intra-pixel gain variation issues. We thus chose the NIRISS SOSS observing mode for this program.

Step 3 - Determine the readout pattern and subarray configuration

See also: NIRISS Detector Readout PatternsNIRISS Detector Subarrays

Words in bold are GUI menus/
panels or data software packages; 
bold italics are buttons in GUI
tools or package parameters.

Since we are observing a relatively bright star, we use a subarray for faster readout to avoid saturation. NIRISS offers two subarray options for the SOSS mode: SUBSTRIP256 and SUBSTRIP96. For this program, we chose SUBSTRIP256 (the default for the SOSS observing mode) which covers all spectroscopic orders and provides more pixels with which to estimate the background compared with SUBSTRIP96.

We chose the NISRAPID readout pattern (where there is 1 frame per group) which is the only permitted readout pattern when using a subarray in the SOSS mode.

Step 4 - Calculate required exposure configuration using the JWST Exposure Time Calculator (ETC)

See also: JWST Exposure Time Calculator OverviewJWST Time-Series Observations TSO Saturation

To determine the exposure parameters for this observation using the JWST Exposure Time Calculator (ETC), please see the article Step-by-Step ETC Guide for NIRISS SOSS Time-Series Observations of WASP-39. Note the ETC is useful for a conservative, average SNR estimate.

Step 5 - Use PandExo for more detailed modeling of spectroscopic exoplanet transits

Interested users are encouraged to use PandExo (Batalha et al. 2017) for detailed modeling of exoplanet transits and to optimize exposure configurations for their observing program.

Step 6 - Determine whether target acquisition is required and use the ETC to determine the appropriate strategy

See also: NIRISS Target Acquisition

target acquisition is required for all SOSS observations observed with a subarray (and strongly encouraged in full frame readout mode) to accurately position the source on the detector. The step-by-step ETC guide for this example science program details how to determine exposure parameters for a successful target acquisition for this program.

Step 7 - Complete the Astronomer's Proposal Tool (APT) template

For details filling out the Astronomer's Proposal Tool (APT) for this example science program, please see the article Step-by-Step APT Guide for NIRISS SOSS Time-Series Observations of WASP-39.


Batalha, N. E., Mandell, A., Pontoppidan, K., et al. 2017, PASP, 129, 064501 (ADS)
PandExo: A Community Tool for Transiting Exoplanet Science with JWST & HST

PandExo homepage

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    Changed target to WASP-39b
Originally published