NIRISS SOSS Time-Series Observations of HAT-P-1
This goal of this example program is to observe the exoplanet transit of HAT-P-1b, and is based on the GTO program "NIRISS Exploration of the Atmospheric diversity of Transiting exoplanets (NEAT)". NEAT is designed to study exoplanet atmospheric composition, energy budget, and dynamics.
Steps for creating observations
Step 1 - Determine the required wavelength coverage: near-infrared or mid-infrared
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 2 visits to achieve the same wavelength coverage. 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
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)
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 HAT-P-1. Note the ETC is useful for a conservative, average SNR estimate.
Step 5 - Use PandExo for more detailed modeling of spectroscopic exoplanet transits
Step 6 - Determine whether target acquisition is required and use the ETC to determine the appropriate strategy
Main articles: NIRISS Target Acquisition
A 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 HAT-P-1.
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