NIRSpec BOTS Observations of WASP-79b

Example Science Program #32

This example science program describes a JWST NIRSpec Bright Object Time Series exoplanet transit observation of WASP-79 b, including Exposure Time Calculation and  Astronomer's Proposal Tool considerations.

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See also: Step-by-Step ETC Guide for NIRSpec BOTS Observations of WASP-79bStep-by-Step APT Guide for NIRSpec BOTS Observations of WASP-79b

The JWST NIRSpec bright object time-series (BOTS) mode is for observations of bright sources that require high throughput and stable time-resolved spectroscopy. This mode is optimized for the study of transiting exoplanets around their bright host stars; such observations are expected to be the primary use of the BOTS mode. BOTS mode can only be used with NIRSpec's S1600A1 aperture. This 1.6" square aperture specifically enables high precision time-series spectroscopy of bright objects. 

Science motivation

WASP-79 b is a hot Jupiter orbiting on a 3.66-day orbit around a F3V type star, which has a planetary equilibrium temperature of about ~1800 K. Previous transit spectroscopy of WASP-79 b with the Hubble Space Telescope (HST) has revealed a large water feature in the 1.1-1.6 micron range, which has made this exoplanet a very interesting target for JWST studies — indeed, this exoplanet is one of the prime targets to be observed by the Director's Discretionary Early Release Science (DD-ERS) Program "The Transiting Exoplanet Community Early Release Science Program". Observations at JWST sensitivities will not only allow for a precise constraint on the water abundance of this exoplanet several factors better than previous studies, but its increased wavelength range will also allow for the detection of molecules that have remained so far undetected on this exoplanet by our current instrumentation, such as CO and CO2. Observations with NIRSpec BOTS, which cover the 1–5 μm range, are particularly interesting for targeting H2O, CO and COfeatures present in exoplanet atmospheres, which can be targeted using a single setting (R=100) prism mode, or in multiple settings with the medium or high resolution gratings. Because WASP-79 has a J-mag (Vega) of 9.3—and the NIRSpec-Prism begins to saturate at a J-mag (Vega) of 10.2 (for Teff = 2,500 K), we opt to choose a higher-resolution grating for these observations. In this particular science program, we will reproduce one of the aims of the DD-ERS Program above, which is to obtain transit spectra in the 3-5 μm range over which features of H2O (from 3-4 μm), CO and CO2 (from 4-5 μm) should be detectable according to predictions made from the HST observations

Relevant star and planet parameters 

Sources: ExoMAST


  • Spectral type = F3
  • Teff = 6600 K
  • Metallicity (Fe/H) = 0.03 solar
  • K = 9.1 (Vega)

Planet WASP-79b

  • Mass = 0.85 MJ
  • Radius = 1.67 RJ
  • Equilibrium temperature (Teq) = 1767 K
  • Period = 3.66238 d
  • Primary transit = 2456215.4556  JD 
  • T14 (total transit duration) = 60.91 min1

1: We note that this transit duration has been taken from Brown et al. (2017), which is inconsistent with the literature value of 3.986 hours (e.g., Smalley et al., 2012) for this exoplanet. In this example program, we nonetheless use the 60.91 min figure.   

Step 1 - Determine required wavelength coverage

Main article: TSO Roadmap

The spectroscopic transitions we aim to detect emit between 3 - 5 μm, requiring near-infrared coverage.

Step 2 - Select an instrument observing mode

Main article: NIRSpec Bright Object Time-Series Spectroscopy
See also: NIRSpec Dispersers and Filters

To acquire spectra in the 3 - 5 μm range with one instrument, the only available choice is NIRSpec Bright Object Time-Series Spectroscopy (BOTS). In particular, the G395H/F290LP grating/filter combination provides a resolution of ~2700 between 2.87 to 5.14 µm. 

Step 3 - Determine readout pattern and subarray configuration

Main articles: NIRSpec Detector SubarraysNIRSpec Detector Readout Modes and Patterns

Two readout patterns are available for this mode: NRSRAPID (1 frame per group) and NRS (4 frames averaged into 1 group). In this case, we select the NRSRAPID mode due to the brightness of the target.

A number of subarrays are available for NIRSpec BOTS mode, each with their own readout time. The user should refer to the above reference pages to learn about the size and restrictions of each subarray. Saturation should be avoided in the observation, so the subarray should be chosen to avoid saturation (the JWST Exposure Time Calculator can help make the appropriate choice). For this observation, we select the SUB2048 subarray, which has a single-frame read time of 0.902 s and which can be used with any of NIRSpec's disperser elements. The BOTS mode always uses the S1600A1 square aperture, which measures 1.6" on the side. The spectra are therefore always placed in the same detector region. 

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

Main article: JWST Exposure Time Calculator OverviewTSO Saturation

The Step-by-Step ETC Guide for NIRSpec BOTS Observations of WASP-79b walks the user through navigating the JWST Exposure Time Calculator (ETC) to determine exposure parameters appropriate for the science goals for this program. Users are recommended to use the JWST ETC for initial estimation of the signal-to-noise ratio in a single integration. 

Step 5 - Calculate optimized exposure configuration using PandExo

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 the appropriate Target Acquisition strategy in ETC

Main articles: NIRSpec Target AcquisitionNIRSpec Wide Aperture Target AcquisitionNIRSpec Target Acquisition Recommended StrategiesJWST ETC NIRSpec Target Acquisition

See also: JWST Pointing Performance

The NIRSpec BOTS mode uses Target Acquisition to place the target accurately in the center of the aperture. The TA method employed is Wide Aperture Target Acquisition, or WATA. The TA procedure is fully performed in the S1600A1 aperture, as the telescope blind pointing accuracy is < 1.6".

The number of groups in the TA exposure is fixed at 3; the exposure time is determined by the detector readout mode and the chosen subarray. Several filters are available. 

The Step-by-Step ETC Guide for NIRSpec BOTS Observations of WASP-79b article shows how to perform this calculation, how setting were chosen, and the outcome of the calculation.

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

The Astronomer Proposal Tool (APT) is used to submit JWST proposals. The Step-by-Step APT Guide for NIRSpec BOTS Observations of WASP-79b provides instructions for filling out the APT observation templates. The exposure parameters determined by the ETC are specified in the APT observation template. 

Related Links

PandExo homepage


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

Bean Jacob L., Stevenson, Kevin B., Batalha, Natalie M., et al. 2018, PASP, 130
The Transiting Exoplanet Community Early Release Science Program for JWST

Brown, D. J. A., Triaud, A. H. M. J., Doyle, A. P., et al., 2017, MNRAS, 464
Rossiter-McLaughlin models and their effect on estimates of stellar rotation, illustrated using six WASP systems

ExoMAST - entry for WASP-79 b

Smalley, B., Anderson, D. R., Collier-Cameron, A., et al., 2012, A&A, 547
WASP-78b and WASP-79b: two highly-bloated hot Jupiter-mass exoplanets orbiting F-type stars in Eridanus


Latest updates
  •  Added warning about WASP-79b's transit duration.