A JWST NIRSpec bright object time-series (BOTS) exoplanet transit observation of WASP-62b is described in this article. It includes Exposure Time Calculation and Astronomer's Proposal Tool considerations.

Example Science Program #9

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The NIRSpec BOTS Prism multistripe mode is designed for observations of bright sources that require high throughput and stable time-resolved spectroscopy over the broad wavelength coverage provided by the Prism (0.6–5.3 μm) disperser, but are brighter than the nominal Prism bright limit of J ~ 11. Instead of reading a single subarray region, the Prism multistripe subarrays split the SUB512 subarray into multiple regions (or "stripes") that are read out individually, with the stripe moving to a different location in the subarray after the integration for the previous stripe is completed. This method allows for the frame time to be dramatically reduced, and bright limit extended to brighter sources, at the expense of reduced exposure efficiency due only to a portion of the subarray being read out at a given time. BOTS multistripe subarrays can only be used with the PRISM/CLEAR disperser-filter combination.

Prism multistripe 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.

Science motivation

WASP-62 b is a hot Jupiter orbiting on a 4.41-day orbit around a F7V type star, which has a planetary equilibrium temperature of about ~1,400 K. Previous transit spectroscopy of WASP-62 b with the Hubble Space Telescope (HST) has revealed the full-amplitude Na absorption feature at ~0.6 μm, which suggest that this planet has a cloud-free, haze-free atmosphere and would therefore be a very interesting target for JWST studies. Observations at JWST sensitivities and wavelengths would allow for precise constraints on the abundances of several carbon- and oxygen-bearing species of this exoplanet. Such information could help to better understand the formation, migration, and evolutionary histories of giant planets as well as the processes underpinning the formation of clouds and hazes in exoplanet atmospheres.

Observations with NIRSpec BOTS, which cover the 1–5 μm range, are particularly interesting for targeting H₂O, CO, and CO₂features present in exoplanet atmospheres. The wavelengths of interest can be accessed using a single setting (R = 100) prism mode, or in multiple settings with the medium or high resolution gratings. Because WASP-62 has a J-mag (Vega) of 9.3, and the NIRSpec Prism begins to saturate at a J-mag (Vega) of 11.1 (for T_eff = 2,500 K), users should choose a multistripe subarray to observe the 1–5 μm range within a single transit. This example science program will provide a step-by-step guide for designing a BOTS Prism multistripe observation for this target. The aim is to obtain transit spectra in the 1–5 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

Star WASP-62

Spectral type = F7
T_eff = 6,230 K
Metallicity (Fe/H) = 0.04 solar
K = 8.9 (Vega)

Planet WASP-62b

Mass = 0.52 M_J
Radius = 1.32 R_J
Equilibrium temperature (T_eq) = 1,394 K
Period = 4.41195 d
Primary transit = 2458850.59934 JD
T₁₄ (total transit duration) = 3.82 hr

Step 1 - Determine required wavelength coverage

The spectroscopic transitions that this program aims to detect are found between 1–5 μm, requiring the broad wavelength coverage provided by NIRSpec Prism.

Step 2 - Select an instrument observing mode

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

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

To acquire spectra in the 1–5 μm range with a single transit of this bright target (J = 9.3), the only available choice is the BOTS prism multistripe mode, which uses the PRISM/CLEAR grating/filter combination, providing a resolution of R ~ 100 between 0.6 to 5.3 µm.

Step 3 - Determine multistripe subarray configuration

All multistripe observations must use the NRSRAPID (1 frame per group) readout pattern.

Four Prism multistripe subarrays are available for NIRSpec BOTS mode, each with their own readout time. The user should refer to the above referenced 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 (ETC) can help make the appropriate choice (see below). For this observation, select the SUB64M8_PRISM subarray, which has a single frame read time of 0.029 s and which can only be used with PRISM/CLEAR. 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)

The Step-by-Step ETC Guide for NIRSpec BOTS Observations of WASP-62b article walks the user through navigating the JWST ETC to determine exposure parameters appropriate for the science goals for this program. Users should use the JWST ETC for estimation of the signal-to-noise ratio in a single integration.

Step 5 - Determine the appropriate target acquisition strategy in ETC

The NIRSpec BOTS mode requires a target acquisition (TA) to place the target accurately in the center of the aperture. The TA method employed for BOTS 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". For the case of stellar source WASP-62, WATA is to be performed on an offset reference source, since the science source is too bright to perform self-WATA.

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-62b article shows how to perform this calculation, how settings were chosen, and the outcome of the calculation.

Step 6 - 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-62b provides instructions for filling out the APT observation templates. The exposure parameters determined by the ETC are specified in the APT observation template.

References

Alam, Munazza K., Lopez-Morales, Mercedes, MacDonald, Ryan J., et al. 2021, ApJL, 906, 2
Evidence of a Clear Atmosphere for WASP-62b: The Only Known Transiting Gas Giant in the JWST Continuous Viewing Zone

ExoMAST - entry for WASP-62 b

Notable updates
Originally published	20 Jul 2026

JWST User Documentation

NIRSpec BOTS Prism Multistripe Observations of WASP-62b