Step-by-step ETC calculation instructions for the JWST NIRSpec BOTS mode observation of WASP-62b are presented and discussed in this article.

See also: NIRSpec Bright Object Time-Series Spectroscopy, JWST Exposure Time Calculator Overview, Proposal Planning Video Tutorials

This article focuses on selecting exposure parameters to detect the exoplanet transit at the desired SNR. An accompanying ETC workbook on which this tutorial is based can be downloaded as a sample workbook from the ETC user interface.

The optimal exposure specifications (e.g., numbers of groups and integrations) are the inputs needed for the Astronomer's Proposal Tool (APT) observation template, which is used to specify an observing program and submit proposals.

Define Source and Scene in the ETC

See also: JWST ETC Scenes and Sources Page Overview, JWST ETC Defining a New Source, JWST ETC Source Spectral Energy Distribution

In the Scene and Sources tab, you can edit the sources within your scene. In the Scene box, there are tabs for setting the source continuum, shape, and flux normalization. Table 1 shows the specifications for this case. The scene was renamed "WASP-62b scene".

Table 1. Source specifications for WASP-62 in the JWST ETC

Select NIRSpec BOTS calculation

See also: JWST ETC Creating a New Calculation, NIRSpec Bright Object Time-Series Spectroscopy, NIRSpec BOTS Operations, NIRSpec Detector Recommended Strategies, JWST ETC Backgrounds

In the ETC, calculations for the NIRSpec BOTS mode are selected via the option Bright Object Time Series from the NIRSpec drop-down menu. 

In the Background tab, enter the target coordinates that were fetched from the Gaia DR3 archive (RA = 05 48 33.5933, Dec = -63 59 18.3884) and select the Low background setting, which corresponds to the 10th percentile of the sky background. This option can be toggled to examine the influence of the background on the observations.

Select instrument parameters

See also: NIRSpec Dispersers and Filters, NIRSpec Fixed Slits, NIRSpec BOTS Wavelength Ranges and Gaps

In the Instrument Setup panel, enter the required filter/grating settings and the aperture in the NIRSpec focal plane. For BOTS mode, when using the multistripe arrays, all observations use the S1600A1 aperture, a 1.6" square-shaped aperture and the Prism/CLEAR grating/filter pair. This provides coverage from ~0.60–5.3 µm with a resolving power of ~100.  The plot in the Instrument Setup panel shows the system throughput over the selected band; this plot is useful for verifying that the wavelength coverage is as expected.

Adjust exposure parameters

See also: NIRSpec Detector Recommended Strategies, JWST Time-Series Observations TSO Saturation, NIRSpec Detector Readout Modes for Full Frame Observations, NIRSpec Detector Subarray Mode

In the Detector Setup panel, select the appropriate multistripe subarray, the readout pattern (which must be NRSRAPID for multistripe observations), number of groups, integrations and exposures. Note that time-series observations (TSOs) are recommended to be executed in a single exposure for optimal stability and efficiency. As the number of integrations is determined by the length of the transit or eclipse event that you want to observe (with each integration treated as an individual observation in the time series), perform the calculation for Integrations per Exposure = 1.

The requested subarray for this BOTS observation is SUB64M8_PRISM, which is selected from the Subarray drop-down menu. If the target is so bright that it will saturate in <3 groups, a smaller subarray should be selected (here, the SUB32M16_PRISM); for WASP-62b this is not the case. As the target is bright, choose the fastest readout pattern and do not average frames into groups; select NRSRAPID.

In the Strategy panel, select the wavelength of interest to be 2.95 µm; both the SNR and Reports pane numbers will be returned for this value. Also in Strategy, set the Aperture Full-Height, over which the flux will be co-added, to 0.30 arcsec. 

As the main concern is to avoid saturation, you can run an initial calculation (Calculation #1) with NGroups = 10 (integration time of 2.72 s). From the Reports pane, you will see that the maximum number of groups before saturation occurs at NGroups = 3. Rerunning the calculation, now using NGroups = 3 (integration time of 1.09 s), produces SNR at 2.95 µm of ~64 per integration. Note that reporting of saturation in the ETC assumes 80% of full well—it includes some safety margin.

Determining number of integrations

You are planning to observe WASP-62b for enough time to allow for sufficient detector settling time (currently estimated to be ~30 minutes; see JWST Time-Series Observations Noise Sources). You will also allow for some margin if the observations do not start exactly when expected or if the transit occurs at a slightly different time than predicted. Use the dwell time (T_dwell) to calculate this exposure time: T_dwell ~ 0.75 hr + MAX(1 hr, T₁₄/2)(before transit) + T₁₄(transit) + MAX(1 hr, T₁₄/2)(after transit) + 1 hr (timing window), where T₁₄ is the transit duration.

For WASP-62b, T₁₄ is 3.816 hours (228.96 minutes), giving a total exposure time of ~9.38 hours.

From entering the SUB64M8_PRISM Subarray and 3 Groups/Int with a Readout Pattern of NRSRAPID in the ETC, you will see that the exposure time for 1 Integrations/Exp is 1.09 s with these exposure specifications. 

Target acquisition

See also: JWST ETC NIRSpec Target Acquisition, NIRSpec Target Acquisition, NIRSpec Wide Aperture Target Acquisition, JWST Pointing Performance

Ideally, for time-series observations, the target should be accurately placed into the aperture or subarray, particularly when the transit event will be observed over multiple epochs. This allows for the best control of systematic errors in the pixels covered by the science target. For NIRSpec BOTS observations, the default target acquisition (TA) mode is the wide aperture target acquisition, or WATA. The user has a choice of filters and subarrays, but the TA image will always consist of a single integration using 3 groups. As the target WASP-62b is bright, select the narrowest available filter (F110W) with the smallest available subarray (SUB32).

In the ETC, these details can be entered in a new calculation from the NIRSpec menu, labelled Target Acquisition. Select the same source, scene and background level as the BOTS calculation, and enter the above selections in the relevant fields under Instrument Setup and Detector Setup. Running the calculation returns a warning regarding saturation: WASP-62b is in fact too bright for a target acquisition with WATA. Therefore, use a nearby source for the target acquisition.

From querying the Gaia DR3 catalog, you'll find a source with a J-band magnitude of 15.69 (Vega): Gaia DR3 4756652365943137920. Fetching the coordinates of this source from the Gaia DR3 catalog (RA = 05 48 28.889, Dec = -63 58 57.216), you will find that it is ~24 arcsec away from WASP-62b. This distance is less than the visit splitting distance in APT, making the position for the source acceptable for a target acquisition.

Create a source and scene for this offset source ("Gaia DR3 4756652365943137920" and "WASP62-TA scene", respectively). Using the atmospheric parameters for this star found in Gaia DR3 (Teff = 4,790 K, log g = 4.157), select the SED to be the Phoenix stellar model for spectral type K2 V (Teff = 4,750 K, log g = 4.5), and use the Gaia DR3 catalog magnitude J = 15.69 mag (Vega). You will find that with the CLEAR filter, the SUB32 subarray and the NRSRAPID readout pattern, you obtain a SNR > 45 on the TA (Calculation #2), ensuring the target acquisition will succeed (a SNR > 20 is recommended for target acquisitions).

Links

2MASS All-Sky Release Database

Gaia DR3catalog