Step-by-Step ETC Guide for NIRISS SOSS Time-Series Observations of WASP-39

A walk through of the JWST ETC for the NIRISS SOSS Example Science Program is provided, demonstrating how to select exposure parameters for this observing program. 

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See also:: NIRISS Single Object Slitless Spectroscopy, JWST Exposure Time Calculator Overview, Proposal Planning Video Tutorials

The JWST Exposure Time Calculator performs signal-to-noise (SNR) calculations for the JWST observing modes. Sources of interest are defined by the user and assigned to scenes which are used by the ETC to run calculations for the requested observing mode.

For the "NIRISS SOSS Time-Series Observations of WASP-39" example science program, we focus on selecting exposure parameters to detect the exoplanet transit at the desired signal-to-noise ratio (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 input needed for the Astronomer's Proposal Tool (APT) observation template, which is used to specify an observing program and submit proposals.

The ETC workbook associated with this example science program is called "#31: NIRISS SOSS Time-Series Observations of WASP-39b and can be selected from the Example Science Program Workbooks dropdown tab on the ETC Workbooks page (after 24 Aug. '23). The nomenclature and reported SNR values in this article are based on ETC v. 3.0 (release date, 24 Aug. '23). There may be subtle differences if using a different version of ETC.

Define Source and Scene in the ETC

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

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

Define source for "WASP-39b" Scene

We first defined a source in ETC that emulates the WASP-39 system. After selecting the source, we opened the Sources and Scenes tab and then updated the default source parameters in the Source Editor pane as follows:

  • ID tab - we updated Source Identity Information to WASP-39b

  • Continuum tab - we selected the Phoenix Stellar Models in the Continuum pull-down menu, and chose a star with spectral type G8V 5500k log(g)=4.5.

  • Renorm tab - we  chose the Normalize in bandpass option, renormalizing the source to a Vega magnitude (vegamag) of H = 10.307 in the 2MASS filter.

  • Lines tab - we left this tab empty since we do not add any emission or absorption lines to the spectrum.

  • Shape tab - we kept the default option of point source .

  • Offset tab - we left this tab empty so the source will be at the center of the scene.

Assign source to "WASP-39b" Scene

We highlighted Scene 1 in the Select a Scene pane and then renamed the Scene Identity Information entry in the ID tab of the Source Editor pane to WASP-39b. Since we updated the default source which was assigned to the default scene, we did not need to define a new ETC scene.

Select NIRISS SOSS Calculation

See also: JWST ETC Creating a New Calculation, JWST Time-Series ObservationsJWST Time-Series Observations Noise Sources, NIRISS Time Series Observations Pipeline Caveats

After selecting SOSS from the NIRISS pull-down menu in the Calculation tab (Calculation #1), we specified the background parameters. Since the JWST Background is position-dependent, fully specifying background parameters is important. We entered the coordinates for WASP-39 (14:29:18.40 -03:26:40.20in the Background tab, and selected Medium for Background configuration, which corresponds to the 50th percentile of the sky background. Note that the background for a specific date can be entered, so based on the exoplanet's transit period and when these windows are visible by JWST, these dates can be entered into ETC to assess the effects the background would have on the transit precision depth. It is important to also note that the ETC currently does not reproduce the exact background shape seen in the SOSS mode, which has an abrupt flux increase near column 700 in the SUBSTRIP256 and SUBSTRIP96 subarrays, impacting wavelengths smaller than about 2 microns much more strongly.

Select instrument parameters

See also: JWST Time-Series Observations TSO SaturationNIRISS SOSS Recommended StrategiesNIRISS Detector SubarraysNIRISS Detector Readout PatternsUnderstanding Exposure TimesWe specify JWST exposures by number of groups and number of integrations. We want to observe a balanced number of groups per integration to maximize both temporal resolution and spectral precision. Previous experience has led the community to sample up the ramp until we reach half the saturation limit. In the context of number of groups for JWST, we will derive the number of groups corresponding to the onset of saturation (NGroups sat) from the Exposure Time Calculator, and choose the number of groups per integration to be NGroups sat/2 (rounding up). We will then choose the number of integrations that fully covers the full transit window.

Calculation #1 represents our initial calculation to determine NGroups sat, where we set the following parameters:

  • Instrument Setup tab - there is only one option in the pull-down menu in the Instrument Setup tab, which is the GR700XD (cross-dispersed) grism. We choose the CLEAR filter.
  • Detector Setup tab -
  • Strategy tab - we kept the Order for spectral extraction at its default value of 1 and left the Wavelength of Interest to its default value of 1.575 μm.

After selecting the Calculate button to perform the calculation with these parameters, we see that the observation does not suffer from saturation, i.e., there is a green checkmark next to Calculation 1 in the Calculations tab and no warnings or errors are reported in the Reports pane. This pane also reports that the number of groups prior to saturation (NGroups sat - 1) as 15. The reported SNR per pixel is ~413. 

Adjust exposure parameters to obtain desired signal-to-noise ratio

See also: JWST ETC Batch ExpansionsJWST ETC Reports

As noted above, the Reports pane tells us that the onset of saturation thus occurs at NGroups sat = 16. Our optimal number of groups per integration ramp (NGroups sat/2) is 8. Our default calculation (Calculation #1) has this number of groups, where as reported above, the SNR per pixel is ~413. From inputting 8 Groups into the Astronomer's Proposal Tool, with the NISRAPID Readout Pattern and SUBSTRIP256 Subarray, we find that 1 integration corresponds to 49.466 s.

Determining number of integrations

We plan to observe WASP-39b for enough time to allow for sufficient detector settling time (currently estimated to be ~30 minutes; see JWST Time-Series Observations Noise Sources). We also allow for some margin if the observations do not start exactly when expected or if the eclipse occurs at a slightly different time than predicted.We use the dwell time (Tdwell) to calculate this exposure time: Tdwell ~ 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-39b, T14 is 2.803 hours, giving us a total exposure time of 7.356 hours. Since each integration ramp for our set-up is 49.466 s, we thus need 535 Integrations/Exp to cover Tdwell.

Interpreting SNR results

See also: JWST ETC Residual Flat Field Errors 

The scientific measurement for an exoplanet transit is the "transit depth", which is a temporal measurement.  The spectroscopic result is, therefore, a relative comparison between a contiguous sequence of time-series measurements, i.e., transit depth over wavelength. It is equivalent to measuring variations in the stellar spectrum over time.

Our goal in this example is to achieve a relative precision better than ~100 parts per million (ppm) on the transit depth per "spectral bin" or "channel," after subtracting the primary transit from the out-of-transit data. Atmospheric models predict that this should provide a useful signal-to-noise on the exoplanet atmospheric spectroscopic signal (~100–250 ppm). A "spectral bin" or "channel" is a set of pixels across the spectrum that we will combine ("bin") to maximize the temporal precision per spectroscopic channel, without losing the spectral features in which we are interested in measuring. For the case of WASP-39b, in particular, a spectral bin of 10 pixels (which decreases the NIRISS SOSS resolution from ~700 to ~70), would be enough to resolve the predicted spectral features in the atmosphere of this planet. 

If we assume a box-shaped transit, the transit depth precision (Precisiondepth) can be approximated by:

Precision_{depth} \sim Precision_{point} \times \sqrt{1/N_{out} + 1/N_{in}} = \sqrt{1/N_{out} + 1/N_{in}}/SNR_{point}

where Precisionpoint is the photometric precision per data point, Nout is the number of datapoints (integrations) out-of-transit, Nin is the number of datapoints (integrations) in-transit, and SNRpoint is the SNR in one integration.

The time spent out-of-transit is 4.553 hours and the time spent in-transit is 2.803 hours. For our example science program and this observational setup (NGroups = 8, NISRAPID readout, and SUBSTRIP256 subarray), these times correspond to  Nout = 331 integrations and Nin = 204 integrations. From the ETC, we found SNRpoint = 413 , such that Precisiondepth is ~ 0.000215 = 215 ppm, prior to binning. When binning by a factor of 10, our Precisiondepth lowers by a factor of √10, such that we achieve a precision of 68 ppm.

Note that the ETC includes an error term for residual flat field errors which affects long exposures. For exposures longer than ~10,000 s, ETC calculations have a "noise floor" above which an increase in exposure time no longer results in an increase in SNR that scales with the square root of the exposure time. Since we are making relative measurements on the same pixels for exoplanet transit spectroscopy, our precision is not affected by the "noise floor" imposed by the residual flat field errors. 

Note also that the ETC does not fully account for systematic errors that affect the real spectrophotometric precision.

Target acquisition

See also: NIRISS Target AcquisitionJWST ETC NIRISS Target Acquisition

target acquisition (TA) must be performed when using a SOSS subarray so that the target is precisely positioned on the detector. A signal-to-noise ratio (SNR) ≥ 30 is recommended to achieve a successful TA, which achieves a centroid accuracy of ≤ 0.15 pixel. Increasing the SNR to 100 improves the centroiding accuracy up to ≤ 0.05 pixel. If the TA fails, the observation will not be performed. Only one integration and one exposure is allowed for a TA. The acquisition mode is either SOSSFAINT (for objects between 6.0 ≤ M ≤ 14.4, Vega mag) or SOSSBRIGHT (for objects between 2.9 ≤ M ≤ 6.0, Vega mag).

Calculation #2, where we selected Target Acquisition under the NIRISS pull-down menu, shows our initial calculation to determine which parameters to specify for TA:

  • Backgrounds tab - we entered the coordinates for WASP-39 (14:29:18.40 -03:26:40.20and selected Medium for Background configuration;
  • Instrument Setup tab - we kept the default selection of SOSS or AMI Faint for Acq Mode since the target is fainter than M > 6.1 (Vega);
  • Detector Setup tab - there are no options for the Subarray, Integrations and Exposures parameters other than the default values. We maximized the number of Groups to a value of 19 and the Readout Pattern to NISRAPID;
  • Strategy tab- the only permissible option for Target Acquisition is Aperture centered on source, which for our case is the science target WASP-39.

We see that with this setup, there is no saturation in the TA and we achieve a SNR of ~200, which ensures the TA will succeed.

See the Step-by-Step APT guide to complete the proposal preparation for this example science program, where we will input the exposure parameters we derived here.

Latest updates
    Changed target to WASP-39b. Calculations made with ETC v.3.0 (ETC 3.0 release: 24 Aug. '23)

    Fixed error in the number of integrations calculation and associated transit precision measurement.

    Update the TA SNR to be consistent with values from ETC v .1.2.2.
Originally published