Step-by-Step ETC Guide for NIRISS SOSS Time-Series Observations of HAT-P-1

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.

On this page

Main article: NIRISS Single Object Slitless Spectroscopy, JWST Exposure Time Calculator Overview
See also: 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 HAT-P-1" 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 HAT-P-1 and can be selected from the Example Science Program Workbooks dropdown tab on the ETC Workbooks page. The nomenclature and reported SNR values in this article are based on ETC v. 1.5. There may be subtle differences if using a different version of ETC.

Define Source and Scene in the ETC

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

Define source for "HAT-P-1" Scene

We first defined a source in ETC that emulates the HAT-P-1 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 "HAT-P-1"

  • "Continuum" tab - we selected the "Phoenix Stellar Models" in the Continuum pull-down menu, and chose a star with spectral type G0V.

  •  "Renorm" tab - we  chose the "normalize in bandpass" option, renormalizing the source to a Vega magnitude of K = 8.858 in the Johnson 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 "HAT-P-1" 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" to  "HAT-P-1". 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

Main article: JWST ETC Creating a New Calculation
See also: JWST Time-Series ObservationsTSO Noise Sources

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 are important. We entered the coordinates for HAT-P-1 (22:57:46.84 +38:40:30.33) in 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.

Select instrument parameters

Main ArticleTSO SaturationNIRISS SOSS Recommended Strategies
See Also: NIRISS Detector SubarraysNIRISS Detector Readout PatternsUnderstanding Exposure Times
We 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.
  • "Detector Setup" tab -
    • subarray is set to SUBSTRIP256;
    • we chose the NISRAPID readout pattern.
    • number of "Groups per integration" was set to 2, the minimum value permitted by the ETC, and the number of "Integrations per exposure"  and number of "Exposures per specification" were kept at the default values of 1.
  • "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 4. The reported SNR per pixel is ~294. 

Adjust Exposure Parameters to Obtain Desired Signal-to-Noise Ratio

Main Articles: JWST ETC Batch ExpansionsJWST ETC Reports

As noted above, the Reports pane tells us that the onset of saturation thus occurs at NGroups sat = 5. Our optimal number of groups per integration ramp (NGroups sat/2, rounding up) is 3. We create a calculation with this number of groups, and find that the SNR per pixel of ~424 (Calculation #2). From inputting 3 Groups into the Astronomer's Proposal Tool, with the NISRAPID Readout Pattern and SUBSTRIP256 Subarray, we find that 1 integration corresponds to 21.996 s.

Determining Number of Integrations

We plan to observe HAT-P-1 for enough time to allow for sufficient detector settling time (currently estimated to be ~30 minutes; see TSO 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 (eclipse) + MAX(1 hr, T14/2)(after transit) + 1 hr (timing window), where T14 is the transit duration.For HAT-P-1, T14 is 2.784 hours, giving us a total exposure time of 7.318 hours. Since each integration ramp for our set-up is 21.996 s, we thus need 1,198 integrations to cover Tdwell.

Interpreting SNR results

Main article: 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 of <50 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 HAT-P-1, 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 4.534 hours and the time spent in-transit is 2.784 hours. For our example science program and this observational setup (NGroups = 3, NISRAPID readout, and SUBSTRIP256 subarray), these times correspond to  Nout = 742 integrations and Nin = 456 integrations. From the ETC, we found SNRpoint = 424 , such that Precisiondepth is ~ 0.000140 = 140 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 44 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

Main articles: 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 6.1 < M < 14.5, Vega mag) or SOSSBRIGHT (for objects between 3 < M < 6.1, Vega mag).

Calculation #3, 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 HAT-P-1 (22:57:46.84 +38:40:30.33) and 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 left the number of groups to the starting value of 3 and the "Readout Pattern" to NISRAPID;
  • "Strategy" - the only permissible option for target acquisition is "Aperture centered on source", which for our case is the science target HAT-P-1.

We see that with this set up, we achieve a SNR ~118, 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
  •  Update the TA SNR to be consistent with values from ETC v .1.2.2
  •  Fixed error in the number of integrations calculation and associated transit precision measurement.