Step-by-Step ETC Guide for NIRCam WFSS Deep Galaxy Observations

A walk-through of the JWST ETC for the NIRCam WFSS Deep Galaxy Observations Example Science Program is provided, demonstrating how to select exposure parameters for this observing program. 

Dated material

This example was created pre-launch, and the ETC has been updated since its creation. You may see differences in the details of the results from the ETC, the information provided, or the appearance of the ETC GUI from what is shown herein.

Please refer to JWST Example Science Programs for more information.

On this page

See also: NIRCam Wide Field Slitless SpectroscopyJWST ETC Exposure Time Calculator OverviewProposal 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.

The ETC workbook associated with this example science program is called #37: NIRCam WFSS Deep Galaxy Observations 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.

After determining the exposure specifications required to meet the science goals (e.g., numbers of groups and integrations), these will be input into the Astronomer's Proposal Tool (APT) observation template, which is used to specify an observing program and submit proposals; for this example program, see the Step-by-Step APT Guide for NIRCam WFSS Deep Galaxy Observations.

Define Sources and Scenes

See also: JWST ETC Scenes and Sources Page OverviewJWST ETC Defining a New Source

We define 2 scenes, each containing one source:

  1. Emission line galaxy (ELG) used for the wide field slitless spectroscopy (WFSS) grism calculation
  2. Galaxy (without emission lines) for the imaging calculations. This extended source will have a width of σ = 0.15", the expected size of galaxies at z ~ 3 (e.g., Ferguson et al. 2004).

1ELGFlat in fnumAB = 26.5 in F356Wcenter = 3.6 µm
width = 200 km/s
strength = 1e-17 erg/cm²/s
point source
2GalaxyFlat in fnumAB = 26.5 in F356W

extended source
2D Gaussian
σx = 0.15"; σy = 0.15"

ETC Calculations overview

See also: JWST ETC Calculations Page OverviewJWST ETC Creating a New Calculation, JWST ETC BackgroundsJWST ETC Outputs Overview

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

Below is a summary of the 4 calculations included in the accompanying ETC workbook. The following sections provide the rationale for each calculation.

For all calculations, we use the following settings for Backgrounds:

  • Ra Dec: 01:00:13.02 28:02:25 (the position of one target in the GTO program)
  • Background configuration: Low (corresponding to the 10th percentile of the background level)


Run ETC calculation for WFSS

See also:
JWST ETC Aperture Spectral Extraction StrategyNIRCam WFSS Recommended Strategies,
NIRCam WFSS SensitivityNIRCam Wide Field Slitless Spectroscopy, JWST ETC Batch Expansion

Recall that the total exposure time (for each mosaic tile) will consist of 24 exposures: 12 dither positions for each of 2 grism observations at long wavelengths, one paired with short wavelength F115W imaging, and the other with F200W. Our initial estimates suggest that roughly 2 hours (~7,200 s) of exposure time will be required, or ~300 s for each of the 24 exposures. Based on the NIRCam imaging sensitivity recommended readout patterns for maximal depth, we find this corresponds roughly to 6 groups of the SHALLOW4 Readout pattern in each exposure. Thus we use that as a baseline for our calculations:

  • Instrument setup:
    • Grism = GRISMR
    • Filter = F356W

  • Detector Setup:
    • Groups per integration = 6
    • Readout patternSHALLOW4
    • Exposure per specification = 24 (to mimic the 12 dither positions × 2 sets of grism observations)

  • StrategyWavelength of interest: 3.6 µm

We require a total exposure time that yields SNR ~ 10. We examine the SNR as a function of exposure time by performing calculations for the range of 3–10 groups of the SHALLOW4 Readout pattern. This is done using the Expand tab at the top of the screen.

  • Expand Groups: Start Value = 3, Step Size = 1, Iterations = 8

The resulting SNR for each exposure time is reported in the Calculations tab. One can also generate a plot of the SNR vs. exposure time by selecting all the entries and viewing the SNR tab in the Plots pane. The results are shown in Figure 1 below.

We find ~8,000 s of exposure time is required to yield a 10-sigma detection of the ELG defined above. In practice, the GTO program opts for 6 groups of SHALLOW4 in each of the 24 exposures, yielding a total exposure time of 7,472.79 s and SNR ~ 9.4.

The final SNR is a slight compromise relative to the initial goal of 10-sigma. After designing the observations in APT, users realize the overhead times charged to their proposed observations. At that point, you will choose either to propose for the full time required for your science goals or compromise the requirements slightly to reduce the total proposal request.

(Note: in the accompanying ETC workbook, rather than providing the full range of calculations, we provide a single calculation with the resulting choice of 6 groups of SHALLOW4.)

The ETC reports signal-to-noise estimates for the module A grism. The throughput for module B is 25% lower. 

Figure 1. SNR vs. exposure time for WFSS observations of the ELG

ETC v1.5 results of SNR vs. exposure time for NIRCam WFSS GRISMR + F356W observations of the ELG described above. Each point represents 24 exposures of 3–10 groups of the SHALLOW4 readout pattern. We find a total exposure time of ~8,000 s yields SNR ~ 10.

Run ETC calculation for direct imaging

See also: JWST ETC Imaging Aperture Photometry StrategyNIRCam Imaging Recommended StrategiesNIRCam Imaging SensitivityNIRCam Imaging

We would like to extract spectra in galaxies as faint as AB mag 26.5. Thus we need to ensure they are detected in the direct imaging to be obtained in F356W. We use a similar setup as above and perform the calculation on the galaxy without any emission line (Calculation #2).

Since this is an extended source, we must increase the size of the photometric apertures to accommodate it. For faint Gaussian sources of width σ, optimal signal to noise is obtained with an aperture of radius ~1.6σ. The width of our source is σ = 0.15", enlarged slightly to 0.158" after convolution with the NIRCam PSF for F356W (σ ~ 0.05").  Thus we will use an aperture radius of 0.25". For the background estimation, we will use large annuli, which is optimistic (assuming this area is free of other sources), but this choice does not affect the results substantially.

  • Instrument setupFilter = F356W

  • Detector Setup
    • Groups per integration = 10 
    • Readout pattern = SHALLOW4 (1 integration, 1 exposure)

  • Strategy:
    • Aperture radius = 0.25 (arcsec)
    • Sky annulus Inner radius = 1.2 (arcsec)
    • Sky annulus Outer radius = 1.95 (arcsec)

The resulting exposure time is 526.10 s, and SNR ~ 8 for the AB mag 26.5 galaxy. That will be sufficient for the direct imaging.

Extended sources require larger photometric apertures than point sources. The ETC default apertures will significantly underestimate signal-to-noise if the source size is larger than the aperture size. We also recommend non-default sizes for point sources (see NIRCam Imaging Sensitivity).

Run ETC calculation for short wavelength imaging

Simultaneous with the WFSS observations, we plan to obtain imaging using the short wavelength filters F115W and F200W, splitting the total exposure time equally between the two (i.e., 12 exposures totaling 3,736.4 s). The specifications for each exposure must be the same as those used for the WFSS observations: 6 groups of SHALLOW4. Thus we define two more ETC calculations on the galaxy as follows:

  • Instrument setup
    • Filter = F115W (Calculation #3)
    • Filter = F200W (Calculation #4)

  • Detector Setup:
    • Groups per integration = 6
    • Readout pattern = SHALLOW4
    • Exposures per specification = 12

  • Strategy:
    • Aperture radius = 0.25 (arcsec)
    • Sky annulus Inner radius = 1.2 (arcsec)
    • Sky annulus Outer radius = 1.95 (arcsec)

The ETC confirms that a small AB mag 26.5 galaxy (without the strong emission line) should be detected with SNR ~ 9 and 14 in the F115W and F200W imaging, respectively.

ETC Results Summary

Below we summarize the ETC inputs and results for the planned observations.

CalculationObservationTargetReadout PatternGroupsExposuresExposure TimeAperture radius /
Sky annulus
inner radius
Sky annulus
outer radius
1GRISMR + F356WELGSHALLOW46247472.790.15"0.3"0.5"


@ 3.6 µm

With the exposure parameters now determined for this program, we can populate the observation template in APT. See the Step-by-Step APT Guide to complete the proposal preparation for this example science program.


Ferguson, H. C. et al. 2004, ApJL, 600, L107
The Size Evolution of High-Redshift Galaxies

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