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A walk through of the ETC for the JWST NIRISS AMI Science Use Case is provided, demonstrating how to select exposure parameters for this observing program.

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Main articles: NIRISS Aperture Masking Interferometry, JWST Exposure Time Calculator Overview
See also: Video Tutorials

The JWST Exposure Time Calculator (ETC) 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 AMI Observations of Extrasolar Planets Around a Host Star" Science Use Case, the target of the observation is HR 8799. The PSF reference star is HD 217783, an F0V star with WISE W2=5.948.

Below we discuss how to navigate the ETC to determine exposure parameters for this science program so that 1010 photons are detected.

ETC Scenes and Sources setup

Main articles: JWST ETC Scenes and Sources Page Overview
See also: JWST ETC Defining a New SourceJWST ETC Defining a New SceneJWST ETC Source Spectral Energy Distribution

ID Tab: Enter identifying information for source and scene

Our scene for this calculation has one source, HR 8799, to be defined in ETC. It has a ground based magnitude of M = 5.26 (Vega) and we will normalize it in NIRISS F480M bandpass. Click on the "Scenes and Sources" tab to set up the parameters of this source (Figure 1). Under the "ID" tab, update the scene name to "HR8799 scene" and source name to "HR8799".

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Figure 1. Source Editor: Assign ID to Scene and Source

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Name source and scene in the source editor.

Continuum Tab: Select source SED

In the "Continuum" tab, select "Phoenix Stellar Models" and "F0V 7250 4.0" as shown in Figure 2.

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Figure 2. Source Editor: Assign SED and spectral type

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Assign spectral energy distribution (SED) and spectral type to the source.

Renorm Tab: Normalize the source SED

In the "Renorm" tab, select "Normalized in bandpass" and enter 5.26 Vegamag for the JWST NIRISS/Imaging F480M filter, as shown in Figure 3. The source has M = 5.26 and we assume M is close to JWST NIRISS F480M bandpass.

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Figure 3. Source Editor: Normalize Source Flux Density

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The source flux density is normalized at 5.26 Vegamag in JWST NIRISS/Imaging bandpass F480M.

Shape Tab: Define the source as a point source

Define the source as a point source, using the "Shape" tab as shown in Figure 4.

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Figure 4. Source Editor: Assign Shape

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Define the shape of HR8799 as a point source.

Offset Tab: Position source in scene

Source can be placed anywhere in the scene by providing offsets as shown in Figure 5. Here we accept the default options that position the source at the center of the scene.

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Figure 5. Source Editor: Position of Source in Scene

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The source is at the center of the scene.

The ETC web interface then simulates a scene based on the properties assigned above and displays it at the bottom of the GUI. The spectrum of the source can be displayed by checking the box under "Plot" listed under "Select a Source". Figure 6 shows the HR8799 Scene Sketch and spectral plot.
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Figure 6. Source and scene setup in ETC for HR8799

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Following the steps in the text to rename the source and scene, define the source SED, and normalize the source in the JWST/NIRISS F480M filter should produce the "Scenes and Sources" view shown above.

Performing ETC calculations

Main article: JWST ETC Calculations Page Overview
See also: JWST ETC Creating a New Calculation 

After defining the source you can run one or multiple calculations by selecting the relevant instrument observing mode under "Calculations". Click on the "Calculations" tab to setup observing parameters for the calculation. Here, we select "AMI" under the "NIRISS" pull-down menu.

Scene Tab: Ensure the calculation is run on the relevant scene

Check that the relevant scene and source, defined above, are selected in the "Scene" tab as shown in Figure 7.

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Figure 7. Calculations: Select Scene

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Select a scene for the calculation

Backgrounds Tab: Specify background levels

The JWST background is position and time dependent. The background level for ETC calculations can be estimated for a specific date or for low, medium, or high levels of background at the target position, corresponding to the 10th, 50th, or 90th percentile, respectively.

For this calculation, we choose no background by selecting "None" in the "Backgrounds" tab as shown in Figure 8.
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Figure 8. Calculations: Select Background

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Choose background level for ETC calculation.

Instrument setup tab: Choose filter for calculation

Under "Instrument Setup", select the filter for the ETC calculation. For this example, we use the F480M filter.

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Figure 9. Select filter in "Instrument Setup" tab

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Use instrument setup tab to choose filter for the observation, which for this case is F480M.

Strategy Tab: Select aperture location and extraction radius

In the "Strategy" tab, choose an aperture location centered on the source, an aperture radius of 1", and a noiseless sky background as shown in Figure 10.

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Figure 10. Strategy tab

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We selected an aperture radius of 1" to include most of the signal in the AMI PSF.

Guidelines for selecting number of Groups (NGroups) in AMI observing mode

Main article: NIRISS AMI Recommended Strategies

The exposure time is determined by the number of "Groups" and "Integrations" in the "Detector Setup" tab. Since AMI uses the NISRAPID readout pattern, the number of "Groups" refer to the number of frames read out in an integration. The number of Groups (NGroups) should be set such that the brightest pixel in the last group of an integration is slightly below the saturation value (sat_e), which we assume to be 72,000 e.


For each filter, we used analytical noiseless NRM PSFs in an aperture of side ~1" (31 × 31 pixels) to estimate NGroups. This PSF can be simulated using the JWST PSF simulation tool WebbPSF (Perrin et al. 2014).

The number of photons per frame in the brightest pixel of the NRM PSF is:

cp_e_per_frame  = cpf × count rate × ph_corr × tframe  (1), 

where cpf is the central pixel fraction; ph_corr (photometric correction) is the NRM throughput relative to the clear pupil, combined with the aperture correction for the 1" aperture; and tframe is the frame time, which for the AMI observing mode using the SUB80 subarray is 0.07544 seconds.

Table 1 gives the values of the central pixel fraction (cpf) for each filter that can be used by the AMI mode, the NRM throughput relative to the clear pupil, and the zero points of the filters. These values can be used to calculate NGroups. The total number of electrons in the central pixels in the entire exposure is:

cptot = cpf × tot_e  (2),

where tot_e is the total number of requested photons in the NRM PSF.

Table 1. Parameters for estimating number of Groups for an AMI observation

Filtercpf a

Photometric correction b


Filter zero point c

a Central pixel fraction.

b NRM throughput relative to the clear pupil/aperture correction for 31 × 31 aperture.

c Using count rates for Vega spectrum scaled to V=9.0.

Depending on the brightness of the source and the number of required photons needed to achieve science goals, there are two scenarios that determines NGroups:

  1. cptot < sat_e This is usually the case for faint objects in which the required number of total photons is reached before the brightest pixel reaches saturation. In this case, NGroups is:
    ngroups = cptot / cp_e_per_frame  (3)
    This value will be NGroups as long as it is less than the maximum number of groups allowed by APT (i.e., < 800). In this case, the observations has one integration.
    If the number of Groups exceeds 800, then more than one integration will be necessary to achieve the exposure time required to detect the necessary number of photons.
  2. cptot > sat_e 

    In this case, saturation in an integration is reached before the requested number of photons are detected and we therefore need an exposure with multiple integrations.

    The time to reach saturation is:

    Tsat = sat_e/(count rate × ph_corr × cpf)  (4)


    ngroups = Tsat/tframe  (5)

In addition to these two scenarios, there are two limiting cases:

  1. Very bright sources: Saturation is reached in under 1 group (e.g., F480M, magnitude 2.41 source). This means the brightness limit for the filter is exceeded and some pixels may saturate.
  2. Very faint sources: Number of integrations exceeds the maximum number of allowed integrations in an exposure (>10000). Another observation can then be created to garner additional photons.

Below, we illustrate how to use these equations and the parameters in Table 1 to estimate a starting point for number of Groups to input into ETC.

Estimating Number of Groups for HR8799

Main article: JWST ETC Batch Expansions

Recall that our aim is to detect 1010 photons from HR 8799 (magnitude M = 5.26, Vega) using NRM and the F480M filter and that saturation occurs at 72000 e-. Using equation (2) and Table 1: 

cptot = cpf × tot_e 

cptot = 0.0194 × 1010

cptot  = 1.94 × 108photons

Since cptot > sat_e, we use equations (4) and (5) and Table 1 to find the NGroups:

Tsat = sat_e / (count rate × ph_corr × cpf) 

Tsat = 72000 e– / (10-(5.26-23.19)/2.5) photons / sec × 0.124  × 0.0194)

Tsat = 2.01 s

ngroups = Tsat / tframe

ngroups = 2.01 sec / 0.07544 s

ngroups = 26.70

This value of NGroups is calculated for a noiseless PSF and should be used only as an initial estimate. The actual value may be slightly lower than this value.

Detector Setup - Determining Number of Groups and Integrations

As a starting point, we enter 26 for the number of groups and leave the number of integrations to 1. The results of this calculation is shown in Figure 11.

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Figure 11. ETC simulation for HR8799 with number of Groups set to 26

Both the main calculations pane and "Reports" pane show a warning for this simulation. Clicking on the "Warnings" tab in the "Reports" pane reveals that 1 pixel is saturated at the end of the ramp (Figure 12).

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Figure 12. The "Warnings" tab in the "Reports" pane reveals that the setup of 26 groups results in 1 saturated pixel at the end of the ramp

We therefore adjust the number of groups so that there are no saturated pixels and the brightest pixel in the last group is just below saturation. This can be done by repeating the calculation for a set of groups below 26 using ETC Batch Expansion, as indicated in Figure 13. At the top of the ETC GUI, click on "Expand Groups" under the "Expand" pull down menu and enter the Start value, step size, and number of iterations. The calculation is now repeated for NGroups = 21, 22, 23,24, and 25.

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Figure 13. Illustration of how to expand over number of groups in ETC to simulate a range of exposures

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The top panel demonstrates how to access the pull-down menu option to expand over number of groups, which opens up a window shown in the bottom panel where the starting value, step size, and number of iterations are specified.

The list of the calculations appear under the "Calculations" tab. Figure 14 shows that for NGroups  = 21, 22, 23 and 24 there are no saturated pixels, indicated by the green check mark in the list of calculations and absence of "Warnings." We will therefore choose NGroups  = 24 as the final value. The calculations for other values of NGroups can be deleted using Edit → Delete Calculation.

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Figure 14. Results of batch expansion over number of groups

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With number of groups set to 24, there are no saturated pixels so we use this value for our ETC simulation.

We now calculate the number of integrations such that we will garner 1010 photons. From the ETC, we see that the extracted flux from the source is reported as 1964733.52 e/s in the "Reports" pane. The exposure time of an observation is given by:

texp = ngroups x nint x tframe

where nint is the number of integrations, and tframe is 0.07544 s with the NISRAPID subarray.

Please note that the exposure time reported by ETC includes reset time, equivalent to one tframe, between each integration. No photons are recorded during this reset time, so this frame should not be included when calculating the total number of photons.

Since the total number of counts detected during this observation is:

photons = flux x ngroups x nint x tframe,

the number of integrations needed to detect 1010 photons is:

nint = 1010 / (flux x ngroups x tframe)

nint = 2811.15

The final number of integrations is thus 2812.

Using a similar method, we calculated the number of groups and integrations for the PSF reference star (Table 2). These values will be used in the APT file for the proposal.

Table 2. Number of groups and integrations for target and PSF reference star

Number of GroupsNumber of Integrations

Target acquisition

Main article: NIRISS Target Acquisition
See also: JWST ETC NIRISS Target Acquisition 

target acquisition (TA) is required in order to precisely position the target 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. 

The only allowed readout pattern for a TA is NISRAPID and the number of integrations and exposures is limited to 1. The acquisition mode is either AMIBRIGHT, for sources with M-band magnitudes between 3.0 ≤ M ≤ 9.3 (Vega), or AMIFAINT, for sources with magnitudes between 9.3 ≤ M ≤ 14.5 (Vega). Since both sources are brighter than M = 8.5, we use the AMIBRIGHT acquisition mode. TA is always performed with the F480M filter.

Using the ETC, we calculate the SNR for target acquisition for both the target and PSF reference star.  In the NIRISS drop-down menu in the calculations pane, select Target Acquisition. For the target (HR 8799), select "SOSS or AMI Bright" in the "Acq Mode" in the "Instrument Setup" tab (Figure 6). The minimum number of groups that is allowed for TA is 3. With NGroups set to 3, the SNR is 105.11, which ensures a successful TA with a centroid accuracy ≤ 0.05 pixel.

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Figure 15. Defining Target Acquisition Mode in ETC

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The "Instrument Setup" tab in the ETC NIRISS Target Acquisition calculation allows the user to define the Acquisition Mode (SOSS or AMI Bright, SOSS or AMI Faint). The filter for TA is fixed at F480M.

Similarly, when using the ETC to simulate the TA on the PSF reference star (HD 217783) with the "SOSS or AMI Bright" acquisition mode, NGroups = 5 gives a SNR of 109.41.

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JWST technical documents

WebbPSF (Perrin, M. D., Sivaramakrishnan, A., Lajoi, C. P, et al. 2014, Proceedings of the SPIE, 9143, 91433X)

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Last updated

Published February 8, 2018


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Published March 2, 2017