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.
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
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".
Continuum Tab: Select source SED
In the "Continuum" tab, select "Phoenix Stellar Models" and "F0V 7250 4.0" as shown in Figure 2.
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.
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.
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.
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.
Performing ETC calculations
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.
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.
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.
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.
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–.
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.
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).
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.
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.
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
A 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.
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.
WebbPSF (Perrin, M. D., Sivaramakrishnan, A., Lajoi, C. P, et al. 2014, Proceedings of the SPIE, 9143, 91433X)