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- Methods and Roadmaps
- JWST Imaging
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A walk-through of the JWST ETC for the NIRISS WFSS Science Use Case is provided, demonstrating how to select exposure parameters for this observing program. Additional calculations are described to highlight considerations of which a user should be cognizant when running ETC calculations for the WFSS observing mode.
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 "Using NIRISS WFSS and NIRCam Imaging to Observe Galaxies Within Lensing Clusters" Science Use Case, we focus on selecting exposure parameters for NIRISS WFSS as the prime observing mode. Direct images are taken before and after each set of dithered grism exposures for the NIRISS WFSS mode.
We start by defining a scene of sources relevant to this science case. We show how to run ETC calculations to achieve the desired SNR for both the direct imaging and grism observations. The optimal exposure specifications (e.g., number 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.
We also discuss ETC calculations which highlight considerations relevant to the WFSS observing mode, namely how extended or nearby sources can lead to spectral confusion, degrading resolution.
Using ETC to derive exposure parameters for proposal submission in APT
Defining sources for the "Multiple Galaxies" scene
We first set up a scene with multiple galaxies with a range of magnitudes and SED types. We define the following sources in ETC:
Galaxy mAB= 26: a point source galaxy with a flat continuum, normalized to mAB = 26 in the NIRISS/Imaging F200W filter (Figure 1);
Galaxy mAB=28: a point source galaxy with a flat continuum, normalized to mAB = 28 in the NIRISS/Imaging F200W filter (Figure 2);
Emission Line Galaxy: a point source emission line only galaxy with no continuum, where emission line wavelengths, widths, and intensities are specified in the Lines tab in the Source Editor (Figure 3) as:
center = 1.15 μm, width = 1,000 km/s, strength = 8e−18
center = 1.5 μm, width = 1,000 km/s, strength = 8e−18
center = 2 μm, width = 1,000 km/s, strength = 8e−18
Starburst Galaxy: an extended (Sersic profile, semi-major axis = 0.3" and semi-minor axis = 0.15") starburst galaxy (using the SED of NGC 3690 from the extragalactic spectral templates available in the ETC) at z = 2, normalized to mAB = 25 in the NIRISS/Imaging F200W filter (Figure 4).
In the "ID" tab of the "Source Editor" pane, update the "Source Identity Information" with the name of each source.
Assigning sources to "Multiple Galaxies" scene
Main articles: JWST ETC Defining a Scene
Create a new ETC scene by clicking "New" in the "Select a Scene" pane. Assign each of the above sources to this scene by highlighting the source, the scene, and clicking "Add Source" in the "Select a Scene" pane (Figure 5, top). In the "ID" tab of the "Source Editor" pane, update the "Scene Identity Information" to "Multiple Galaxies" (Figure 5, bottom).
Now that the sources are assigned to a scene, we can define offsets with respect to the center of the scene. In the "Offset" tab of the "Source Editor" pane, specify the following offsets and orientations:
Galaxy mAB=26: X offset = 0.7", Y offset = -0.5";
Galaxy mAB=28: X offset = 1", Y offset = -1.5";
Emission Line Galaxy: X offset = 0, Y offset = 0.5";
Starburst Galaxy: X offset = -1.5", Y offset = 1.5", Orientation = 30°.
Note that since the first three galaxies are point sources, orientation need not be specified in the "Offset" tab. The position of the sources in the scene can be viewed in the lower left "Scene Sketch" pane. By checking the checkbox in the "Plot" column in the "Select a Source" pane, the SEDs of the selected sources can be plotted. Figure 6 shows the scene sketch and the SEDs of the four galaxies in the scene, where the x-axis in the plot window is restricted to the wavelength range relevant for the NIRISS WFSS mode (0.8 – 2.2 μm).
Running ETC calculation for direct imaging
A direct image is taken before and after each set of dithered grism exposures in NIRISS WFSS mode. This program uses both the GR150R and GR150C grisms, which disperses the light in orthogonal directions. There are therefore four direct image exposures per filter. The F115W, F150W, and F200W filters are used in this program.
Our goal is to detect Galaxy mAB = 28 at a SNR ~10 among the filters, so we run ETC calculations to determine the exposure parameters we need to specify to achieve this SNR.
In the "Calculations" tab, select "Imaging" from the "NIRISS" drop-down menu. This step triggers an ETC NIRISS Imaging calculation for the default scene using default parameters.
In the "Scene" tab, select the "Multiple Galaxies" scene in the "Select Scene for Calculation" pull-down menu (Figure 7, top). In the "Backgrounds" tab, update the position to use the coordinates of one of the HST Frontier Fields (04:16:09.40 – 24:04:04.00) for an accurate SNR estimate since the JWST background is position dependent. Select "Medium" for "Background configuration", which corresponds to the 50th percentile of the sky background (Figure 7, bottom).
In the "Instrument Setup" tab, keep the default filter specification of F200W (Figure 8, top). In the "Detector Setup" tab, ensure that the subarray is set to "Full" (only full frame readout is supported for NIRISS imaging) and choose the NIS Readout Pattern (where four frames are averaged in a group, making this Readout Pattern the preferred option for longer exposures). Set the number of exposures ("NExposures") to 4 since four direct images will be taken within each filter. Keep the number of groups ("NGroups") at the default value of 10 and the number of integrations ("NIntegrations") at the default value of 1 (Figure 8, bottom).
To calculate the SNR from the mAB = 28 Galaxy, click on the "Strategy" tab and select the "Centered on Source" option and "Galaxy mAB = 28" from the drop-down menu (Figure 9). Keep aperture radius set to 0.1" and the default values for the Sky Annulus parameters for background subtraction (inner radius = 0.22", outer radius = 0.4").
Click "Calculate", which initiates the ETC calculation with these updated parameters. The SNR, ~10.9, is reported in the upper left "Calculations" pane and the bottom right "Reports" pane.
To calculate the SNR in the other filters, select "Copy Calculation" in the "Edit" pull-down menu. Copy this calculation twice, and update the filters in the "Instrument Setup" tab for the new calculations to F115W and F150W. When running these new calculations on the updated filters, the SNR is under 10.
The SNR in the F150W filter is the median value, so we want to determine the number of groups needed to achieve a SNR ~10 in this filter. To efficiently run this calculation for a range of groups, where only NGroups is varied, use Batch Expansion. Highlight the calculation for the F150W filter and select "Expand Groups" in the "Expand" pull-down menu (Figure 10, top). In the "Batch Groups Configuration" menu that appears (Figure 10, bottom), update the start value to 11 and keep step size and number of integrations at their default values of 1 and 5, respectively. Click "Submit."
With NGroups ≥ 12, we achieve a SNR > 10.
Since this program is a coordinated parallel program with NIRCam imaging, there is a balancing act when choosing exposure times. The exposure times for the coordinated mode (including overheads) can not exceed the exposure time of the prime observing mode. However, minimizing dead time, when the coordinated mode is not observing, is also important. From experimentation in APT, we find that choosing 13 groups for NIRISS WFSS direct imaging allows us to achieve our SNR goals while making efficient use of simultaneous NIRCam imaging observations (see the Step-by-Step APT Guide for the corresponding NIRCam specifications). In general, determining optimal exposure parameters may involve some iteration between ETC and APT.
The "Images" pane shows the 2D SNR image for this calculation with the specified exposure parameters (Figure 11, top). Copy the NGroups = 13 calculation twice, and update the filters to F115W and F200W. By selecting the check-box next to the calculations corresponding to these exposure specifications (NGroups = 13, NIntegrations = 1, NExposures = 4) for the various filters, we can compare the predicted SNR through these calculations in the "Plots" pane (Figure 11, bottom).
Running ETC calculation for WFSS
Main article: JWST ETC Aperture Spectral Extraction Strategy
This program uses an 8-step dither pattern for each filter. Our goal is to obtain a SNR ~ 3 per pixel in the emission lines from the Emission Line Galaxy.
To begin the calculation, select "WFSS" in the NIRISS drop-down menu. Update the scene in the "Select Scene for Calculation" pull-down menu to "Multiple Galaxies." Similar to the direct imaging calculation, enter the coordinates of the field and select "Medium" for "Background Configuration" in the "Backgrounds" tab. In the "Instrument Setup" tab, keep the default grism choice (GR150 row-dispersed) and filter (F115W). In the "Detector Setup" tab, keep the default values for "Subarray" (Full; only full-frame readout is supported for WFSS), Readout Pattern (NIS), NGroups (10) and NIntegrations (1). Since this program uses eight dither steps, update the number of exposures to 8.
To calculate the SNR for the Emission Line Galaxy, select this source in the "Centered on Source" drop-down menu in the "Strategies" tab, making sure to select the "Centered on Source" option (Figure 12). Keep the "Aperture Half-Height" at its default value of 0.15" and the "Sky sample region" to its default values for start region (0.2") and end region (0.5").
Click "Calculate." The SNR for this calculation is ~1.9, which is too low. Similar to the direct imaging calculation, use batch expansion to repeat the calculation, increasing only the number of groups. Use a starting value of 11, 15 iterations, and a step size of 1. It is recommended to limit NGroups to 25 with the NIRISS NIS Readout Pattern to mitigate the impact of cosmic ray hits which can result in discarded frames.
With NGroups ≥ 22, the SNR exceeds ~2.8, which is close to our target of 3. Similar to the experimentation we did to match up parallel NIRCam Imaging exposures with NIRISS WFSS direct imaging exposures in APT, we strike a balance between maximizing NIRCam exposure time within the exposure time window allowed by the prime NIRISS WFSS exposures. We find that for NGroups = 23, we achieve an acceptable NIRISS WFSS SNR while minimizing dead time with a simultaneous NIRCam observation.
Copy the NIRISS WFSS calculation through the F115W filter twice and update the filter to F150W and F200W. Note that when updating the filter, the "Wavelength of Interest" in the "Strategy Tab" (Figure 12) has to be updated by hand, in this case, to the central wavelength of the filter. The SNR with this exposure set-up (i.e., NGroups = 23, NIntegrations = 1, NExposures = 8) is about 5 and 6 in the F150W and F200W filters, respectively.
See the "Step-by-Step APT Guide" for a walk-through of how to fill out the APT observation template and enter the exposure parameters derived here.
Illustrative ETC calculations: complicated source morphology, high-z galaxy, and effects of background aperture and nearby sources on SNR and spectral resolution
The NIRISS WFSS mode will be used to observe fields with multiple sources. Here, we illustrate how source morphology, background aperture region, and nearby sources can affect the SNR and spectral resolution, which are important effects to consider when designing a WFSS observation. We also demonstrate the capabilities of the NIRISS WFSS observing mode to detect a high-z galaxy using the exposure specifications derived above.
Extended source morphology
Create Lensed Galaxy
To mock up a lensed galaxy, which is relevant to the CANUCS program to observe galaxies within lensed clusters, create 3 arc segments that will be assigned to a scene in a lens configuration. Define a source that uses the SED of NGC 3079 as a template, set the redshift to 2, and normalize to 50 nJy at 2 μm. In the "Shape" tab, select "extended", "flat distribution," and set the semi-major axis to 0.1" and the semi-minor axis to 0.005." Keep “Normalization” choice at the default value of “integrated flux" (see Figure 13). Name this source "arc 1" in the "ID" tab of the "Source Editor" pane.
Making sure that the source "arc 1" is highlighted in the "Select a Source" pane, select "Copy Source" twice from the Edit pull-down menu, to create a total of three arc segments. Name the two newly created arc segments "arc 2" and "arc 3."
In the "Select a Scene" pane, click "New" to create a new scene. Then click "Add Source" to add each of the arc segments to this scene. In the "ID" tab of the "Source Editor" pane, name this scene "Lensed Galaxy" (Figure 14).
By defining the offsets of the arc segments relative to the center of the scene in the "Offset" tab in the "Source Editor" pane, we will produce a facsimile of a lensed galaxy. Define the following offsets for the arc segments:
arc 1: X offset = 0", Y offset = 0", Orientation = 90°
arc 2: X offset = 0.06", Y offset = 0.16", Orientation = 50°
arc 3: X offset = 0.06", Y offset = -0.16", Orientation = -50°
After specifying these offsets and orientations, the "Scene Sketch" in the lower left pane should look like Figure 15.
Calculating SNR for different grism options: effect of dispersion direction on spectral resolution
Main article: NIRISS GR150 Grisms
The NIRISS WFSS mode had two grism choices: GR150R, which disperses the light in the fast readout direction, and GR150C, which disperses the light in the slow readout direction. In ETC, these options are implemented as "GR150 row-dispersed" and "GR150 column-dispersed." Since the lensed galaxy is elongated along the Y-direction, the GR015C grism will disperse the light along the elongated arc, degrading spectral resolution as we will see below.
To initialize a WFSS calculation, select "WFSS" from the NIRISS pull-down menu. Update the "Scene for Calculation" to Lensed Galaxy (Figure 16, top). In the "Backgrounds" tab, enter the coordinates above (04:16:09.40 – 24:04:04.00) and select "Medium" configuration (see Figure 7, bottom). In the "Instrument Setup" tab, keep the default grism option ("GR150 row-dispersed") and update the filter to F200W. Note that the "wavelength of interest" in the "Strategy" tab has to be manually updated to 2 μm.
Retain the default parameters in the "Detector Setup" tab: "FULL" Subarray, "NIS" Readout Pattern, 10 groups per integration, 1 integrations per exposure, and 1 exposure per specification. In the "Strategy" tab (Figure 16, bottom), make sure the option to "Specify offsets in scene" is selected and is set to 0: this option places the extraction aperture at the center of the scene. Retain the default parameters for "Aperture Half-Height" (0.15") and "Sky sample region" (start region = 0.2", end region = 0.5"). Click "Calculate."
In the "Edit" pull-down menu, select "Copy Calculation." In the new calculation, update the grism in the "Instrument Setup" tab to "GR150 column-dispersed."
To compare the spectral resolution for these two calculations, click the checkbox next to both calculations in the "Calculations" pane and inspect the SNR plot in the bottom middle "Plots" pane (Figure 17). Since the dispersion direction for the GR150 column-dispersed grism calculation is along the elongated arc of the lensed galaxy, the resolution of the spectrum is degraded compared with the GR150 row-dispersed grism calculation.
Effect of background region on SNR
In the previous calculation, the background is sampled from a region from 0.2" – 0.5." Portions of the lensed galaxy extend from 0.2" – 0.3," and is thus included as background in the calculation. To see how this background over-subtraction affects the results, we perform a calculation where the background is extracted from a source-free region in the scene.
Copy the GR150 row-dispersed grism calculation, and in the "Strategy" tab, update the start region to 0.3" and the end region to 0.6." Click "Calculate." Compare the SNR plot for both calculations (Figure 18): with a larger background sample region that does not include source flux, the SNR is higher.
Care must be taken when defining the background sample region to exclude emission from an extended source, or nearby sources. Alternatively, the "noiseless sky background" option can be chosen, mitigating contamination concerns in the background region.
Main article: JWST ETC User Supplied Spectra
Create high-z galaxy scene
We create a scene with a high-redshift galaxy and a foreground galaxy to estimate the SNR at which we would detect emission lines from the high-z galaxy using the exposure parameters derived in Part 1 of this guide and the effects of a neighboring source in creating spectral confusion.
Create a new source, with a user-supplied spectrum as the SED template. The spectrum for this example, which is of an emission line galaxy at z = 8.76, can be found here. Click on the "Upload Spectra" tab and upload this spectrum (Figure 19, top).
Create a new source in the "Select a Source" pane and name it "high-z galaxy." In the "Continuum" tab, select "Uploaded File." The file imported in the previous step should be visible in the pull-down menu (Figure 19, bottom).
In the "Renorm" tab, normalize the continuum to 25.8 (abmag) in the NIRISS F200W filter. In the "Shape" tab, set the shape to "Extended" with a Sersic profile and normalization choice to "integrated flux." Set both the semi-major and semi-minor axes to 0.065. Keep Sersic index at 1.
Create a new source which will serve as a foreground galaxy in this scene. In the "Continuum" tab, select extragalactic spectra and choose NGC 4552 as the SED. Set the redshift to z = 0.45. In the "Renorm" tab, normalize the continuum to 22 (abmag) in the NIRISS F200W filter. Similar to the high-z galaxy above, define the source as extended with a Sersic profile, Sersic index of 1, and integrated flux normalization. Set the semi-major axis to 0.3" and semi-minor axis to 0.2."
Define a new scene and add the high-z galaxy and foreground galaxy to this scene. Name the scene "high-z galaxy." Assign the following offsets in this scene:
High-z Galaxy: X offset = 0, Y offset = 1.6";
Foreground Galaxy: X offset = 0, Y offset = 0, Orientation = -45°.
The scene sketch (bottom left pane) and source spectrum plots (bottom middle pane, after restricting the x-axis from 1 – 2.6 μm) are shown in Figure 20. Note the high-z galaxy has prominent Lyα and CIII] emission lines, observed at 1.19 μm and 1.86 μm, respectively.
Determine SNR of high-z galaxy emission lines
In this series of calculations, we determine the SNR of the Lyα and CIII] emission lines in the high-redshift galaxy, using the exposure parameters we derived for this observing program.
To begin, select "WFSS" from the NIRISS pull-down menu. In the Scene tab, select the "high-z galaxy" scene. Update the "Background" configuration to match that from previous calculations: set coordinates to 04:16:09.40 – 24:04:04.00 and select "Medium" configuration (see Figure 7, bottom). In the "Instrument Setup" tab, select "GR150 row-dispersed" grism and the F115W filter.
In the "Detector Setup," specify the same exposure parameters derived above: "FULL" subarray, "NIS" Readout Pattern, 23 groups per integration, 1 integration per exposure, and 8 exposures per specification (Figure 21, top). In the "Strategy" tab, select "Centered on Source" and make sure the high-z galaxy is selected from the pull-down menu. Keep the aperture half-height (0.15") and background sky sample region parameters (start region = 0.2", end region = 0.5") at their default values. Click Calculate.
Copy this calculation twice and update the filters to F150W and F200W, and the wavelength of interest in the "Strategy" tab to 1.5 and 2.0 μm, respectively.
Plot the output of this calculation to inspect the predicted SNR of the emission lines (Figure 22).
Effects of nearby sources
In the above calculations, the dispersion direction was along the rows in the ETC scene. In this scene, there is a foreground galaxy below the high-redshift galaxy. Here, we explore the effects of dispersing the spectrum along the columns of the scene.
Copy the calculation above with the F115W filter, update the grism to "GR150 column-dispersed" in the "Instrument Setup" tab, and click Calculate.
Compare the SNR image from the GR150 row-dispersed grism (Figure 23, top) with that from the GR150 column-dispersed grism (Figure 23, bottom). Note that when dispersing along the ETC rows (Figure 23, top), the spectral traces from both sources are distinct, while they are confused when dispersing along the ETC columns (Figure 23, bottom).
The spectral confusion is visible in the 1-D SNR line plot (Figure 24): the spectrum from the GR150 column-dispersed grism is contaminated by the foreground galaxy, which is evident when comparing with the spectrum from the GR150 row-dispersed grism. To mitigate spectral confusion when observing with the WFSS mode, use both the GR150R and GR150C grisms.