See also: NIRISS Wide Field Slitless Spectroscopy, JWST ETC Exposure Time Calculator Overview, Proposal 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 "Using NIRISS WFSS and NIRCam Imaging to Observe Galaxies Within Lensing Clusters" example science program focuses on selecting exposure parameters for NIRISS WFSS as the prime observing mode.

Start by defining a scene of sources relevant to this science case. This example shows you how to to achieve the desired SNR for both the direct imaging and grism observations. 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., 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.

Define Sources and Scene in the ETC

See also: JWST ETC Scenes and Sources Page Overview

Define sources for the "Multiple Galaxies" scene

See also: JWST ETC Defining a New Source, JWST ETC Source Spectral Energy Distribution

First, set up a scene with multiple galaxies with a range of magnitudes and SED types. For this example, define the following sources in ETC in the Source Editor pane: 

• Galaxy m_AB= 26: a point source galaxy with a flat continuum in F_ν, normalized to m_AB = 26 in the NIRISS/Imaging F200W filter;

• Galaxy m_AB=28: a point source galaxy with a flat continuum in F_ν, normalized to m_AB = 28 in the NIRISS/Imaging F200W filter;

• Emission Line Galaxy: a point source emission line-only galaxy with no continuum and not renormalized (i.e., Do not renormalize is selected in the Renorm table), where emission line wavelengths, widths, and intensities are specified in the Lines tab in the Source Editor pane as:

  • Line Center = 1.15 μm, Line Width = 1,000 km/s, Line strength = 8e−18 erg/cm²/s

  • Line Center = 1.5 μm, Line Width = 1,000 km/s, Line strength = 8e−18 erg/cm²/s

  • Line Center = 2 μm, Line Width = 1,000 km/s, Line strength = 8e−18 erg/cm²/s

• Starburst Galaxy: an Extended (Sersic (effective radius), 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 in the Continuum tab) at Redshift = 2, normalized to m_AB = 25 in the NIRISS/Imaging F200W filter.

Assign sources to "Multiple Galaxies" scene

See also: JWST ETC Defining a Scene

After assigning these sources to one ETC scene, by highlighting them one-by-one and clicking the Add Source button in the Select a Scene tab, and renaming the scene Multiple Galaxies, apply the following offsets to the sources within the scene using the Offset tab in the Source Editor pane:

• Galaxy m_AB=26: X offset =  -1.5", Y offset = -1.2";

• Galaxy m_AB=28: X offset = 2", Y offset = 1.7";

• Emission Line Galaxy: X offset = 0, Y offset = 0.5"; 

• Starburst Galaxy: X offset = -2.7, Y offset = 2.7", Orientation = 30°.

Note that since the first 3 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 overplotted and easily compared (note: it may be helpful to limit the wavelength axis to the range relevant to the NIRISS WFSS mode, i.e., 0.8–2.2 µm). 

Run ETC calculation for direct imaging

See also: JWST ETC Calculations Page Overview, JWST ETC Creating a New Calculation, JWST ETC Imaging Aperture Photometry Strategy, NIRISS Imaging Recommended Strategies, NIRISS Imaging, JWST ETC Backgrounds, JWST ETC Outputs Overview, JWST ETC Batch Expansion

Select NIRISS Imaging Calculation

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 4 direct image exposures per filter. The F115W, F150W, and F200W filters are used in this program. 

The goal is to detect Galaxy m_AB = 28 at a SNR ~8 among the 4 coadded images in each of the filters; therefore, run ETC calculations for NIRISS/Imaging for the 3 filters above to determine the exposure parameters needed to achieve this SNR.

Since the JWST background is position dependent, fully specifying background parameters are important for the most accurate SNR calculation. Therefore, enter the coordinates of one of the HST Frontier Fields (04:16:09.40 -24:04:04.00) in the Backgrounds tab, and select Medium for Background configuration, which corresponds to the 50th percentile of the sky background.

Select instrument parameters

Calculation #1 represents your initial calculation to assess the SNR with (mostly) default parameters, as follows:

• Instrument Setup tab - keep the default Filter specification of F200W.
  
  
• Detector Setup tab - 
  • Subarray is set to Full (only full frame readout is supported for NIRISS imaging);
  • For this example, choose the NIS Readout Pattern since faint objects are being observed; 
  • Number of Groups per integration is kept at the default value of 10 and the number of Integrations per exposure is kept at the default value of 1 (for NIRISS imaging and WFSS modes, it is recommended to maximize the number of groups per integration (up to a limit of 25 for the NIS Readout Pattern to mitigate cosmic ray hits) to provide better sampling up-the-ramp);
  • Number of Total Dithers is set to 4 since four direct images will be taken within each filter. 
    
    
• Strategy tab -
  • Select the Centered on source option for Aperture location, choosing Galaxy m_AB = 28 from the drop-down menu, so that the SNR is calculated for this source.
  • Aperture radius is set to 0.175", which represents the 80% encircled energy fraction in the F200W filter (see NIRISS Imaging Recommended Strategies for filter-dependent choices for the aperture extraction radius and background annulus radii for point sources).
  • Sample the background from an annulus around the source, choosing an Inner radius and Outer radius that is 2× and 4× the source extraction radius, or 0.35" and 0.7", respectively.

Run ETC Calculation

Running the calculation with these parameters (by clicking the Calculate button) gives a SNR of ~8.5, as 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 (Calculations #2 and #3, respectively), and set the aperture radius to 0.169" and 0.167", respectively, in the Strategy tab which represents the 80% encircled energy fraction in the PSF for these filters. The radii for the sky annulus from which the background is extracted is set to 2× and 4× the source extraction radius, as above. Running these new calculations on the updated filters shows the SNR is under 8 for both filters.

Adjust exposure parameters to obtain desired signal-to-noise ratio

The SNR in the F150W filter is the median value, therefore you want to determine the number of groups needed to achieve a SNR ~ 8 in this filter. To efficiently run this calculation for a range of groups, where only the number of groups is varied, use batch expansion by selecting Expand Groups from the Expand pull-down menu. Calculations #4 through #8 shows the results of this exercise, where the start value of number of groups in batch expansion was updated to 11 and kept the step size and number of iterations at their default values of 1 and 5, respectively.

Note that with number of Groups per integration ≥ 11 (Calculations #4 through #8), a SNR ≥ 8 is achieved. 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, you will find that choosing 13 Groups per integration for NIRISS WFSS direct imaging allows you to achieve the 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.

To determine the SNR for the other filters, copy the calculations where the number of groups equals 13 (Calculation #6) twice, and update the Filter to  F115W and F200W (Calculations #9 and #10, respectively); the aperture radius to 0.168" and 0.175", respectively; and the radii for the background sky annulus to 0.338" and 0.676" (F115W) and 0.35" and 0.7" (F200W). By selecting the check box next to the calculations corresponding to these exposure specifications (number of Groups per integration = 13, number of Integrations per exposure = 1, number of Exposures per specification = 4) for the various filters (Calculations #6, #9, and #10), you can compare the predicted SNR through these calculations in the Plots pane.

Run ETC calculation for WFSS

See also: JWST ETC Aperture Spectral Extraction Strategy, NIRISS WFSS Recommended Strategies

Select NIRISS WFSS Calculation

This program uses an 9-step dither pattern for each filter, using the custom 9-POINT-MEDIUM-NIRCam dither pattern created for coordinated parallel observations with NIRISS WFSS as the primary observing mode and NIRCam imaging as the parallel observing mode (see NIRISS WFSS Recommended Strategies for a discussion about the trade-offs between dither size and number of dither steps). Our goal is to obtain a SNR ~6 per pixel in the emission lines from the emission line galaxy from the coadded dithered WFSS exposures.

Initiate a NIRISS/WFSS calculation and update the Backgrounds tab as above for the direct imaging calculations (i.e., the coordinates were set to 04:16:09.40 -24:04:04.00 and select Medium for Background configuration).

Select instrument parameters

Calculation #11 represents an initial calculation to assess the SNR with (mostly) default parameters, as follows:

• Instrument Setup tab -
  • Grism is set to GR150R. (Note that grism GR150C has higher throughput in order 1 compared with GR150R, so this choice provides a conservative SNR estimate);
  •  Keep the default Filter specification of F115W.
    
    
• Detector Setup tab - 
  • Subarray is set to Full (only full frame readout is supported for the NIRISS WFSS mode);
  • Choose the NIS Readout Pattern since faint objects are being observed; 
  • Number of Groups per integration is kept at the default value of 10 and the number of Integrations per exposure is kept at the default value of 1;
  • Number of Total Dithers is set to 9 since the program uses a 9-point dither pattern. 
    
    
• Strategy tab
  
  • Select the Centered on source option for Aperture location, choose Emission Line Galaxy from the drop-down menu, so that the SNR is calculated for this source.
  • Wavelength of Interest is set to the wavelength of the emission line, i.e., 1.15 µm.
  • Aperture Full-Height is set to 0.338", which is twice the 80% encircled energy radius for a point source.
  • The sky sample start region and end region are set to 2× and 4× the aperture full-height value (0.676" and 1.352", respectively).
    

Run ETC Calculation

With these parameters, you'll find an SNR of ~4, which is too low.

Adjust exposure time to obtain desired signal-to-noise ratio

Similar to the direct imaging calculation, use batch expansion to repeat the calculation, increasing only the number of Groups per integration, using a starting value of 16, 9 iterations, and a step size of 1 (Calculations #12 - #20). It is recommended to limit the number of groups to 25 with the NIRISS NIS Readout Pattern to mitigate the impact of cosmic ray hits which can result in discarded frames.

You'll find that with number of groups ≥ 23 (Calculation #19), the SNR ≥ 6. Similar to the experimentation done to match up parallel NIRCam imaging exposures with NIRISS WFSS direct imaging exposures in APT, strike a balance between maximizing NIRCam exposure time within the exposure time window allowed by the prime NIRISS WFSS exposures. You'll find that for 23 groups, you make the most efficient use of a simultaneous NIRCam observation while achieving a WFSS SNR of ~6 (Calculation #19).

To determine the SNR in filters F150W and F200W for this exposure specification, copy Calculation 19 twice, update the Filter to F150W and F200W (Calculations #21 and #22, respectively), and set the wavelength of interest in the Strategy tab to the wavelengths of the emission lines (i.e., 1.5 µm and 2 µm). Also update the Aperture Full-Height values to 0.334" and 0.350", respectively which corresponds to twice the 80% encircled energy radii for the PSF through these filters, and the background sky sample start region and end region to 0.668" and 1.336" (F150W) and 0.7" and 1.4" (F200W), i.e., 2× and 4× the aperture full-height value. You will find a SNR of > 7 through both filters with this exposure set-up.

Examine signal-to-noise ratio for parallel NIRCam imaging observations

See also: NIRCam Imaging, NIRCam Detector Readout Patterns, NIRCam Point Spread Functions, NIRCam Filters

NIRcam imaging observations are taken of a nearby field during the NIRISS WFSS exposures. As discussed in more detail in the step-by-step APT guide, there is a set of NIRCam exposures in the short wavelength channel and long wavelength channel for each set of NIRISS WFSS Direct Image → GR150 → Direct Image exposures. 

NIRCam imaging observed in parallel to NIRISS grism exposures

The longest NIRCam imaging exposure sequence is observed in parallel with the set of dithered NIRISS GR150 exposures. Calculations #23 and 24 show NIRCam imaging exposures for one of these sets of parallel observations to the dithered GR150 exposures for illustrative purposes. The set up is as follows:

• Backgrounds tab - the position is set to the same position as the prime field, with the Medium background level chosen;
  
  
• Instrument Setup tab - the Filter× is set to F090W for NIRCam short wavelength imaging (Calculation #23) and to F277W for NIRCam long wavelength imaging (Calculation #24);
  
  
• Detector Setup tab - 
  • Subarray is set to Full since faint galaxies are being observed and you'll need the maximum field of view;
  • The readout pattern is set to MEDIUM8 to obtain the highest S/N for faint objects, with enough groups per integration (≥ 4) and a short enough integration ramp (< 1,000s) to mitigate the impact of cosmic rays;
  • Total Dithers  is set to 9 to mimic the 9-POINT-MEDIUM-NIRCam dither pattern, Groups per integration is set to 9 since experimentation with APT shows that this exposure time best matches the time available during the parallel WFSS GR150 observation, and Integrations per exposure is left to 1;
    
    
• Strategy tab -
  • Select the Centered on source option for Aperture location, choose Galaxy m_AB = 28 from the drop-down menu, so that the SNR is calculated for this source.
  • Aperture radius is set to 0.066" for the F090W filter and 0.184" for the F277W filter, representing approximately 2x the PSF full width half maximum in these filters (see NIRCam Point Spread Functions).
  • The background Sky Annulus radii are set to approximately 2× and 4× the source aperture radius, or 0.132" and 0.264" for the F090W filter and 0.368" and 0.736" for the F277W filter.

From this exercise, you'll see that if a galaxy with m_AB = 28 is in the field, it would be detected with a SNR of >17 in the F090W filter (Calculation #23) and the F227W filter (Calculation #24) when using these exposure parameters.

NIRcam imaging observed in parallel to NIRISS direct image exposures

One NIRCam imaging exposure is obtained in parallel with each NIRISS direct image exposure. Calculations #25 and 26 show NIRCam imaging exposures for one of these sets of parallel observations for illustrative purposes. The set up is as follows:

• Backgrounds tab - the position is set to the same position as the prime field, with the Medium background level chosen;
  
  
• Instrument Setup tab - the Filter is set to F150W for NIRCam short wavelength imaging (Calculation #25) and to F300M for NIRCam long wavelength imaging (Calculation #26);
  
  
• Detector Setup tab - 
  • Subarray is set to Full since faint galaxies are being observed and you'll need the maximum field of view;
  • The readout pattern is set to MEDIUM8 to obtain the high quality data of faint objects for a shorter exposure.
  • Total dithers is set to 1 since there is only one NIRCam exposure per NIRISS direct image, Groups per integration is set to 5 since experimentation with APT shows that this exposure time best matches the time available during the parallel NIRISS direct image observation, and Integrations per exposure is left to 1;
    
    
• Strategy tab -
  • Select the Centered on source option for Aperture location, choose Galaxy m_AB = 26 from the drop-down menu, so that the SNR is calculated for this source.
  • Aperture radius is set to 0.1" for the F150W filter and 0.2" for the F300M filter, representing 2× the PSF full width half maximum in these filters.
  • The background Sky annulus radii are set to 2× and 4× the source aperture radius, or 0.2" and 0.4" for the F150W filter and 0.4" and 0.8" for the F300M filter.

From this exercise, you'll see that if a galaxy with m_AB = 26 is in the field, it would be detected with a SNR of ~24 in the F150W filter (Calculation #25) and with a SNR of ~14 in the F300M filter (Calculation #26) when using these exposure parameters.