The JWST Exposure Time Calculator (ETC) allows observers to simulate sky scenes and perform signal to noise calculation for a given telescope/instrument configuration. We show the step-by-step instructions to perform ETC calculations for observations following a real science case, namely IR imaging and spectroscopy IFU observations of Supernova 1987A.

Defining the sources

SN1987A consists of a bright ring, enclosing the dust ejecta material. The outer region of the dust ejecta includes several knots of shocked emission. The geometry of the dust source could be defined as:

A dust ring with a diameter of ~1.1" and a thickness of ~0.2" for MIRI calculations.

Using the ETC web interface, it is possible to define sources within a scene. The ring part of the supernova is defined as an extended source with a flat-flux distribution and using a circular shape with a radius of 0.63".

The Source Editor panel (right side of Figure 1) includes a set of tabs that allow the user to define the characteristics of the sources in the scene. Figure 1 shows the first step in defining a new source in the scene. We start by assigning it a source identification ID, in this case the ID "ring".

Figure 1. Source Editor

ETC workbook view where a single extended source (the ring of the SN) is identified by name.

The continuum tab allows us to select an SED for the source. In this case we model the source with a Blackbody Spectrum of 400 K as shown in Figure 2.

Figure 2. Source Editor

The source (ring) is assigned a black body SED of 400 K in the Continuum tab.

In the normalization (Reform) tab, we can normalize the spectrum to have a flux density of 80 mJy at 10 μm as shown in Figure 3.

Figure 3. Source Editor

The source flux density is normalized at 10 μm.

The shape of the source is defined in the Shape tab. In this case we select an extended source with a flat flux distribution and we give it the shape of a disk with a 0.63" radius. See Figure 4.

Figure 4. Source Editor

The source is considered extended, then a flat circle with a radius of 0.63" should represent it well.

The ETC web interface creates a simple sketch of the scene and displays it at the bottom of the GUI. The spectrum of the source is also plotted when selected as shown in Figure 5.

Figure 5. Source Editor

The ETC workbook shows a simple plot of the scene (in the case an extended source) and also the source spectrum (in this case normalized at 10 μm with a flux density of 80 mJy).

Following the same procedure, it is possible to define another source in the scene, in this case the "ejecta", which we can assume is a point source with a Blackbody SED of 100 K, and with its flux density normalized to 0.1 mJy at 10 μm. Figure 6 shows the ETC workbook with both sources selected.

Figure 6. Source Editor

ETC workbook view after adding the second point source that represents the ejecta.

A third source in the scene is defined with a name "shock knot1. This will be a source of pure line emission at 2.12 μm to simulate shocked molecular hydrogen in an extended knot. This source is added in a similar manner to the instructions described above. The detailed steps for this are:

Add a "new" source in the 'select a source' region of the ETC.

In the ID area for the source, name the new source "shock knot1"

Choose 'no continuum' in the "Continuum" tab for the source definition.

In the "Renorm" tab, select 'Do not renormalize'.

In the "Lines" tab, fill out the appropriate parameters for molecular hydrogen H2 emission: 2.12 μm wavelength, line width of 50km/s (a possible value for shocked H2 lines), and a line flux of 1.0e-16 ergs/cm2/s. The intrinsic flux of the expected emission may not be well known. The ETC can be used to estimate S/N on a range of source fluxes by changing this line emission flux, or by adding additional sources of varying flux levels.

Click "Add" to add the line to the source. Additional lines can be added to build up a more complex emission line spectrum, but here we describe only the 2.12 μm H2 feature.

In the "Shape" tab, keep the default point source for a compact H2 knot of emission.

In order to offset this simulated H2 line to the outer region of the dust source, the 'shock knot1' source must be added to the scene. Highlight 'shock knot1' in the "Select a Source" region, and add it to the SN1987A using the "Add a Source" (blue) button in the "Select a Scene" area.

Now, offset the source in the scene to the outer region of the dust emission. Do this in the "Offset" tab, by adding and saving x, y offset values of 0.4, -0.4.

Figure 7 shows the ETC workbook with all sources selected.

Figure 7. Adding an emission line source

ETC workbook view after adding the offset line emission source that represents a shocked knot of molecular emission (highlighted in yellow in the scene view).

Overview of NIRSpec calculations

Once the sources are defined, it is possible to perform many signal-to-noise calculations on the scene using the different JWST instruments. This is carried out under the Calculations tab in the ETC view. For each calculation, few additional parameters need to be set under the following tabs: Backgrounds, Instrument Setup, Detector Setup, and Strategy.

Backgrounds tab:

For this example we used the Medium background configuration as shown in Figure 8.

Figure 8. Calculation Editor

Designing the ETC calculation: Background tab

Instrument Setup tab:

Based on the science goals, the selection for Grating/Filter pair should be G235M/F170LP as shown in Figure 9. Note that the ETC automatically shows the Total System Throughout for this configuration in its corresponding wavelength range.

Figure 9. Calculation Editor

Designing the ETC calculation: Instrument tab

Detector Setup tab:

In order to achieve the desired exposure time, the number of Groups is set to 10, and the number of Integrations is set to 2. Multiple integrations are used in this case to ensure that uncertain source fluxes will not saturate in the allocated exposure time. The Readout pattern used for this example is NRS IRS2. See Figure 10. The exposure time is automatically calculated and displayed. For this example the total exposure time is ~25 minutes.

Figure 10. Calculation Editor

Designing the ETC calculation: Detector tab

Strategy tab:

In the Strategy tab it is possible to select the type of nodding with the options "In Scene" and "Off scene". We select "Off scene" so that this extended source has no residual subtraction effects. We select the wavelength 2.12 μm to correspond to run the calculation on the molecular hydrogen emission line. Note that this value has to be in the range determined by the grating/filter combination, in this case 1.66–3.17 in units of μm. We define the calculation to have offsets in x and y that correspond to the location of the extended knot of H2 emission modeled in the 'shock knot1' source. The top section of Figure 11 shows the Strategy tab and the IFU Nod selection.

Once all parameters are in place, simply click on the Calculate button to perform the calculation. This might take some time because the NIRSpec IFU ETC calculates information on the full IFU data cube (which can be saved to disk in the 'downloads' section of the ETC). Figure 11 shows the results of the calculation, a signal-to-noise of 21 is achieved on the H2 emission line. A detailed description of the ETC outputs are described in the article JWST ETC Outputs Overview.

Figure 11. Calculation Editor

Designing and Running the NIRSpec ETC calculation: Strategy tab

Overview of MIRI calculations

Following a similar approach, we can perform a couple of calculations for the JWST instrument MIRI. We begin with a signal to noise calculation on the mid-infrared dust morphology using the MIRI Imaging mode.

We create a new calculation by selecting MIRI→ Imaging. This sets default values in the calculation space that need to be customized to our specific needs for this project. The few additional parameters that need to be set are under the following tabs: Backgrounds, Instrument Setup, Detector Setup, and Strategy.

In the Backgrounds tab we set the value to 'Low'. In the Instrument Setup tab we select the MIRI filter F560W. Under the Detector Setup, there are a few options for subarray. We select BRIGHTSKY for this project. From the Readout pattern we select FAST. The number of groups is set to 16 to avoid saturation and eight is the number of integrations. Leave number of exposures equal to one.

Under the Strategy tab, there is only one option which is Imaging Aperture Photometry. We center on the "ring" and modify the radii for the aperture and sky annulus as follows: aperture radius 0.6", inner sky radius 0.7", and outer sky radius 0.9". Once these parameters are set, we can perform the calculation. Figure 12 shows a screen grab of the ETC GUI with the MIRI imaging mode highlighted. The plot in the bottom left shows a recreation of what the scene looks like at the detector level. the signal to noise for this calculation yields a value of 4936.5 in a total exposure time of 355.20 s.

The warning on this calculation can be viewed in the 'Warning' tab in the Reports area. In this case, the warning simply states that the background region used for the subtraction is smaller than the science aperture area. This can lower the signal to noise in this calculation on an extended source area. In actual data processing, a larger spatial region can be defined that will improve noise results in background subtraction.

We create a new calculation by selecting MIRI→ MRS. This sets default values in the calculation space that need to be customized to our specific needs for this project. The few additional parameters that need to be set are under the following tabs: Backgrounds, Instrument Setup, Detector Setup, and Strategy.

In the Backgrounds tab we set the value to 'Low'. In the Instrument Setup tab we are presented with four channels. The science line of interest is the 12.8 μm neon line, so we select Channel 3 that spans the wavelength range 11.53-13.48 in units of μm. We leave the Disperser as the default value of 'Short'. Under the Detector Setup, there is only the FULL subarray option. From the Readout pattern we select SLOW. The number of groups is set to 7 to avoid saturation and three is the number of integrations. Leave number of exposures equal to one.

Under the Strategy tab, there are two methods: 'Nod In Scene', or 'Nod Off Scene'. We choose to 'Nod Off Scene' and use and aperture radius of 0.1". We center the calculation on the "ring" source. Once these parameters are set, we can perform the calculation. Figure 13 shows a screen grab of the ETC GUI with the MIRI MRS mode highlighted in yellow. The plot in the bottom left shows a recreation of what the scene looks like at the detector level. The signal to noise for this calculation yields a value of 110.88 in a total exposure time of 501.69 s.

Figure 13. MIRI MRS mode

Final calculation results for the MIRI Medium Resolution Spectroscopy mode.