This walk-through of the JWST Exposure Time Calculator (ETC) demonstrates how to select exposure parameters for example science program #22: NIRCam Deep Field Imaging with MIRI Imaging Parallels.

Example Science Program #22 ETC Guide

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To achieve the goals of this example science program, the ETC is used to compute signal-to-noise ratios (S/N) for a z = 8 galaxy and a z = 1.5 galaxy in all of the desired filters. Exposure times are determined by working iteratively with both the ETC and APT to define the observations. See the companion APT article for this science program: Step-by-Step APT Guide for NIRCam Deep Field Imaging with MIRI Imaging Parallels.

The ETC workbook associated with this example science program is called #22: NIRCam Deep Field Imaging with MIRI Imaging Parallels and can be selected from the Example Science Program Workbooks dropdown tab on the ETC Workbooks page. The nomenclature and reported SNR values in this article are based on ETC v. 3.2. There may be subtle differences if using a different version of ETC.

Define Scene and Sources

Words in bold are GUI menus/
panels or data software packages;
bold italics are buttons in GUI
tools or package parameters.

In the Scene and Sources tab, you can edit the sources within your scene. In the Source Editor pane, there are tabs for setting the source continuum, shape, and flux renormalization. Below are the specifications. The resulting spectra are shown in Figure 1.

Table 1. Input source parameters

Source	Continuum	Redshift	Normalization	Shape
1	Galaxy Spectra from Brown et al. (2014): UGCA 219 (Blue Compact Dwarf)	8	10 nJy at 2 µm	Extended 2D Gaussian σ_x = 0.1" σy = 0.05"
2	Galaxy Spectra from Brown et al. (2014): NGC 4552 (E w/ UV upturn)	1.5	250 nJy at 2µm	Sersic (Scale Radius) Semi-Major Axis: 0.5" Semi-Minor Axis: 0.2" Sersic index: 1.3

After creating the sources, add Source #1 to the Scene. All calculations (defined below) will be performed on that source. To perform calculations on Source #2, add that source to the scene and remove Source #1. All calculations will then automatically recalculate for Source #2.

Alternatively, you may wish to add both sources to a single scene, including spatial offsets so they don't overlap. For an example of this setup, see Step-by-Step ETC Guide for NIRISS WFSS and Parallel NIRCam Imaging of Galaxies in Lensing Clusters.

The spectra of the z = 8 galaxy (blue, fainter source) and the z = 1.5 galaxy (green, brighter source). The galaxy spectra come from Brown et al. (2014), in this case using the UGCA 219 (blue compact dwarf) template for redshift 8 and the NGC 4552 (E w/ UV upturn) template for redshift 1.5. This figure was generated by the ETC.

Define exposure calculations

The signal-to-noise estimates are carried out under the Calculations tab. This program requires NIRCam short wavelength (SW) imaging, NIRCam long wavelength (LW) imaging, and MIRI imaging calculations. For each calculation, you must specify, in the Calculations editor panel, the Scene (which was defined above), Background, Instrument Setup, Detector Setup, and Strategy. The aim will be to detect both galaxies with NIRCam exposure times from about 30 to 65 ks, where these approximate depths were pre-determined based on the numbers of high-z galaxies expected (see NIRCam Deep Field Imaging with MIRI Imaging Parallels).

The following table summarizes input parameters to be used with multiple filters.

Table 2. Input Calculation parameters

Tab	Parameter	Instrument	Value
Background	Position (Ra Dec)	NIRCam & MIRI	03:32:28 -27:48:30
Background	Background configuration	NIRCam & MIRI	Medium
Detector Setup	Readout pattern	NIRCam	DEEP8
Detector Setup	Readout pattern	MIRI	SLOWR1
Strategy	Aperture radius	NIRCam SW	0.13"
Strategy	Aperture radius	NIRCam LW	0.16"
Strategy	Aperture radius	MIRI	0.3"
Strategy	Perform Background Subtraction Using	NIRCam & MIRI	noiseless sky background

Backgrounds

See also: JWST ETC Backgrounds

The backgrounds will vary significantly depending on the location of the sky and time of year. In the Backgrounds tab, you can specify the position Ra Dec as 03:32:28 -27:48:30, which is in the center of GOODS-S. Since you don't know when the observations will be scheduled, select Medium as the Background configuration (50^th percentile for all dates with visibility).

Instrument setup

See also: JWST ETC Batch Expansions

Under the Instrument Setup tab, specify the filter used in each calculation. This program uses 10 filters: 4 NIRCam SW, 5 NIRCam LW, and one MIRI filter—one calculation is needed for each. See Table 3 for the filter specifications.

These can either be set up individually, or users can set up one and then choose to Expand over filters from the Expand menu at the top of the ETC window. This option copies the selected calculation once for every available filter (more filters than needed here). See JWST ETC Batch Expansions.

Detector setup

Under the Detector Setup tab, exposure times and specifications are defined. Shorter exposure times are needed at 2–3 µm where NIRCam imaging sensitivity is best. Longer exposure times are required at λ > 4 μm (due to lower filter throughputs and higher backgrounds) and at λ < 1 μm to significantly detect the Lyman break, distinguishing blue z ~ 8 galaxies from red z ~ 1.5 galaxies.

Here, estimate the expected S/N for galaxies in the scene to be ~30–60 ks of total exposure times with NIRCam. The F770W MIRI filter will be observed in parallel with all observations for a total of about 200 ks (see APT section), so the S/N for that exposure time is computed as well.

To set up these exposure times in the ETC, you need to choose readout patterns as well as the numbers of groups, integrations, and exposures. Use the

NIRCam DEEP8 readout pattern and MIRI's SLOWR1 readout pattern, both of which are required to reduce data volume for this long observing program.

Also limit integration times to less than 1,000 s. In longer integrations, cosmic rays will affect many pixels. See discussions in NIRCam Imaging Recommended Strategies and MIRI Recommended Strategies.

When setting up the observations in APT, you will have to adjust the final exposure times for various reasons. See Step-by-Step APT Guide for NIRCam Deep Field Imaging with MIRI Imaging Parallels. In practice, you will work iteratively with both the ETC and APT.

Ultimately, you will arrive at the exposure times and specifications shown in Table 3. Exposures refer to the number of dithers in the ETC (one exposure is executed at each dither position). The total exposure times are ~184 ks for NIRCam SW, NIRCam LW, and MIRI.

Each MIRI integration time must be less than the NIRCam integration time since it is obtained in parallel, as explained in the APT Guide.

Table 3. Input exposure specifications and resulting output S/N

Calculation	Mode	Filter	Groups	Integrations	Exposures	Total time (s)	z = 8 S/N	z = 1.5 S/N
1	nircam_sw_imaging	F090W	5	1	72	68028	6.04	0.84
2	nircam_sw_imaging	F115W	5	1	72	68028	10.85	4.04
3	nircam_sw_imaging	F150W	5	1	72	68028	15.06	11.00
4	nircam_sw_imaging	F200W	5	1	36	34014	12.76	15.25
5	nircam_lw_imaging	F277W	5	1	36	34014	12.41	31.74
6	nircam_lw_imaging	F335M	5	1	36	34014	9.97	24.31
7	nircam_lw_imaging	F356W	5	1	36	34014	16.10	39.21
8	nircam_lw_imaging	F410M	5	1	72	68028	11.64	32.81
9	nircam_lw_imaging	F444W	5	1	72	68028	16.22	40.28
10	miri_imaging	F770W	39	1	252	234791	2.26	5.36

Strategy

The Strategy tab specifies the aperture information used to measure the source photometry and compute the S/N. The Aperture radius should be large enough to encompass the source, yet not too small as to yield an overly optimistic S/N estimate. For the source at z = 8, use an aperture size of r=0.13" for the NIRCam short wavelength imaging, r=0.16" for the NIRCam long wavelength imaging, and r=0.3" for MIRI imaging. For the z = 1.5 source, larger apertures should be used given its larger angular size.

Extended sources require larger photometric apertures than point sources. The ETC default apertures will significantly underestimate signal to noise if the source size is larger than the aperture size. For faint Gaussian sources of width σ, optimal signal to noise is obtained with an aperture of radius ~1.6σ.

Allow the ETC to subtract the known (noiseless) sky background, as recommended for aperture photometry, in a range of filters (see JWST ETC Imaging Aperture Photometry Strategy). Note the sky noise is still factored into the SNR calculation, just not the subtraction of the flux. Similar results (with slightly lower SNR) are achieved by using large annuli to measure the sky background.

S/N for the z = 8 galaxy (top) and z = 1.5 galaxy (bottom) vs. wavelength in each filter with the exposure specifications listed in Table 3. These plots were generated by the ETC.

Discussion of the results

For the z = 8 galaxy, S/N > 4 at all NIRCam wavelengths, and S/N = 2 in the MIRI image. MIRI is therefore better suited to detect slightly lower redshift galaxies in this deep field. The S/N is better than about 9 for 1.5 μm < λ < 4 μm, ensuring that galaxies at z = 8 will be detected. The z = 1.5 galaxy is well detected at all wavelengths except F090W, owing to a sharp downturn in the spectral energy distribution at λ < 1 μm for the adopted galaxy template (NGC 4552).

To check how well these observations will detect a fainter galaxy at higher redshift, the z = 8 galaxy can be changed to have a redshift of 10 and normalized flux of 5 nJy. In that case, the galaxy is still detected with S/N > 3 in all filters except F090W and F770W.

Keep in mind that the depth will not be uniform across the observed field. Significant areas will have more or less exposure time than assumed here, as shown in the APT Guide.

Also note that the NIRCam and MIRI observations defined in this program do not overlap with one another. The JWST GTO JADES program will make use of shallower NIRCam imaging of a wider field to accompany their deep MIRI imaging, as shown on the main page for this example science program.

Continue the Tutorial

With the exposure parameters now determined for this program, you can populate the observation template in the Astronomer's Proposal Tool (APT). See the step-by-step APT guide to complete the proposal preparation for example science program #22.

References

Brown, M. J. I. et al. 2014, ApJS, 212, 18
An Atlas of Galaxy Spectral Energy Distributions from the Ultraviolet to the Mid-Infrared

Pontoppidan, K. M., Pickering, T. E., Laidler, V. G. et al., 2016, Proc. SPIE 9910, Observatory Operations: Strategies, Processes, and Systems VI, 991016 ,
Pandeia: a multi-mission exposure time calculator for JWST and WFIRST

Presentations from the May 2017 JWST Proposal Planning Workshop

Notable updates	11 Jun 2024 Updated for ETC v3.2 16 Dec 2019 Integration times reduced < 1000 s. Reduced data volumes with MIRI SLOW. Simplified and synced with APT. Aperture sizes increased for extended sources. 29 Apr 2019 Revisions include matching exposures to those in the APT Guide.
Originally published	14 Sep 2017

JWST User Documentation

Step-by-Step ETC Guide for NIRCam Deep Field Imaging with MIRI Imaging Parallels