Step-by-Step ETC Guide for NIRCam Deep Field Imaging with MIRI Imaging Parallels
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.
To achieve the goals of this example science program, we will use the ETC 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. 1.5.1. There may be subtle differences if using a different version of ETC.
Define Scene and Sources
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
|1||Galaxy Spectra from Brown et al. (2014):|
UGCA 219 (Blue Compact Dwarf)
|8||10 nJy at 2 µm|
Extended 2D Gaussian
|2||Galaxy Spectra from Brown et al. (2014):|
NGC 4552 (E w/ UV upturn)
|1.5||250 nJy at 2µm|
Sersic (Scale Radius)
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.
Define Exposure Calculations
Main article: JWST ETC Calculations Page Overview
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, we must specify, in the calculations editor panel, the Scene (which we 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 the input parameters to be used with multiple filters. Below, we justify these selections and provide additional input parameters for the individual filters.
Table 2. Input Calculation Parameters
|Background||Position (Ra Dec)||03:32:28 -27:48:30|
|Detector Setup||Readout pattern||NIRCam||DEEP8|
|Detector Setup||Readout pattern||MIRI||SLOW|
|Strategy||Aperture radius||NIRCam SW||0.13"|
|Strategy||Aperture radius||NIRCam LW||0.16"|
|Strategy||Perform Background Subtraction Using||noiseless sky background|
Main article: JWST ETC Backgrounds
The backgrounds will vary significantly depending on the location of the sky and time of year. In the Backgrounds tab, we can specify the position Ra Dec as 03:32:28 -27:48:30, which is in the center of GOODS-S. Not knowing when the observations will be scheduled, we select Medium as the Background configuration (50th percentile for all dates with visibility).
Main article: 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 1 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 Expansion.
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, we will estimate the expected S/N for the galaxies in our scene for ~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 we compute the S/N for that exposure time as well.
To set up these exposure times in ETC, we need to choose readout patterns as well as the numbers of groups, integrations, and exposures. We use the NIRCam DEEP8 Readout pattern and MIRI's SLOW Readout pattern, both of which are required to reduce data volume for this long observing program.
We also limit integration times to less than 1,000 s. In longer integrations, the majority of pixels would be affected by cosmic rays. See discussions in NIRCam Imaging Recommended Strategies and MIRI Recommended Strategies.
When setting up the observations in APT, we 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, we work iteratively with both the ETC and APT.
Ultimately, we 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 obtained in parallel, as explained in the APT Guide.
Table 3. Input exposure specifications and resulting output S/N
|z = 8|
|z = 1.5|
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 point sources, we recommend apertures with radii of 2.5 pixels, where the pixel sizes are 0.031" (NIRCam SW), 0.063" (NIRCam LW), and 0.11" (MIRI). For the extended source at z = 8 considered here, we require a larger aperture size for the NIRCam short wavelength imaging (r = 0.13" instead of 0.08"). For the z = 1.5 source, still larger apertures should be used.
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.
We 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.
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 and F115W, 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
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