NIRCam Parallel Imaging and NIRISS WFSS of Galaxies in Lensing Clusters

Example Science Program #33

This example science program provides a walk-through of developing a JWST observing program using NIRISS Wide Field Slitless Spectroscopy (WFSS) as a prime observing mode and NIRCam Imaging as a coordinated parallel mode. 

See also: Step-by-Step ETC Guide for NIRISS WFSS and Parallel NIRCam Imaging of Galaxies in Lensing Clusters,
Step-by-Step APT Guide for NIRISS WFSS and NIRCam Parallel Imaging of Galaxies in Lensing Clusters

The overarching science goals are from the GTO program "NIRISS Unbiased Cluster Survey (CANUCS)" to provide context for navigating the Exposure Time Calculator and setting up the observation templates in the Astronomers Proposal Tool. 

The CANUCS program is designed to study galaxies within and lensed by galaxy clusters. The main science goals are to:

  • measure physical properties (star formation rates, metallicities, abundances) of dwarf galaxies from 1 < z < 5;
  • spatially resolve emission lines, metallicities, and star formation rates of lensed galaxies at > 1;
  • detect extreme emission line galaxies from 1 < < 8 and determine their evolution in their numbers and their properties;
  • discover and characterize galaxies in the era of reionization (z > 7) via spectral diagnostics (continuum breaks, Lyα emission lines; broad-band dropouts).

The CANUCS program will observe five strong lensing galaxy clusters from the HST Frontier Fields. For illustrative purposes, we focus on one of these clusters for this example science program.



Step 1 - Choose the instrument to use for the science case (NIRISS, NIRCam or both), based on the wavelength coverage

Main article: JWST Slitless Spectroscopy Roadmap
See also: NIRISS Wide Field Slitless SpectroscopyNIRCam Wide Field Slitless SpectroscopyCoordinated Parallels Roadmap

NIRISS WFSS is the only JWST observing mode that permits slitless spectroscopy between 0.8–2.2 μm and is well-suited to measure emission lines from a large sample of galaxies over a range of redshifts.  A direct image is taken before and after each set of dithered grism exposures to determine object positions, define the wavelength zeropoint of the dispersed spectra, and to facilitate modeling of spectral overlaps (i.e., "contamination"). 

NIRCam imaging, executed as a coordinated parallel observation in this program, is designed to detect galaxies with strong emission lines based on their medium-band filter excesses in regions several arcminutes away from and adjacent to the lensing clusters (here "adjacent" is defined by the relative locations of NIRISS and NIRCAM in the JWST focal plane and the spacecraft roll angle).



Step 2- Choose the blocking filters that cover the wavelengths of interest

Main article: NIRISS FiltersNIRCam Filters

The CANUCS program will observe with the F115W, F150W, and F200W NIRISS filters to optimize emission line and wavelength coverage. In this example program, the parallel field will be observed with 12 wide and medium NIRCam filterssix in the short wavelength channel (F090W, F115W, F150W, F182M, F210M, F140M) and six in the long wavelength channel (F277W, F250M, F300M, F335M, F360M, F410M)



Step 3 - Check the direct image and grism (line and continuum) sensitivities in the WFSS mode(s) of interest

Main article: NIRISS SensitivityNIRCam Sensitivity

Use of the Exposure Time Calculator (ETC) is required for the most accurate signal-to-noise ratio (SNR) calculations for an observing program. Based on the predicted performance of NIRISS and NIRCam, the sensitivity limits to achieve a SNR of 10 for a 10 kilosecond observation can be estimated. Users are referred to the NIRISS Sensitivity and NIRCam Sensitivity articles for more information.



Step 4 - Choose one or both of the orthogonal grisms

Main article: NIRISS GR150 Grisms

This program will observe a galaxy cluster field, which is crowded. Thus the use of both orthogonal GR150R and GR150C grisms is critical for removing contamination from overlapping spectra. 



Step 5 - Decide on dither pattern

Main articles: NIRISS WFSS Recommended StrategiesDithers Customized for Coordinated Parallel Observations
See also: NIRISS DithersNIRISS WFSS Dithers 

Since the NIRISS WFSS point spread function (PSF) is undersampled, dithering of NIRISS grism exposures is required to properly sample the PSF and to mitigate bad pixels. As discussed in the NIRISS WFSS Recommended Strategies, the MEDIUM dither step size (~0.6") is optimal for extragalactic studies of the moderate-to-high redshift universe and is thus appropriate for this example science program.

Since this program is a coordinated parallel observation, we choose a custom dither pattern that is designed to provide optimal pixel phase sampling for both the primary and parallel instrument. A 9-step dither strategy is chosen to provide increased photometric depth in individual exposures and improve PSF sampling by increasing astrometric and photometric precision. In the Astronomer's Proposal Tool, this dither pattern is called 9-POINT-MEDIUM-NIRCam.

We note that the CANUCS program uses the standard NIRISS WFSS 8-step MEDIUM dither pattern, while we use the custom dither pattern created for coordinated parallel observations for illustrative purposes.



Step 6 - Decide whether mosaicking is required to cover the target field for the science program

Main Articles: NIRISS Mosaics

The primary science goals of this example program require the spectra of distant galaxies that are lensed by the cluster core region which lies within the 2.2' x 2.2' FOV of the NIRISS detector. The ancilliary NIRCam imaging addresses the science goals related to the galaxies in the cluster outskirts, along with selection of emission line galaxies through the medium band imaging. Mosaicking or multiple pointings is not essential for this program, unlike observing programs where a wide area coverage is important to address the science goals.



Step 7 - Decide the readout pattern to use

Main articles: NIRISS Detector Readout PatternsNIRCam Detector Readout Patterns
See also: JWST APT Coordinated Parallel Observations

NIRISS offers two readout patterns: NIS (four frames are averaged in a group) and NISRAPID (one frame per group). NIS is the preferred readout pattern for long observations and is thus used for this program.

Since NIRCam imaging observations are performed as a coordinated parallel observation to the NIRISS WFSS observations, the NIRCam readout patterns are chosen to best match the available observing time to the contemporaneous NIRISS observation element (see Step-by-Step APT Guide for NIRISS WFSS and NIRCam Parallel Imaging of Galaxies in Lensing Clusters). For the direct imaging part of the NIRISS WFSS exposure, a NIRCam readout pattern of MEDIUM8 is chosen for the parallel NIRCam imaging observations since the NIRISS direct imaging exposures are relatively short.  For the NIRCam imaging observations that are performed parallel to the NIRISS grism exposures, the DEEP8 readout pattern is chosen since this readout pattern is optimized to provide the highest signal-to-noise data for the faintest objects and the parallel NIRISS grism exposures are long enough to support this contemporaneous NIRCam readout pattern.



Step 8 - Use the Exposure Time Calculator (ETC) to determine the exposure parameters for the direct images and for the dispersed images from the grisms

To determine the exposure parameters for this observation using the Exposure Time Calculator (ETC), please see the article Step-by-Step ETC Guide for NIRISS WFSS and Parallel NIRCam Imaging of Galaxies in Lensing Clusters.



Step 9 - Fill out the Astronomers Proposal Tool (APT)

For details filling out the Astronomers Proposal Tool (APT) for this example science program, please see the article Step-by-Step APT Guide for NIRISS WFSS and NIRCam Parallel Imaging of Galaxies in Lensing Clusters.



References

CANUCS program information

http://www.stsci.edu/jwst/observing-programs/program-information?id=1208



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