(Delete) APT Mosaic Strategy for NIRCam WFSS Deep Galaxy Observations

The article describes the mosaic observing strategy to be implemented in APT for the NIRCam WFSS Deep Galaxy Observations Example Science Program.

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Choice of grism cross filter

Main article: NIRCam WFSS Field of View
See also: Step-by-Step APT Guide for NIRCam WFSS Deep Galaxy Observations

Each NIRCam module provides a set of row- and column-direction grisms (GRISMR and GRISMC, respectively). These can be used to obtain spectra of sources along different orientations to mitigate the effects of contamination. Observations obtained using multiple orientations can also be essential to unambiguously identify bonafide emission lines. Due to the reduction in the effective field of view when crossing either grism with a filter, there are no ways to assume that every source in the field can be dispersed along different directions. In Figure 1, we show one example of the reduced effective field of view for GRISMR combined with the F356W filter.

Alternatively, one can overlap the full spectrum field of view from Module A and Module B by using a large pointing offset. The overlapping field will have GRISMR spectra dispersed in opposite directions from each module. In this way, the observer can maximize the area of overlap containing full spectra dispersed in 2 directions, at the expense of reduced depth for the areas without overlap.

Figure 1. Field of view of the GRISM R/F356W combination

The NIRCam Module A and B LW detectors are shown (black squares), along with larger area covered by the NIRCam optics including the footprint of the coronagraph. The areas marked with solid colors are fields of view where full spectra are generated for sources in GRISM R/F356W observations. Sources within the hashed regions will generate truncated spectra. Positions of sources are shown as stars, and the extent and direction of their dispersed spectra are shown as black arrows. The wavelength increases from left to right in module A (left panel) and right to left in module B (right panel). Similar figures for other combinations of grisms and filters are shown in NIRCam WFSS Field of View.

APT mosaic strategy

The goal is to obtain deep spectroscopic observations of Lyα emitters around known z > 6 quasars. Each field will be covered by 4 overlapping pointings which, when combined, will be deep enough to detect faint Lyα emission lines.

Unlike other JWST instruments, the 10 NIRCam detectors (2 LW and 8 SW) are split between 2 different modules (A and B) which are separated by a large (~45") gap. In order to obtain a contiguous mosaic, 3 intramodule primary dithers are required to fill the SW detector gaps. Larger offsets are then required to fill the large gaps between the 2 LW channels. In addition to this, 4 standard subpixel dithers are added to help mitigate the effects of bad pixels as well as to improve the final spatial sampling in the final combined images. Altogether, this yields a total of 12 dithered exposures.

A larger area can be covered by observing each field 4 times and offsetting each visit by a quarter of a single LW field of view (i.e., ~60’’). By allowing the data to be taken ~6 months apart, the position angle (PA) of 2 of these 4 overlapping visits can be 180° apart, ensuring that the central area, containing the z > 5 quasars, will be observed with the GRISMR of each module pointing in different directions and the maximum depth of this survey.

There will be a small area of overlap (approximately 210 pixels (15’’) wide by 2040 pixels (132’’) high) for an area of about 0.57 arcmin2 where objects will produce full spectra in 2 different directions. This is illustrated in Figure 2 where we show a single NIRCam pointing in panel (A). Adding a second observation that is flipped 180° and offset by about half of the size of the LW field of view results in the pattern shown in panel (B). Adding another identical pair of exposures but now offset in the other direction results in what is shown in panel (C). The black arrow points to the narrow region near the center of the field where we can expect objects to produce fully dispersed spectra in both directions. 

We have decided to offset each pointing so that the 45'' gap between the module A and module B LW channels is filled. A portion of the observations will be to full depth. Other portions will be observed at either 1/2 or 1/4 depth. In this example, as well as the other example below, we show the location of a source as a black dot in each module and the resulting dispersed spectrum as a black line. The source location corresponds to a wavelength of approximately 4 μm. The spectra span a range of 3.1–4.1 μm, as F356W is being used. We use different hashed patterns so that we can readily see overlapping observations taken using different spectral dispersion directions. The hash patterns are arbitrary and are not related to the actual dispersion direction but are useful to distinguish the different dispersion orientations. 

Figure 2. Observation pattern using GRISM R only

Observation pattern using 4 GRISM R pointings. Different dispersion directions are indicated using different shading so that areas where objects will be observed using multiple dispersion directions can be readily seen. Panel (A) shows a single pointing. Panel (B) shows 2 pointings, offset by 1/2 of the field of view (FOV) of a single LW channel and flipped 180°. Panel (C) shows the final, 4-pointings mosaic. There is a small portion of the field which is observed in all 4 pointings and using 2 different dispersion directions (black arrow). It is in the center of a shallower band which is observed in 2 of the 4 pointings but again using 2 different dispersion directions.

Alternative strategies considered

It is difficult to obtain a contiguous mosaic of dispersed spectra with different dispersion directions. We illustrate this further by showing alternative possible choices using GRISM C observations (Figure 3), an alternative use of the GRISM R (Figure 4), and finally using a combination of GRISM R and C observations (Figure 5). The gap between the 2 modules and the relative directions of the dispersed spectra in each module result in mosaics with either non-contiguous coverage or regions where objects will only produce spectra dispersed in a single direction. To satisfy our science case, we picked the pattern shown in Figure 2 because it gives the best compromise between a deep uniform coverage of the field around the quasars while also allowing multiple orientation slitless spectroscopy at the center of the field.

Figure 3. Observation pattern using GRISM C only

Observation pattern using 4 GRISM C pointings. Using sets of offsets GRISM C exposures flipped 180° would allow us to obtain contiguous coverage of our field, but in this particular example the deepest coverage would be on each side of the center of the field. Panel (A) shows a single GRISM C/F356W exposure. Panel (B) shows how 2 exposures, flipped 180° and offset to overlap the effective field of view would allow us to obtain different slitless dispersion for every source. Panel (C) shows an option were we repeat the exposures shown in Panel (B) but offset them to cover the gap between Module A and Module B.
Figure 4. Observation pattern using GRISM R to increase number of sources observed with multiple dispersion directions

In this example, we again only use the GRISM R grisms (Panel A). We however offset a second exposure so that the fields of view of module A and B overlap. We then add another 2 exposures to reach our full depth (Panel C). This example has the advantage of allowing for a larger area where multiple dispersion directions will be used but has the disadvantage of not producing a contiguous set of observations.
Figure 5. Observation pattern combining GRISM R and GRISM C

In this example, we combine a GRISM R and GRISM C exposure, (Panel A). The area of overlap of the 2 grisms is relatively small, covering about 1/4 of the field. We can however do the same thing by observing the same field with a position angle that is flipped 180°, (Panel B). By offsetting these 2 sets of observations we can obtain spectra with multiple dispersion directions for a large portion of each module. As shown in Panel C, approximately half of the field will be observed using different dispersion directions.



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