Step-by-Step APT Guide for NIRCam WFSS Deep Galaxy Observations

Instructions are providing for filling out the JWST APT observing template for the NIRCam WFSS Deep Galaxy Observations Example Science Program, consisting of NIRCam WFSS observations.

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Exposure Specifications

Based on our ETC Calculations for this program, we require 8000 s of observation per pointing with GRISM F356W. Simultaneously, we will obtain NIRCam Imaging in F115W (4000 s), followed by F200W (4000 s). Each of these 4000 s observations will be further divided among the 12 dither positions, such that each exposure will be 333 s. Consulting Table 1 of NIRCam Imaging Sensitivity, we find that 6 groups of the SHALLOW4 readout pattern will optimize signal-to-noise. The total resulting exposure time will be 7473 s with the GRISM F356W, and half that for the imaging in each filter: F115W and F200W.

Following these exposures will be a direct image and two out-of-field images in the same long wavelength filter as the grism observations.

SequenceExposure TypeLong WavelengthShort WavelengthReadout PatternGroupsDithersTotal Exposure Time
1Grism & ImagingGRISMR + F356WF115W imagingSHALLOW46123736 s
2Grism & ImagingGRISMR + F356WF200W imagingSHALLOW46123736 s
3Direct ImageF356W imagingF200W imagingSHALLOW4101526 s
4Out of Field Image #1F356W imagingF200W imagingSHALLOW4101526 s
5Out of Field Image #2F356W imagingF200W imagingSHALLOW4101526 s



Mosaic Strategy

The above observation sequence will be repeated 4 times in a 2 × 2 mosaic. Various combinations of the mosaic with GRISMR and/or GRISMC are explored in APT Mosaic Strategy for NIRCam WFSS Deep Galaxy Observations. The goal is to optimize the number of sources that will yield complete spectra (not truncated by the effective field of view) and dispersed in multiple directions (to mitigate confusion from overlapping spectra of nearby sources). Ultimately, the decision is made to observe with GRISMR only for this program. Figure 2 on that page shows the ideal strategy, including observations flipped by 180°. However, that requires a ~6 month delay between two epochs of observations, as well as increased overheads charged for the additional slew to target. In practice, a similar observing strategy to be executed in 1 epoch without the 180° flip (Figure 4 on that page) is implemented in GTO program #1243.

The resulting mosaic coverage is shown below in Figure 1. It is implemented in APT as follows:

Mosaic Properties

  • Rows: 2
  • Columns: 2
  • Row Overlap: 45%
  • Column Overlap: 72%
  • Row shift: 0
  • Column shift: -1.2° (rotation)


Figure 1. Mosaic to be observed (Long Wavelength channel)


APT screenshot of the final 2 × 2 mosaic, with each tile consisting of 12 dither positions, 1 direct image, and 2 out of field images. Darker regions correspond to greater exposure time in the long wavelength channel. The short wavelength observations are not shown, for clarity.



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