Step-by-Step APT Guide for NIRCam WFSS Deep Galaxy Observations
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.
|Sequence||Exposure Type||Long Wavelength||Short Wavelength||Readout Pattern||Groups||Dithers||Total Exposure Time|
|1||Grism & Imaging||GRISMR + F356W||F115W imaging||SHALLOW4||6||12||3736 s|
|2||Grism & Imaging||GRISMR + F356W||F200W imaging||SHALLOW4||6||12||3736 s|
|3||Direct Image||F356W imaging||F200W imaging||SHALLOW4||10||1||526 s|
|4||Out of Field Image #1||F356W imaging||F200W imaging||SHALLOW4||10||1||526 s|
|5||Out of Field Image #2||F356W imaging||F200W imaging||SHALLOW4||10||1||526 s|
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:
- Rows: 2
- Columns: 2
- Row Overlap: 45%
- Column Overlap: 72%
- Row shift: 0
- Column shift: -1.2° (rotation)