NIRCam Imaging APT Template

Instructions for designing JWST NIRCam imaging observations using APT, the Astronomer's Proposal Tool.

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Main article: NIRCam Imaging
See also: Imaging RoadmapNIRCam Imaging Recommended StrategiesNIRCam Deep Field Imaging with MIRI Imaging ParallelsNIRCam Parallel Imaging and NIRISS WFSS of Galaxies in Lensing Clusters

Imaging is one of the 5 NIRCam Observing Modes. Each mode has a corresponding template in the Astronomers Proposal Tool, APT, for users to design their observing programs. Instructions for NIRCam Imaging are given below. Complete listings of allowed values are documented in the NIRCam Imaging Template parameters page.



Step-by-step APT instructions

Generic

The following parameters are generic to all templates, and are not discussed in this article: observation Number, observation Labelobservations CommentsTarget name, ETC Wkbk. Calc ID (in the Filters dialog box), Mosaic Properties, and Special Requirements.

Module

Main article: NIRCam Modules

Under the NIRCam Imaging* tab, set Module to ALL to observe with both NIRCam modules, returning data from the full field of view using all 10 detectors. To obtain data from only a single module or smaller detector subarrays, select Module B.

Subarray

Main article: NIRCam Detector Subarrays

When using Module B, users may opt to either read out all 5 detectors completely (FULL) or the smaller detector subarrays. Subarrays are read out more quickly than the full detector, providing brighter saturation limits for a given number of reads (see below). The available subarrays are shown in Figure 1 and listed in Table 1.

The central subarrays were designed for bright extended sources (e.g., Jupiter or large star-formation regions). They return data from 4 short wavelength detectors and one long wavelength detector, with similar combined areas on the sky in each wavelength channel. Note that there is a gap in the coverage in the short wavelength channel; these gaps can be covered with dithering or mosaics (see below). The corner subarrays are designed for small objects (named "P" for point source) and return data from just 2 detectors, one from each wavelength channel.

Figure 1. NIRCam detectors and imaging subarrays

Layout in the NIRCam field of view of the 10 detectors and Module B subarrays. The short and long wavelength channels are shown in blue and red, respectively. Note each long -wavelength subarray covers 4 times the area as the corresponding short-wavelength subarray with the same number of pixels, because the short-wavelength pixels deliver twice the spatial resolution along each axis.
Table 1. Imaging subarrays

Imaging
subarray
Size in pixels
Nrows × Ncolumns
Short wavelength
FOV (each side)
Long wavelength
FOV (each side) 
Frame
time (s)
Noutputs
FULL2048 × 20482 × 64" + 4–5" gap129"10.736774
SUB640
640 × 6402 × 19.9" + 4–5" gap40.4"4.185841
SUB320320 × 3202 × 9.9" + 4–5" gap20.2"1.069041
SUB160160 × 1602 × 5.0" + 4–5" gap10.1"0.278641
SUB400P400 × 40012.4"25.0"1.656241
SUB160P160 × 1605.0"10.0"0.278641
SUB64P64 × 642.0"4.0"0.050161

Subarrays ending in "P" only use a single detector in the short wavelength channel. The other subarrays use all four short wavelength detectors; the resulting images include 4–5" gaps along the center of both axes.

The overlapping area between the short wavelength SUB64P subarray and the long wavelength SUB64P subarray is smaller than JWST's 2-σ pointing accuracy. Set the Primary Dither Type parameter to SUBARRAY DITHER to increase the spatial coverage and ensure the target is observed in both channels.


* Bold italics style indicates words that are also parameters or buttons in software tools (like the APT and ETC). Similarly, a bold style represents menu items and panels.



Dither parameters

Dithering (multiple exposures with shifted overlapping pointings) is required for NIRCam imaging. Dithering mitigates bad detector pixels and improves flat-fielding and PSF sampling by imaging each portion of sky with multiple regions of the detector. Larger primary dithers and smaller subpixel dithers also serve additional purposes described below.

Primary dithers

Main article: NIRCam Primary DithersNIRCam Subarray Primary Dithers
See also: NIRCam Imaging Recommended Strategies: Dither pattern

Primary dithers serve to fill gaps in sky coverage in the field of view between the detectors and to mitigate flat field uncertainties. The primary dither patterns were designed for different purposes (and the choice is restricted depending on the module selected). 

  • FULL (Module = ALL): Large fields, covering all gaps between detectors and the ~45" gap between modules. Designed for use with mosaics (tiled pointings) of larger areas, but inefficient, requiring visit splitting and large overheads (see below).

  • FULLBOX (Module = ALL): More efficient than FULL. Covers a rectangular field of view without gaps when performing 4 or more dithers.

  • INTRAMODULE (Module = ALLor B): Targets smaller than the field of view of a single module (< 110" across). Covers the 5" gaps between the short-wavelength detectors, but not the 44" gap between modules.

  • INTRAMODULEX (Module = ALLor B): Similar to INTRAMODULE, but more efficient when performing 4 or more dithers.

  • INTRAMODULEBOX (Module = ALL or B): Covers two square regions without gaps when performing 4 dithers. More compact than INTRAMODULE(X), yielding more area with maximal depth.

  • INTRASCA (Module = B): Compact targets that can be imaged at the four corners of the detector (and in between) to mitigate flat field uncertainties. The object size should be smaller than the the field covered by an individual detector (<50" or <100" across for short or long-wavelength observations, respectively).

  • SUBARRAY_DITHER (Module = B; designed primarily for SUB64P): NIRCam Subarray Primary Dithers, spanning about ±0.6" in each axis (x and y), were designed to ensure that sources are observed in both wavelength channels when using SUB64P.

  • NONE (Module = ALLor B): Only recommended if used only in conjunction with an appropriate mosaic or subpixel dithering (see below).

When subarrays (smaller than the full detectors) are used, then fewer primary dither options are available:

  • INTRASCA (SUB640, SUB320, SUB160): as described above, available for extended-source subarrays only

  • INTRAMODULEBOX: as described above, available for all subarrays.

  • SUBARRAY_DITHER (designed primarily for SUB64P): NIRCam Subarray Primary Dithers, spanning about ±0.6" in each axis (x and y), were designed to ensure that sources are observed in both wavelength channels when using SUB64P.

  • NONE: only recommended in conjunction with subpixel dithering (see below)

When dithering, consider the Slew Times and Overheads. Note that the FULL dithers always require visit splitting (new guide star acquisitions), increasing overheads significantly.

Dithering results in uneven depth (exposure time) across the final combined image. Only the INTRASCA dither patterns are designed to provide uniform depth (full coverage in every exposure) across the small area containing the science target.

Figure 2. Example NIRCam primary dither patterns

Exposure map for dithered observations in the short-wavelength channel. Corresponding long-wavelength observations are not shown. [Figures appear in three columns in a wide browser window.]

1) Top left: FULL with 3TIGHT Primary Dithers
2) Top right: FULL with 9 Primary Dithers and a Mosaic of 3 rows × 3 columns (or greater)
3) Middle left: FULLBOX with 6TIGHT Primary Dithers
4) Middle right: INTRAMODULEX with 4 Primary Dithers
5) Bottom left: INTRAMODULEBOX with 4 Primary Dithers
6) Bottom right: INTRASCA with 5 Primary Dithers. The science target should fit within one of the larger black regions.

Subpixel dither type

Main article: NIRCam Subpixel Dithers

Smaller subpixel dither patterns include subpixel shifts designed to optimally improve image sampling and resolution. This is especially useful for images with undersampled PSFs below the Nyquist wavelengths: 0.6–2 µm in the short wavelength channel and 2.4–4 µm in the long wavelength channel. Subpixel dithering is not implemented by default but may be added to an observing program by selecting more than one subpixel position.

In addition to the standard subpixel dither patterns, more compact small grid dither (SGD) patterns are also offered. SGD patterns are executed more quickly and precisely using the fine steering mirror (FSM) without slewing the telescope. They are limited in size to 0.06" or less, and are expected to be 2 times more precise than regular subpixel dithers. New SGD patterns were introduced in APT 25.4. They preserve the optimal subpixel sampling of the original pattern, while reducing overheads but offering less mitigation of bad pixels. Prior to APT 25.4.1, the SGD patterns designed for coronagraphy were available for imaging as well.

When NIRCam imaging is used as prime mode in a coordinated parallel combination, additional customized subpixel dither patterns become available which work well for both NIRCam Imaging and the parallel instrument mode. The NIRCam-specific subpixel dither patterns can still be selected in that case by specifying “NIRCam Only”, which results in an additional pull-down selector “NIRCam Positions” that can be used to specify the number of subpixel positions.



Filters and exposures

Main article: NIRCam FiltersUnderstanding JWST Exposure Times

NIRCam uses a dichroic to observe simultaneously in the short wavelength channel (0.6–2.3 µm) and long wavelength channel (2.4–5.0 µm). For each exposure, users select 2 NIRCam filters (one in each channel) as well as parameters which control the exposure time (identically for both channels) via readout patterns, which serve to reduce data volume. The exposure hierarchy is:

  • Exposure – with chosen filter pair; all instruments stationary and telescope locked on target
    • Integrations – each ends in a detector reset
      • Groups – saved data: average of one or more reads
        • Reads – non-destructive; charge continues to accumulate

Figure 3. NIRCam filters

Preliminary total system throughput for each NIRCam filter. Filters marked "P" are located in the pupil wheel, requiring transmission through a second filter in the filter wheel, either F150W2, F322W2, or F444W. In these cases, the combined transmissions are plotted. Use of F150W2 on its own (with a CLEAR pupil wheel filter) is discouraged. High photometric precision will not be supported in F150W2 due to the transmission dips and the variable PSF across the wide wavelength range.


Readout patterns

Main article: NIRCam Detector Readout Patterns
See also: NIRCam Imaging Recommended Strategies: Readout pattern

Use the Exposure Time Calculator (ETC) to determine which readout pattern, number of groups, and number of integrations will avoid saturation and/or achieve the signal-to-noise required for your science.

Multiple integrations may be most useful for brighter sources to avoid saturation. However for fainter sources, multiple dithers are generally preferable, with one integration per exposure per dither position.

Multiple groups are desirable to enable "up-the-ramp" fitting to observed count rates. This facilitates cosmic ray rejection, reduces the effective read noise for the integration, and increases the dynamic range of the final image (sampling count rates of bright sources before they saturate).

The 9 readout patterns are detailed below, including integration times for full detectors, which are read out in 10.73677 s. Subarray integration times are shorter. Five group sizes are designed for short to long integrations: RAPID, BRIGHT, SHALLOW, MEDIUM, and DEEP. Based on current assumptions, RAPID, BRIGHT2, SHALLOW4, MEDIUM8, and DEEP8 are recommended as yielding higher signal to noise for faint sources  (Robberto 2009, 2010; and more recent tests with the ETC).


Table 2. NIRCam Readout patterns and rounded integration times for full detectors

Readout pattern
Reads per group
Frames co-added in each group
Time of first group (s)Time of each subsequent group (s)
Groups per integration
Integration time (s)
RAPID1110.710.71–2 (Module = ALL)
1–10 (Module = B)
10.7–21.5
10.7–107
BRIGHT12110.721.51–1010.7–204
BRIGHT22221.521.52–10 (Module = ALL)
1–10 (Module = B)
42.9–215
21.5–215
SHALLOW25221.553.71–1021.5–505
SHALLOW45442.953.71–1042.9–526
MEDIUM210221.5107.41–1021.5–988
MEDIUM810885.9107.41–1085.9–1052
DEEP220221.5214.71–2021.5–4101
DEEP820885.9214.71–2085.9–4166

The RAPID pattern is limited to 2 groups per integration (to limit the data rate) when reading out the full detectors in both modules. This limit increases to 10 groups when using a single module. DEEP2 and DEEP8 allow up to 20 groups. Scroll right to view full table, if needed.



Mosaics

Main article: NIRCam Mosaics

Mosaics are used primarily to cover areas larger than the 5.1' × 2.2' NIRCam field of view (including the ~45" gap between modules). For NIRCam mosaics, the spatial extent of each tile is defined as 5.115033' × 2.221150' in APT. Tile overlaps as well as shifts (pattern rotation angle) may be adjusted for each axis. Primary dithers should be used in conjunction with mosaics to fill the gaps between detectors.

FULLBOX primary dithers are recommended for this purpose. If these patterns would split visits, then INTRAMODULEBOX should be considered as well.

A less efficient option is the original FULL pattern, which was designed specifically to yield roughly even observing depth over large areas with mosaics. Use FULL with mosaic tile spacings of 5.8' × 2.25', input as –13.4% column overlap and—1.3% row overlap in APT. The negative overlap leaves a gap between each tile; these gaps are filled by the FULL dither pattern. This strategy was designed by Anderson (2009) before the observing overheads were known. More efficient mosaics are possible using other dither patterns, though these provide less even depth across the field. Additional background information on principles of dithered observations with JWST are also described in Koekemoer & Lindsay (2005), Anderson (2011)Anderson (2014), and Coe (2017).

Figure 4. NIRCam Mosaic Examples

Examples of 3 × 3 NIRCam mosaics combined with primary dither patterns. Top: FULLBOX 6TIGHT with 0% overlap in rows and columns. Bottom: FULL 3TIGHT with –1.3% and –13.4% overlap in rows and columns, respectively. This yields relatively uniform depth across the field, at the expense of significantly higher overheads.


References

Anderson, J. 2009, JWST-STScI-001738
Dither Patterns for NIRCam Imaging

Anderson, J. 2011, JWST-STScI-002199
NIRCam Dithering Strategies I: A Least Squares Approach to Image Combination

Anderson, J., 2014, JWST-STScI-002473
NIRCam Dithering Strategies II: Primaries, Secondaries, and Sampling

Coe, D. 2017, JWST-STScI-005798
More Efficient NIRCam Dither Patterns

Koekemoer, A. M. & Lindsay, K. 2005, JWST-STScI-000647
An Investigation of Optimal Dither Strategies for JWST

Robberto, M., 2009, JWST-STScI-001721
NIRCAM Optimal Readout Modes

Robberto, M., 2010, JWST-STScI-002100
NIRCAM Optimal Readout II: General Case (Including Photon Noise)




Published

 

Latest updates
  •  
    Added Subarray Primary Dithers

  •  
    Added new dither patterns available in APT 25.4.

  •  
    Clarified that Primary Dithers NONE may be recommended in some cases.