Instructions for designing JWST NIRCam imaging observations using APT, the Astronomer's Proposal Tool, are provided in this article.

See also: NIRCam Imaging, Imaging Roadmap, NIRCam Imaging Recommended Strategies, NIRCam Deep Field Imaging with MIRI Imaging Parallels, NIRCam 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. Step-by-step APT instructions for NIRCam imaging are given below. Complete listings of allowed values are documented in the NIRCam Imaging Template parameters page.

 

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Generic parameters

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

After selecting Template: NIRCam Imaging, the Coordinated Parallel checkbox appears on the form.

Coordinated Parallel checkbox

See also: JWST Parallel Observations

NIRCam imaging supports coordinated parallel observations with MIRI or NIRISS. When the Coordinated Parallel checkbox is checked, one of these 3 parameter values can be selected:

1. NIRCam-MIRI Imaging
2. NIRCam-NIRISS Imaging
3. NIRCam Imaging-NIRISS WFSS

Note that the default option is None Selected.

The lower part of the form, following the diagnostic boxes about Visit Splitting, Duration, and Data Volume, presents 4 tabs for detailed observation parameters. They are described below. 

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NIRCam Imaging tab

Module

See also: 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

See also: NIRCam Detector Subarrays

When using module ALL, all 10 detectors are completely read out and Subarray is set to FULL by default. 

When using module B, users may opt to either read out all 5 detectors completely (FULL) or one of the 7 smaller detector subarrays. Subarrays are read out more quickly than the full detector, providing brighter saturation limits for a given number of reads and smaller data volume (see below). The locations of the subarrays cannot be changed; they are shown in Figure 1 and listed in Table 1.

The 3 central subarrays, SUB160, SUB320 and SUB640, are designed for bright extended sources (e.g., Jupiter or large star-formation regions). They return data from the 4 short wavelength detectors and the long wavelength detector of module B, with similar combined areas on the sky. Note that there are gaps in field coverage for the short wavelength channel; these gaps can be filled with dithering or mosaics (see below).

The 3 corner subarrays, SUB64P, SUB160P and SUB400P,  are designed for small objects (subarray names are postfixed with "P" for point source) and return data from just 2 detectors, B1 for the short wavelength and B5 for the long wavelength channel. 

A 7th subarray, FULLP, covers the full area of B1 and B5, with the target placed in the upper right region where image quality and detector response are expected to be optimal. 

Table 1. Imaging subarrays

Target Placement

See also: NIRCam Apertures, NIRCam Imaging Recommended Strategies

When observing with both NIRCam modules (Module = ALL), the science target may be centered at the following locations defined by Target Placement:

• Module Gap (default) – centered in the ~44” gap between the modules in the field of view
• Module A (A3 corner) – near the center of module A and the corner of the A3 detector
• Module B (B4 corner) – near the center of module B and the corner of the B4 detector

These locations are shown on NIRCam Apertures and designated ALL, As, and Bs, respectively. The pointing may be tweaked by combining with a special requirement Offset.

Dither Parameters

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

Primary dithers

See also: NIRCam Primary Dithers, NIRCam Subarray Primary Dithers, NIRCam Imaging Recommended Strategies: Dither pattern

Primary dithers serve to fill gaps in the field of view between the detectors and to mitigate flat field uncertainties. The primary dither patterns are optimized for different science goals (and the choice is restricted depending on the module selected). The primary dithers are labeled according to the following Primary Dither Type parameters:

• FULL (only for Module = ALL)
  • for large field targets, large moves allow coverage of all gaps between detectors as well as the ~45" gap between modules
  • optimal for use with mosaics (tiled pointings) of larger areas, but inefficient. Because of the large telescope moves, guide star reacquisition is necessary, causing visit splitting and large overheads (see below)
  • There is a choice between 9 types of FULL primary dithers, depending on the number of telescope moves: 3TIGHT, 3, 6, 9, 15, 21, 27, 36, 45.

• FULLBOX (for Module = ALL) 
  • more efficient than FULL
  • covers a rectangular field of view without gaps when performing 4 or more dithers; recommended for wide area mosaics (see below)
  • There is a choice between 8 types of FULLBOX primary dithers, depending on the number of telescope moves: 2TIGHTGAPS, 3TIGHTGAPS, 4TIGHT, 4, 5TIGHT, 6TIGHT, 6, 8NIRSPEC

• INTRAMODULE (for Module = ALL or B)
  • for 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
  • There is a choice between 8 types of INTRAMODULE primary dithers, depending on the number of telescope moves: 3, 4, 6, 8, 12, 16

• INTRAMODULEBOX (for Module = ALL or B)
  • covers two square regions without gaps when performing 4 dithers
  • more compact than INTRAMODULE or INTRAMODULEX , yielding more area with maximal depth
  • There is a choice between 15 types of INTRAMODULEBOX primary dithers, corresponding to any number of telescope moves between 2 and 16

• INTRAMODULEX (for Module = ALL or B)
  • similar to INTRAMODULE, but more efficient when performing 4 or more dithers
  • Also in this case there a choice between 15 types of INTRAMODULEX primary dithers, corresponding to any number of telescope moves between 2 and 16

• INTRASCA (for Module = ALL or B)
  
  • compact targets that can be imaged at the 4 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)
  • There is a choice between 8 types of INTRASCA primary dithers, depending on the number of telescope moves: 2, 3, 4, 5, 7, 9, 13, 17, 25

• NONE (for Module = ALL or B) 
  • avoiding  primary dithers is only recommended if used in conjunction with appropriate mosaic or subpixel dithering (see below).

The seven Primary Dithers listed above are available when MODULE = ALL and SUBARRAY = FULL (the only choice with MODULE = ALL) are selected.

When MODULE = B is selected, regardless on the subarray used, the primary dithers FULL and FULLBOX are not available; however, a new Primary Dither Type is added:

• SUBARRAY_DITHER (only for Module = B; designed primarily for SUB64P) 
  • allows to select on of the NIRCam subarray primary dithers, spanning about ±0.6" in each axis (x and y). Designed to ensure that sources are observed in both wavelength channels when using SUB64P
    
  • There is a choice between 3 types of Subarray Dithers, with Primary Dithers values of 2, 3, and 4.

When module B and subarrays smaller than the full detectors (SUB-type) are used, fewer primary dither options are available:

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

JWST offers a large range of possibilities; the user can acquire familiarity with the different options using the View in Aladin tool in APT. 

When dithering, besides field coverage, also consider the JWST slew times and overheads. Note in particular that FULL dithers always require visit splitting (new guide star acquisitions), and the associated overheads significantly decrease observing efficiency.

Dithering results in uneven depth (exposure time) across the final combined image. The INTRASCA dither patterns are especially designed to provide uniform depth (full coverage in every exposure) across a small area centered on a compact science target.

Subpixel Dither Type

See also: 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, i.e., for filters falling in the 0.6–2 µm region 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, with Subpixel Dither Type  = STANDARD,  Subpixel Positions > 1  (up to 64).

In addition to the standard subpixel dither patterns, selecting Subpixel Dither Type = SMALL-GRID-DITHER allows users to choose more compact small grid dither (SGD) patterns. SGD patterns are executed more quickly and precisely using the fine steering mirror (FSM), i.e., 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. The maximum number of FSM moves is 9.

When NIRCam imaging is used as prime mode in a coordinated parallel combination (when the Coordinated Parallel box is checked), additional customized subpixel dither patterns (e.g., Subpixel Dither Type = 3-POINT-WITH-MIRI-F560W) become available which work well for both NIRCam imaging and the parallel instrument mode. Still, a NIRCam-specific subpixel dither patterns can be selected 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 (Filters entry window)

See also: NIRCam Filters, Understanding 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 must select 2 NIRCam filters: Short Filter and Long Filter.

The parameters setting the exposure time will be identical for both channels). The first parameter to set is one of the 9 readout patterns. The remaining parameters detail the exposure following this hierarchy:

• Exposure—with chosen filter pair; all instruments stationary and telescope locked on target. 
  • Integrations—an exposure can contain one or more integrations. An integration starts with a detector reset
    • Groups—integrations are composed of multiple groups. These are the data transmitted to the ground. A group can correspond to one read or result from the average of multiple reads (either 2, 4 or 8).
      • Reads—a single sampling of all pixel data (either full array or subarray), taken in non-destructive mode; after a read the charge continues to accumulate, until a final reset.

These parameters are entered in the Filters window specifying 

• Groups/int: number of groups per integration
• Integrations/Exp: number of integrations in an exposure

The fields Total Dithers, Total Integrations, and Total Exposure Time are presented to summarize the configuration of the observation in the selected pair of filters.

Readout patterns

See also: NIRCam Detector Readout Patterns, NIRCam Imaging Recommended Strategies: Readout pattern

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 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 11 readout patterns are detailed below, including integration times for full detectors, which are read out in 10.73677 seconds. Subarray integration times are shorter. Six group sizes are designed for short to long integrations: RAPID, BRIGHT, SHALLOW, MEDIUM, MEDIUMDEEP, and DEEP. Based on current assumptions, RAPID, BRIGHT2, SHALLOW4, MEDIUM8, MEDIUMDEEP8, 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

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.

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Other tabs

Mosaic Properties

See also: NIRCam Mosaics
See also:  Specifying Mosaics in APT 

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).

Special Requirements

A variety of observatory level Special Requirements may be chosen under the Special Requirements tab.

Comments

The Comments field (under the Comments tab) should be used for observing notes.

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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)

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