Dither pattern

See also: NIRCam Dithers and Mosaics

Dithering is required to mitigate bad pixels and improve overall image quality. Larger primary dithers are useful to fill gaps in sky coverage. Smaller subpixel dithers are optimized to improve sampling in stacked images. Programs can use both types of dithers. But programs covering very large areas, or targeting single compact sources, may elect to implement only primary or secondary dithers, depending on their science goals and the observing overheads for each dither and exposure. Primary dithers also offer some improvement in image sampling, so these may be preferred unless optimal image sampling is the highest priority.

Primary dithers

Of the primary dither patterns, some examples of the options recommended for general use are as follows:

FULLBOX 6TIGHT

FULLBOX 6TIGHT fills all gaps between detectors and modules; it requires greater overheads than the INTRAMODULE dither types.

INTRAMODULEBOX 4 

INTRAMODULEBOX 4 fills the short wavelength gaps, leaving the gap between modules; it maximizes the deep area with full exposure time.

INTRAMODULEX 3 or 4

INTRAMODULEX 3 or 4 fill the short wavelength gaps, leaving the gap between modules; they have larger but more uneven areal coverage than INTRAMODULEBOX 4.

Subpixel dithers

Of the subpixel dither patterns, SMALL-GRID-DITHER has lower overheads and is recommended for use when primary dithers are also being obtained. If no primary dithers are performed, then the larger STANDARD dithers are recommended to better mitigate bad pixels.

Subpixel dithering is particularly relevant when the PSF is undersampled by the detector pixels, i.e., when the pixels are too large to provide adequate Nyquist sampling of the PSF in a single exposure. In such cases, multiple exposures obtained with a subpixel dither pattern provide additional information about the PSF structure, enabling reconstruction of an improved PSF that is closer to the intrinsic one. For NIRCam, the PSF is well sampled at wavelengths longer than about 2 µm and 4 µm in the short and long wavelength channels, respectively, by design.

For shorter wavelengths, where undersampling can become significant, a good rule of thumb is that the number N of subpixel dithers should scale approximately as the square of the wavelength ratio, relative to the well-sampled regime. For example, images obtained in the short wavelength channel with F115W are undersampled by a factor ~ 2 µm / 1.15 µm, i.e., a factor of ~1.73 (relative to the well-sampled 2 µm regime), so a minimum of (1.73)², i.e., 3 subpixel dithers, would be needed to recover the PSF information. In practice, observers may wish to consider 4 subpixel dithers, which can provide somewhat more regular subsampling of the square detector pixels on a 2 × 2 half-pixel grid. For more detailed background information on principles of dithered observations with JWST, see Koekemoer & Lindsay (2005), Anderson (2011), and Anderson (2014).

Note that when performing coordinated parallel observations, additional custom subpixel dither patterns are available that achieve optimal pixel phase sampling for both the prime and parallel instruments. However, NIRCam's small grid dithers are not available for parallel observing, since they are not large enough for good sampling of the PSF for other instruments.

Position angle

Aladin will show the default orientation PA = 0 unless a different PA is specified using special requirements. Note that PA = 0 may not be an allowed angle for your specific target. If the PA is adjusted manually within the Aladin Viewer by clicking and dragging the field of view around, note that these changes are not automatically applied to the observation. See the APT Aladin Viewer article for more details on adjusting the PA using special requirements.

Mind the gaps 

We recommend using the APT Aladin Viewer to check the target placement and dither coverage. Keep in mind that primary dithers fill the gaps between NIRCam detectors but result in uneven depth across the stacked image. Consider this when calculating the required exposure time using the Exposure Time Calculator.

When observing with both (ALL) NIRCam modules, the science target is, by default, centered in the ~44” gap between the modules in the field of view. Depending on the size of the target, the following strategies are recommended to ensure that the target does not fall between the gaps:

(a) Large scene

If the goal is to image a scene larger than 5' × 2' without gaps:

• Use the FULLBOX primary dithers to fill the gaps. This will cover the full area, though with non-uniform depth across the field of view.

(b) Mid-size target

For a target that fits within one module (2.2' × 2.2'), well within the field of view of an HST camera, we recommend centering the target within module B by either:

• Using the Target Placement option to center the target roughly within module B (in the corner of detector B4). The pointing may be tweaked by combining with a special requirement Offset.
  
  
• Adding a special requirement Offset of (X,Y) ~ (81”, -4"). Here, X & Y are given in the “Ideal” (X, Y) coordinate system, which is fixed relative to the NIRCam detectors. (Note with this offset, the center of the target will be near the gap between the short wavelength detectors. Primary dithering can be used to fill these gaps.)

• Observing with NIRCam module B only. (The center of the target will be near the center of module B, in the corner of detector B4.)

In either case, we recommend primary dithering with INTRAMODULEBOX or INTRAMODULEX to fill the short wavelength chip gaps. Most of the area imaged by the 2 modules will be at full depth. Observing overheads will also be lower than when using FULLBOX dithers to cover the full scene.

(c) Compact target

For a target that fits within one short wavelength detector (64" × 64"), one may choose to center it within the B1 detector using the Detector B1 option in the Target Placement or center it within one of the other detectors by adding a special requirement to provide an Offset. To choose a different detector, or if observing with one module only, one may calculate the offset using Table 2 and 3 in NIRCam Apertures, or simply use the Aladin viewer to manually move the target.)

This strategy ensures the small target is observed at full depth by avoiding the gaps between the short wavelength detectors. INTRAMODULEBOX, INTRAMODULEX, or SMALL INTRASCA primary dithers and/or subpixel dithers are recommended in this case. Even point sources will benefit as dithering mitigates bad pixels and flat field uncertainties.

For a compact target that fits onto a single short wavelength detector, other considerations also affect the choice of detector. For example, scattered light features such as claws and wisps (see NIRCam Scattered Light Artifacts), and detector-level features such as NIRCam Persistence, all manifest to different extents on the different detectors. When considering the combination of these effects across all the available short wavelength detectors, it has been found that the B1 detector shows a good combination of low persistence and low susceptibility to claws and wisps, therefore observers may wish to consider the use of the B1 detector for observations of compact targets that fit onto a single short wavelength detector.

Integration times

We recommend integration times less than 1,000 s. For longer integrations, the majority of pixels would likely be affected by cosmic rays. Total exposure times can be made longer by using dithers and/or by increasing the number of integrations. See the discussion in MIRI Cross-Mode Recommended Strategies.

Readout pattern

To achieve a desired exposure time and signal-to-noise ratio, users must choose one of the 11 available NIRCam detector readout patterns, as well as numbers of groups (of each pattern) and integrations at each dither position. We provide the following guidance:

• The readout patterns RAPID, BRIGHT2, SHALLOW4, MEDIUM8, and MEDIUMDEEP8 maximize the signal-to-noise ratio for a given integration time <1,000 s. See NIRCam Imaging Sensitivity for details.
  
  
• Greater numbers of groups are preferred to mitigate cosmic rays for all pixels. The minimum number should be 5 groups, except for the shortest exposures, according to the recommendations for maximum sensitivity. For example, we recommend 8 groups of SHALLOW4 rather than 4 groups of MEDIUM8, as long as the data volume is manageable.
  
  
• A single integration at each dither position will be sufficient for most programs. Given the choice between extra integrations or extra dithers, dithers are preferred as they improve data quality in multiple ways. The one drawback is that dithers increase overheads.
  
  
• DEEP8 and DEEP2 result in integrations >1000 s, which are expected to be substantially affected by cosmic rays. These patterns are thus not recommended, but may be required in some cases to reduce data volume.

Data volume & data excess

APT places some limits on data excess and volume. Exceeding these limits can cause warnings or errors in APT. The data volume and excess can be viewed by clicking on the individual visits in the sidebar on the left of the APT window.

To reduce the data volume, try the following (assuming a fixed total exposure time to achieve the required signal to noise):

• Use a longer readout pattern that generates less saved data, for example MEDIUM8 instead of SHALLOW4. 
• Observe with fewer detectors.
  • Observing with both (ALL) modules yields data from 10 detectors.
  • Observing with module B only and full subarrays yields data from 5 detectors.
  • Some NIRCam subarrays yield data from as few as 2 detectors.
  • Note that choosing a smaller subarray does not necessarily decrease the data volume / total charged time significantly, since smaller subarrays are read out more quickly.
• If necessary, obtain fewer dithers and opt instead for a longer exposure. This may sacrifice data quality somewhat, and depends on the goals of each specific program. One benefit is that observing overheads will decrease.

Overheads

If reducing overheads is desired, users are advised to simply "sit and stare." Minimize the numbers of filter changes, dithers, exposures, and (importantly) visits requiring new guide stars after larger pointing shifts (see JWST Slew Times and Overheads). These operations may take several minutes (see JWST Instrument Overheads). Of course, filter changes and dithers improve data quality and may be required to achieve the scientific goals of the program.  Large overheads may not be avoidable for some science programs.

It should be noted that the dither pattern(s) will be repeated for each filter pair (short and long wavelength). Larger dithers with larger overheads should be avoided if possible, in part by using the dither patterns introduced in APT 25.4.1 (Coe 2017). Also note that more compact dither patterns will not split the observation into multiple visits, where the details depend on the visit splitting distance assigned to the observation. Targets at lower Galactic latitude will have larger visit splitting distances and may be more efficient to observe, especially with the FULLBOX dither pattern that fills all gaps between detectors and the modules.

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