NIRCam Imaging Recommended Strategies
Guidance is provided for astronomers preparing JWST NIRCam Imaging observations using the Astronomers' Proposal Tool (APT).
Here we give advice on handling the gaps between detectors, selecting dither patterns and readout patterns, and reducing data volume and overheads.
Dithering is required to mitigate bad detector pixels and improve overall image quality. Larger primary dithers are useful to fill gaps in sky coverage. Smaller subpixel dithers are optimized to improve resolution in stacked images. Ideally programs will do both, but smaller programs may have to choose between primary and secondary dithers, given the observing overheads added for each dither and exposure. Primary dithers may offer some improvement in image resolution, so these may be preferred unless image resolution is the highest priority.
In general, it is preferable to divide each exposure into as many dithers as feasible (given the overheads) to improve image quality.
Of the primary dither patterns, a few options recommended for general use are:
- FULLBOX 6TIGHT – fill all gaps between detectors and modules; greater overheads than the INTRAMODULE dither types
- INTRAMODULEBOX 4 – fill short wavelength gaps, leaving the gap between modules; maximal deep area with full exposure time
- INTRAMODULEX 3 or 4 – fill short wavelength gaps, leaving the gap between modules; irregular areas with uneven coverage
Note depth across the stacked image will be uneven, especially for FULLBOX or FULL dithers. This should be considered when calculating your required exposure time with the JWST ETC.
For the INTRAMODULE pattern types, a minimum of 4 dithers is preferred, but 3 may be acceptable if needed to minimize overheads:
- 4 INTRAMODULEBOX dithers cover all area by at least 2 dithers.
- 4 INTRAMODULEX or INTRAMODULE dithers cover all central areas by at least 2 dithers, while leaving some areas along the edges covered by only 1 dither.
- 3 INTRAMODULEX or INTRAMODULE dithers cover all short wavelength detector gaps, though some central areas are covered by only 1 dither.
- Any area covered by only 1 dither cannot be corrected for bad pixels. This may leave holes in the final stacked image.
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 can provide the necessary additional information about the PSF structure, enabling reconstruction of an improved PSF that is closer to its intrinsic properties. 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, since the FWHM of the PSF becomes larger than ~2 pixels at these wavelengths.
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 ~ 2µm / 1.15µm, ie a factor of ~1.73 (relative to the well-sampled 2 µm regime), so a minimum of (1.73)2, ie 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 2x2 half-pixel grid. For more detailed background information on principles of dithered observations with JWST, see Koekemoer & Lindsay (2005), Anderson (2011), and Anderson (2014).
Mind the gaps
We recommend using the APT Aladin Viewer to check the target placement and dither coverage. Primary dithers fill the gaps between NIRCam detectors, resulting in uneven depth across the stacked image. Consider this when calculating your required exposure time using the Exposure Time Calculator Old.
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, we recommend the following strategies to ensure your target does not fall between the gaps:
If the goal is to image a scene larger than 5' × 2' without gaps:
- Use FULLBOX primary dithers to fill the gaps.
This will cover the full area while sacrificing depth in some areas.
For a target that fits within one module (2' × 2'), we recommend centering the target within Module B by either:
- Adding a Special Requirement OFFSET of ~82” in X. Here, X is 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 in the gap between the short wavelength detectors, but primary dithering should 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 two modules will be at full depth. Observing overheads will also be lower than when using FULLBOX dithers to cover the full scene.
For a target that fits within one short wavelength detectors (1' × 1'), you may choose to center it within one of those detectors using a different OFFSET. For example:
- Add a Special Requirement OFFSET of ~55” in X and ~35" in Y to center the target within detector B3 while observing with both modules. (To choose a different detector, or if observing with Module B only, you may calculate the offset using Table 1 in NIRCam Apertures, or simply use the Aladin viewer to move your target manually.)
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.
To achieve a desired exposure time and signal-to-noise, users must choose among the 9 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 DEEP8 maximize signal-to-noise for a given exposure time. When observing with all 10 detectors, BRIGHT1 is often required instead of BRIGHT2, which is limited to 4 groups. See NIRCam Imaging Sensitivity for details.
- Greater numbers of groups are preferred to mitigate cosmic rays for all pixels. The minimum number will generally be 5 groups, except for the shortest exposures, when following recommendations for maximum sensitivity. For example, we recommend 8 groups of SHALLOW4 rather than 4 groups of MEDIUM8, so long as the data volume is manageable.
- A single integration at each dither position will be preferred for most programs. Given the choice between extra integrations or extra dithers, the latter are preferred as they improve data quality in multiple ways. The one drawback is overheads.
APT places some limits on Data Rates and Volume. For each visit, the Data Volume may not exceed 58 GB, roughly the memory capacity of the onboard Solid State Recorder. Exceeding this limit will generate an error in APT.
Users should also check the ratio of Data Volume / Total Charged Time for each observation. If this ratio exceeds 0.654 MB/s for a total time of ~12 hours or more, then the program will likely be difficult or prohibitive to schedule. APT does not issue a warning in this case.
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 DEEP8 instead of MEDIUM8. Consider doing the same for any other instrument obtaining coordinated parallel observations.
- 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, each with a longer exposure. This may sacrifice data quality somewhat. One benefit is that observing overheads will decrease.
Advice to reduce observing overheads and improve efficiency can be summarized simply as: 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). Note each of these operations take several minutes (see JWST Instrument Overheads). Of course filter changes and dithers improve data quality and may be required for your science.
Bear in mind your 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). Whenever possible, use a more compact dither pattern that does not split your observation into multiple visits. This will depend on the Visit Splitting Distance assigned to your 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