NIRCam Small Grid Dithers
JWST NIRCam small grid dithers are quick sub-arcsecond pointing offsets used primarily to improve coronagraphic PSF subtraction.
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Small grid dithers (SGD) are very small, fast, and precise pointing offsets of a target image. They are implemented in NIRCam coronagraphic imaging to obtain multiple images of a reference star to optimize the PSF subtraction (Lajoie et al. 2016).
Unlike other JWST dithers, which are executed by slewing the telescope, small grid dithers are executed using the fine steering mirror while remaining under control of the JWST Fine Guidance Sensor (FGS). The range of motion is restricted to ~1 FGS pixel (~69 mas on a side), but the pointings are performed very quickly (in a few seconds) and accurately (~2 mas uncertainty per axis).
By comparison, JWST slews of 25" or less (small angle maneuvers, or SAMS) take over 48 s to execute (~20 s + overheads) and have expected accuracies of ~5 mas per axis. Such slews are used in coronagraphic imaging to position science targets and PSF reference stars behind the occulters.
Ideally the science and PSF targets would be positioned at exactly the same place behind the occulter. In practice, the subpixel pointing differences of ~5 mas per axis significantly alter the scattered light speckle pattern. Small grid dithers can be used to place one of those targets (usually the PSF reference) at multiple positions behind the occulter (see Tables 1 and 2 and Figure 1), and that grid of PSF images can be used to improve the PSF subtraction from the science image (Lajoie et al. 2016).
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The complete list of dither patterns are also given as ASCII tables in arcseconds in the .zip file:
This .zip file contains a collection of separate files for all the NIRCam-related dither patterns. Specifically, relevant to the subpixel dither patterns described here, it contains these subpixel imaging dither patterns (mapping their names to the APT pattern names):
Filename | APT dither pattern type and pattern name |
|---|---|
| NircamImagingSmallSubpixel.txt | Subpixel Dither Type: SMALL-GRID-DITHER |
| NircamImagingSubpixel.txt | Subpixel Dither Type: 3-POINT-BAR, 5-POINT-BAR, 5-POINT-BOX, 9-POINT-CIRCLE, 5-POINT-DIAMOND |
Coronagraphic imaging
Figure 1. shows the 5 offered SGD patterns for NIRCam coronagraphic imaging. Tables 1 and 2 lists all their coordinates in milliarcseconds (mas). Observing a PSF reference star with more SGD positions ensures a better PSF subtraction (improved diversity) but increases the overall observing time for a given program. It is a trade-off. Certainly, the largest number of positions is recommended for the most challenging programs (e.g., when the object to be imaged is in the close vicinity of the mask, typically from within the inner working angle to one arcsecond).
Table 1. Small grid dither offsets for round masks
| Position | Offsets (mas) | Position | Offsets (mas) | |||
|---|---|---|---|---|---|---|
| # | X | Y | # | X | Y | |
| 5-POINT-BOX | 9-POINT-CIRCLE | |||||
| 1 | 0.0 | 0.0 | 1 | 0.0 | 0.0 | |
| 2 | +15.0 | +15.0 | 2 | 0.0 | +20.0 | |
| 3 | -15.0 | +15.0 | 3 | -15.0 | +15.0 | |
| 4 | -15.0 | -15.0 | 4 | -20.0 | 0.0 | |
| 5 | +15.0 | -15.0 | 5 | -15.0 | -15.0 | |
| 5-POINT-DIAMOND | 6 | 0.0 | -20.0 | |||
| 1 | 0.0 | 0.0 | 7 | +15.0 | -15.0 | |
| 2 | 0.0 | +20.0 | 8 | +20.0 | 0.0 | |
| 3 | 0.0 | -20.0 | 9 | +15.0 | +15.0 | |
| 4 | +20.0 | 0.0 | ||||
| 5 | -20.0 | 0.0 | ||||
Table 2. Small grid dither offsets for bar masks
| Position | Offsets (mas) | Position | Offsets (mas) | |||
|---|---|---|---|---|---|---|
| # | X | Y | # | X | Y | |
| 3-POINT-BAR | 5-POINT-BAR | |||||
| 1 | 0.0 | 0.0 | 1 | 0.0 | 0.0 | |
| 2 | 0.0 | +15.0 | 2 | 0.0 | +20.0 | |
| 3 | 0.0 | -15.0 | 3 | 0.0 | +10.0 | |
| 4 | 0.0 | -10.0 | ||||
| 5 | 0.0 | -20.0 | ||||
Click on the figure for a larger view.
Measured position of Round mask small grid dithers using the fine guiding sensors (left, many SGDs) and NIRCam itself. SGD are very precise to ~2–3 milliarseconds.
Imaging
NIRCam imaging small grid dithers can be used in lieu of subpixel dithers. They perform the same optimal pixel subsampling as subpixel dithers, but with smaller patterns. That is, the fractional pixel steps are preserved, but fewer whole pixel steps are taken. Small grid dithers can still achieve some mitigation of bad pixels, especially in the short wavelength channel. The overheads are reduced and pointings are more precise compared to standard subpixel dithers. The 9 available patterns are shown below, and the complete list of pointing offsets in arcseconds for all dithers are given as ASCII tables in arcseconds in the .zip file:
Click on the figure for a larger view.
In these illustrations of the SMALL-GRID-DITHER patterns for NIRCam imaging, short and long wavelength pixels are shown in blue and pale red, respectively. Dither points are black and numbered. The dither path is colored yellow – brown – black. Light gray points mark the subpixel phasing covered by each pattern. Reproduced from Coe 2017.
References
Coe, D. 2017, JWST-STScI-005798
More Efficient NIRCam Dither Patterns
Lajoie, C-P et al., 2016, SPIE, 99045K
Small-grid dithers for the JWST coronagraphs
Lajoie, C-P, Soummer, R., Pueyo, L., and The JWST Coronagraphs Working Group, 2016, STScI Newsletter, Vol 32, Issue 02
Improving Webb Coronagraphic Performance with Small-Grid Dithers
Soummer, R. et al. 2012, SPIE, 91433V
Small-grid dithering strategy for improved coronagraphic performance with JWST