JWST Small Grid Dither Technique
Small grid dithering (SGD) is an optional technique in JWST coronagraphic imaging that obtains multiple data sets on a target using the fine steering mirror to make very small offsets between exposures. This technique avails various post-processing options that permit higher fidelity PSF subtractions at the cost of additional observing time.
Target acquisition (TA) is an important factor that contributes to the contrast performance in high-contrast imaging applications and typically depends on the specific instrument. In the case of JWST, coronagraphic TAs rely on measuring the image centroid of the target at a position away from the focal plane mask, and performing a small angle maneuver (SAM) to place the target behind a selected coronagraphic mask. Therefore, the accuracy of the TA is directly limited by the SAM accuracy, which is expected to be ~6–8 mas per axis (1-sigma radial) (see JWST Pointing Performance table for details).
For JWST high contrast imaging observing programs, it is standard operating procedure to obtain and subtract a scaled image of a point spread function (PSF) reference star to remove the speckles that remain in an occulted science target observation. However, the accuracy with which a science target and a subsequent PSF reference target can be placed behind an occulter is limited by the accuracy of the target acquisition procedure. Since the placement of a science target behind a given occulter may be slightly different from the placement of the PSF reference target, the speckles in the corresponding observations may be slightly different, thus compromising the quality of the PSF reference subtraction from the science target observation.
In cases where the highest quality PSF matching is required, a technique called small grid dithering (SGD) can be invoked. The technique uses the fine steering mirror (FSM) to make a number of very small (5–10 mas) offsets of the target in a grid pattern around the nominal TA position, with an observation executed at each step, thus creating a mini library of PSFs obtained at the same epoch as the science observation. Post-processing of the ensemble of observations can be used to model a more precise speckle pattern to use for the subtraction, at the expense of the additional observational overheads. An excellent article by LaJoie et al. (2016) is available for those who need to know the details of the technique, where it can be most effectively used, and the tradeoffs involved. See also the technical report by Soummer et al. (2014).
It is anticipated that most users of this technique will apply it only to the PSF reference target, and then only in cases where the highest quality PSF subtractions are needed for the science use case of interest. Applying this technique to the science target does not allow for an improved fit of the PSF reference observation. Use of the SGD technique on a science target is not disallowed within APT, however, in case the community devises a use case where it would be beneficial.
Specifying SGDs in APT
The expected use case for SGDs will be to take the science observation(s), and then use an SGD dither pattern to obtain a set of slightly offset exposures on the PSF reference star. The ensemble of PSF star SGD exposures covers the expected region of possible misalignments from the science observation TA.
The SGD technique is expected to have the most benefit for MIRI 4QPM observations, owing to the great sensitivity of target placement relative to the apex of the mask. However, simulations have shown significant benefit for all coronagraphic modes with MIRI and NIRCam (LaJoie et al. 2016).
The MIRI and NIRCam instrument teams have created a set of pre-defined SGD options that can be selected from within the appropriate APT observation template, using the pull-down menu for specifying dithers. The table below shows the available options for each mode, including the dither name used in APT.
Table 1. SGD Dither option names in APT
|Coronagraph/filter||APT dither type SGD options|
|4QPM/F1065C 1||5-POINT-SMALL-GRID; 9-POINT-SMALL-GRID|
|Coronagraphic mask||APT dither pattern SGD options|
|MASKSWB (wedge)||3-POINT-BAR; 5-POINT-BAR|
|MASKLWB (wedge)||3-POINT-BAR; 5-POINT-BAR|
|MASK210R||5-POINT-BOX; 5-POINT-DIAMOND; 9-POINT-CIRCLE|
|MASK335R||5-POINT-BOX; 5-POINT-DIAMOND; 9-POINT-CIRCLE|
|MASK430R||5-POINT-BOX; 5-POINT-DIAMOND; 9-POINT-CIRCLE|
From this table, it should be clear that use of SGD comes with a price: depending on the grid chosen, the number of grid points (and hence observations) of the PSF reference star can be as high as 9 instead of one. The good news is that the FSM offsets are tiny compared with an FGS guider pixel, and so no reacquisition of the guide star is needed. The FSM motions themselves take relatively little time, so it is mainly the additional observation time that is required. Because of this efficiency hit, you should only select the SGD technique in cases where the highest suppression of target star light is needed, but in those cases, significant improvements can be garnered (LaJoie et al. 2016).
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Use of SGD data in the data processing
For observational sequences that include SGD data on the reference star, there are two possible processing algorithms to derive an improved PSF reference model for subtracting the residuals in the science image. The two options are known as KLIP (Karhunen-Lo`eve image projection) or LOCI (locally optimized combination of images). Initially, the pipeline will use the KLIP algorithm as part of standard processing. These algorithms use the small variations in the PSF speckle pattern from each SGD step to produce a model PSF that best matches the speckle pattern in the science target observation.
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Small-grid dithers for the JWST coronagraphs
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Small-Grid Dithering Strategy for Improved Coronagraphic Performance with JWST