Dithering with JWST NIRISS is required for the Wide Field Slitless Spectroscopy (WFSS) mode in order to improve the sampling of the point spread function and mitigate the effects of bad pixels and variations in sensitivity across the detector.
Dithering in NIRISS observing modes
Dithering is performed by moving the telescope slightly between exposures using Small Angle Maneuvers (SAMs), so that the astronomical scene is offset to different locations on the detector in the different exposures. Dithering is generally motivated by various benefits that improve the photometric and astrometric quality of astronomical images:
- It allows one to improve the effective sampling of Point Spread Function (PSF) at short wavelengths where the detector pixel size causes undersampling. Since the NIRISS detector is critically sampled at 4 μm, the features within an astronomical scene are undersampled in many NIRISS images if no dithering is used.
- It can mitigate effects of undesired features such as bad pixels, coronagraphic spots, persistence from previous exposures, or small scratches on the NIRISS pick-off mirror.
- It helps reduce flat-fielding errors by placing the target on different pixels, thereby reducing the flat-fielding errors by the square root of the number of dithers.
- It improves noise properties of images corrected for geometric distortion compared to a stack of images taken with single pointing by mitigating correlated sampling errors.
- It mitigates the photometric and astrometric errors induced by the sensitivity variations across individual pixels or Intra-Pixel Sensitivity (IPS) of the NIRISS detector .
Dithering is required for the wide field slitless spectroscopy mode and is supported, but is not encouraged, for the aperture masking interferometry mode. Dithering is not allowed for the single object slitless spectroscopy mode since these observations require high precision differential spectrophotometry, where all systematic differences between individual exposures in a sequence need to be minimized. NIRISS imaging is an observing mode that can only be executed as a coordinated parallel with NIRCam or MIRI as prime instrument. As such, it will follow the dither pattern selected by the prime instrument's observing mode.
NIRISS Dither Sequences
Users have to choose from a restricted set of "pre-defined" dither patterns, which are described in the articles for the individual observing modes. The total exposure time for a NIRISS observing sequence is determined by three parameters:
The dither pattern determines the number of exposures, and the readout (sub)-array and MULTIACCUM parameters (readout pattern, number of groups per integration, number of integrations per exposure) determine the integration time per exposure.