NIRCam Features and Caveats

Based on pre-launch testing and in-flight commissioning data, NIRCam has known features, characteristics, and data artifacts that observers should be aware of when planning observations and interpreting data.

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Scattered light artifacts

Dragon's breath type I artifacts are due to bright sources just off the edge (within 2") of the NIRCam field of view scattering light onto the NIRCam detectors.

Dragon's breath type II artifacts are caused by bright sources ~12" from the detectors in the field of view. They are only seen in short wavelength images.

Claws are artifacts created by an extremely bright star (K ~< 3 Vega mag) ~10 degrees from the target observation in the +V3 direction.

Wisps are stationary features that always appear in the same locations, most prominently on the B4 detector, and sometimes detected on the A3 and B3 detectors. During data analysis, they may be subtracted by a template that has been developed and is available in the NIRCam Claws and Wisps article.

Ginkgo Leaf artifacts have been observed in long wavelength module A images due to a star out of the field to the left (+V2) and ~24.5" below the top edge (in V3) of the detector A5.

Tadpoles are artifacts in NIRCam WFSS data that may be mistaken for emission line galaxies. They are most prominent and ubiquitous in grism C module B data.

Shells are fainter WFSS artifacts that appear occasionally due to scattered light from very bright sources.



Snowballs

Extreme cosmic ray impacts introduce large artifacts in near-IR detectors named snowballs that are not currently corrected by the pipeline. Most snowballs appear round. Some are elongated. Some have long tails or streaks. None of these features are properly flagged as cosmic rays by the current pipeline. Four or more dithered exposures enable the pipeline to reject these features as outliers.



Persistence

Bright sources produce faint residual images that persist in subsequent integrations. This persistence declines exponentially in the detector pixels subjected to illumination.



Bad pixels

Ground testing revealed isolated bad pixels and some clumps up to 1.7" across, as seen in pre-launch NIRCam flat fields. Small high priority targets should avoid these regions. Primary dithers are recommended to fill in the large clumps of bad pixels in most exposures (any pattern is large enough).



WFSS field of view

NIRCam wide field slitless spectroscopy observations yield spectra from sources within a field of view that varies with the selected filter and grism. Additional spectra are introduced by out-of-field objects, requiring additional imaging. The coronagraph masks and substrate block portions of some spectra in most Grism C observations.




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