Data Artifacts and Features

Different kinds of artifacts have been identified in data files from pre-launch testing as well as on-orbit operations.  This article will point you to the current support information to understand these effects.

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See also: A separate set of articles describing JWST Calibration Pipeline Caveats

Ground  testing and in-flight operations have identified a number of features that can appear in JWST data.  The frequency with which each artifact appears depends on the instrument, observing mode, and random external factors such as the possible location and brightness of a spoiler star that may cause a glint of scattered light.  Some types of artifacts can appear in multiple instruments while others are specific to a given instrument.  The links below provide access to more detailed information about each artifact type.

This article will be updated frequently as new information becomes available.

Multi-instrument artifacts


Snowballs are attributed to particularly prominent cosmic ray impacts on the near IR detectors in various JWST instruments.   A saturated core is surrounded by a multicomponent halo effect, as shown in the accompanying article.  Proper dithering may help minimize the impacts.

Scattered light

Testing on the ground and during commissioning has uncovered a number of ways that unwanted light can find its way onto the detectors, causing various flavors of scattered light.  Because these different flavors manifest themselves differently, they have been given names to identify the observed effects; thus, claws, the light saber, dragon's breath, glow sticks, and short streaks have entered the JWST lexicon.

Some of these problems are caused by light from very bright stars offset from the field of view that enters an instrument by a "rogue path" instead of reflections off the primary and secondary mirrors.  Others are due to bright stars just outside the detector field of view. In the case of the glow sticks in MIRI, the origin of the light is emission from the observatory itself.

As we learn more from analysis of commissioning data as well as in real operations, it is likely that careful pre-planning may be able to avoid or at least minimize some of the effects, but they may be observed in early data until such time that they can be fully characterized and planned around.  The individual instrument articles below report on these features in each instrument's data, as appropriate.  If any of these effects show up on your data, contact the JWST Help Desk as your experience will help us better define and ultimately mitigate them as much as possible.

Instrument-specific features and caveats 

Each science instrument's documentation contains a "Features and Caveats" page for their particular instrument that provides information about which users should be aware.  The following list provides quick access to these instrument-specific articles.

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