NIRCam Claws and Wisps

Claws are artifacts that occasionally appear due to scattered light from extremely bright stars very far (10 degrees) from the field of view that can be avoided in observation planning. Wisps are stationary artifacts always present, though not always visible, and may be corrected by templates provided here.

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Claws

Claws are a type of scattered light phenomenon (Figure 1 and 2) that is observed in NIRCam when an extremely bright star (K ~< 3 Vega mag) lands in a specific susceptibility region, which is located very far from the actual instrument field of view, at about +10 degrees in the V3 direction, a huge distance compared to the ~2 arcminutes effective extent of the NIRCam field of view (Figure 3). Light from this region enters the NIRCam instrument directly through the aft-optic-system (AOS) mask, located in front of JWST tertiary, without first bouncing off the primary and secondary mirror. This light path is called the rogue path. The collecting area through the AOS mask is effectively small (about 0.01 square meters), moreover the light in this case is not focused by the primary and secondary, and it undergoes several reflections within the NIRCam instrument before hitting the detector. As a consequence, the total amount of scattered light is reduced by about 18 magnitudes with respect to the bright star that causes it. There is some variability in this dimming factor due to the exact placement of the stars within the rogue path. 

Figure 1. Claws and wisps

Examples of the morphology and detector location of the stray light features known as wisps and claws are shown in this figure. Multiple dithered exposures are median-stacked to remove astronomical sources, and to enhance the stray light features. Note that while the wisps are fixed in detector space, and their morphology does not change across observations (although their strength does), the claws visible in the figure are peculiar to this particular observation and differ between observations (and are completely absent in most of them).
The resulting feature is an extended image with a claw-mark-like appearance. The characteristic shape is imprinted by machining features on one of the surfaces involved. The claws are observed exclusively in the short wavelength channel. When present, claws occur primarily in the A1 or B4 detectors, affecting roughly 5% of the pixels on those detectors.

Claws will have different appearances and morphologies depending on the location of the bright star in the susceptibility region. For a given set of observations, the appearance and location on the detectors may remain roughly constant.

The claws do not always appear in the same areas of the NIRCam detectors, but they move as the responsible source moves within the susceptibility region. Given that the claw images are not observed via the nominal telescope path, the amount of claw motion is not 1:1 with the amount of motion of the source within the rogue path. Moving the telescope boresight by several arcminutes results in a claw motion of a few arcseconds. Typically, claws do not move in an appreciable manner within dithered sequences, but they are observed to move across multiple mosaic tiles.

The effect of the claws is to increase the background by about 10% on a typical area of 50,000 pixels. This factor is estimated at the ecliptic (larger than average background) using the F200W filter and a very red star. Variations are expected as a function of the SED of the bright star and of the background specific to each observation. Note that the affected area is a few percent of a single NIRCam short wavelength detector (2048x2048 pixels).

Given the size of the susceptibility region and based on a conservative estimate of the minimum stellar brightness necessary to produce this feature, the NIRCam team estimates that up to about 10% of observations may be affected. In practice, during the JWST commissioning period, the occurrence of claws has been significantly lower than this. The NIRCam team is both developing mitigation strategies and actively monitoring data as they are acquired, to further characterize this phenomenon and further reduce its potential impacts to science.

Figure 2. Claw stacked image

Example of a claw on detector A4. This image is obtained by stacking multiple exposures  in a dithered sequence, in order to remove stars and galaxies and increase the SNR in the claw feature itself. The colorbar indicates the ratio with respect to the median value of the image. In this specific instance of a very strong claw, the surface brightness in the claw region is about 15-20% higher than the average background.

Figure 3. Claws susceptibility region

The shape of the susceptibilty zone for claws. Given the different optomechanical characteristics of the 2 NIRCam modules, the regions for Modules A and B are similar and almost specular, but they do differ in the details. Note the axes are in degrees; the NIRCam field of view would appear very small (~5 × 2 arcmin) at the origin.


Wisps

Wisps (see Fig. 1) are faint, diffuse stray light features present in the same detector locations in all NIRCam exposures. They are most prominent on the B4 detector, with variable brightness that is typically about 10% of the zodiacal background. Fainter wisps have also been seen in detectors A3, A4, and B3, affecting a few percent of the pixels in each detector.

Similarly to claws, the wisps are caused by light going through a rogue path. The light bypasses the primary and secondary mirrors and enters NIRCam through the aft-optic-sytem (AOS) mask in front of the tertiary.

Differently from the claws, however, the wisps are not caused by light coming directly from a bright source. Wisps are instead caused by light coming off-axis, and bouncing off the top secondary mirror strut. Therefore, with respect to the NIRCam FoV, the illuminating source is always at the same location, albeit its brightness varies.

Wisps can be thought of as an additive feature that can be subtracted from each exposure by using a universal wisp template whose strength can be scaled to match the observed strength. The NIRCam team developed initial templates provided here, along with preliminary guidance on how to process NIRCam images to remove this feature.

Wisp templates available here may be subtracted from count rate (slope) files (stage 1 output) using code provided:

https://stsci.box.com/s/1bymvf1lkrqbdn9rnkluzqk30e8o2bne




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