NIRCam Dragon's Breath Type II

Scattered light from bright stars ~10"–12" from the short wavelength detectors can produce an artifact called type II dragon's breath. Discovered during flight, it is similar to the original dragon's breath artifact identified in ground testing and caused by stars ~2" from the field of view of the detector

Type II dragon's breath was first observed in JWST commissioning program 1165 in module B images. It was characterized further by NIRCam commissioning program 1067 in module A. Avoidance zones have yet to be fully mapped.

Type II dragon's breath is caused by light scattering off a knife edge installed to block a stray light path to the short wavelength detectors. This edge is not present in the long wavelength light path, and the artifact is not seen in long wavelength images.

The artifact spans the length of 2 short wavelength detectors. Half resembles a diffraction spike extending roughly along the V2 axis (rotated 3°). The other half is a bright glint ~6" wide, containing ~2% of the flux of the star. The bright glint has been observed most prominently in short wavelength detectors B3 and A4 due to stars ~12" from the edge of the field of view of detectors B1 and A2, respectively. Less severe glint has also been observed in detector A1.

Figure 1. Type II dragon's breath

This type II dragon's breath observed during commissioning is due to a bright star (K ~ 9 Vega mag) 12" from the edge of the B1 detector (in the negative V2 direction). The bright glint shown here is 6" wide, 45" long in B3 plus another 10" in B1, and contains ~2% of the flux of the star. A gap of a few arcseconds in B3 is a shadow due to stray light being blocked by the FPA mask. Another linear feature spans the length of B1 pointing back to the star. It resembles the two PSF diffraction spikes also pointing to the star. The full B3 and B1 detectors are shown here, each 64" across. This image is from commissioning program 1165, observation 3. Type II dragon's breath has been observed in other short wavelength detectors, produced by stars 10"–12" away.

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