NIRCam Dragon's Breath

Bright sources off the edge of the JWST NIRCam field of view scatter light onto the NIRCam detectors. This effect is called dragon's breath.

Scattered light on the detectors from sources placed just outside of the NIRCam field of view is known as "dragon's breath." It is caused by light scattering from the inner wall of the mask immediately in front of the focal plane array. Users can minimize dragon's breath by avoiding the placement of bright objects near the edges of NIRCam detectors. 

In the long wavelength (LW) channel, the integrated intensity of the scattered light is up to ~1.5% of the total intensity from an equivalent in-field source, with the peak pixel including up to ~0.75% of the source intensity. The total intensity of the scattered light depends on the distance of the source from the edge of the detector, with the strongest effect occurring near ~1.9" from the detector edge in the LW channel.  The light scatters about 200–250 pixels into the field, with the peak intensity occurring within ~50 pixels (1 LW pixel = 0.063″). The effect is significantly smaller in the short wavelength (SW) channel, where the integrated intensity of the scattered light is only up to ~0.4% of the total intensity of the equivalent in-field source, with the peak pixel including ~0.01%.

Figure 1. Dragon's breath

This is an example of dragon's breath, including the diffraction spikes. Left: a short wavelength channel dragon's breath, for a source placed 2.1″ from the frame edge. Right: a long wavelength channel dragon's breath, for a star placed ~1.7″ from the frame edge. Both images are 512 × 506 pixels; since the pixels are twice as large in the LW channel, the angular extent of the LW scattered light is larger than it is in the SW channel.


Avoidance zones  

The dragon's breath avoidance zones are around the perimeters of all 10 NIRCam detectors. In the LW channel, dragon's breath is strongest for sources positioned ~1.9″ from the detector edge and for those within ±0.8″ from that point, forming an avoidance annulus around the detector. In the SW channel, the avoidance zone is centered at ~2.1″ from the frame edge and has a width of ±0.5″, including the region between SW detectors in each module. 

Figure 2. Avoidance zones

A zoomed-in view of the upper part of the module A detectors, showing the avoidance zones for the SW channel (blue) and the LW channel (red). The avoidance zones are the hatched regions, with ±0.5″ (±0.8″) width for the SW (LW) detectors.



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