There are two sources of spiking. The sixfold spiking is due to diffraction in the optics, either from the hexagonal mirrors or from the tripod mount in front of the mirrors (you’ll always get an even number of spikes, so threefold symmetry leads to sixfold symmetry). There’s also a fourfold spiking, that’s in the detector, because oversaturated pixels bleed over into adjacent pixels.
This is an image of a single point-source star. Bright point sources are good for calibrating a telescope, but they’re not all that interesting scientifically (at least, not until after you feed them through a spectroscope). The objects that JWST will be imaging as its actual mission will be too diffuse to cause these effects.
Compared to CCD detectors, IR detectors like the one used in the WFC3 IR channel show higher read noise and dark current. Unlike CCDs, however, IR detectors allow the accumulated signal to be read out non-destructively multiple times, without affecting other pixels. The capability to sample the signal multiple times during the integration can be exploited to reduce the effective read-out noise significantly. IR detectors are also immune to the charge bleeding exhibited by CCDs at high signal levels; however, saturation may still be a problem because pixels subject to the highest signal levels show higher dark-current rates (“image persistence”) in subsequent exposures.
This also makes sense based on the physical design. The detectors have a separate absorber layer divided into pixels:
Even if a pixel in the absorber layer saturates, it should not bleed into adjacent pixels. The readout sensor is separate and should be able to read the full range of possible voltage levels. That’s in contrast to a CCD where once the charge builds up too much, it has no place to go besides adjacent pixels.
I believe that’s from the third/top strut. Two of the struts are placed in the same orientation as the edges of the mirror segments. The third strut is not–it’s halfway between:
Where is Webb website now reporting MIRI at 6K. Makes me feel like shivering, even though spring is sprung here and we’ve got a whopping 291K outside just now. I believe everything is now at final operating temps.
I’ve just discovered that among the 248 or so approved projects for the JWST will be a search for life on the Trappist-1 system!
Trappist-1 was an exciting find a few years ago because it consists of seven planets orbiting close to a cool star, of which three (Trappist-1e, -1f, and -1g) are thought to be in the habitable zone. This made the Trappist-1 system really fascinating. And now the full power of the JWST will be brought to bear on those three planets, through spectroscopic analysis of their atmospheres for signatures of life like water vapour and free oxygen.
It’s a long shot, but the first signature of (at least probable) extraterrestrial life could possibly be found soon.
