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CNN's February 24, 2023 Astrophysicist says 'there is nothing wrong with not knowing' is basically an opportunity for Neil deGrasse Tyson to give us a pep talk and reminder about the exciting process of science.

In the beginning there is a JWST photo where bright unresolved objects (foreground stars?) show JWST's classic six-plus-two diffraction spikes.

But one of them (slightly bluish in the false color image) has two copies, with a rotation angle of roughly 13° between them.

At first I thought maybe exposures at different wavelengths for this false color image were done at different times with different spacecraft attitudes (because of the way JWST works it will point in a given direction with different axial rotations depending on what the sunshield has to do) but I noticed that, at least to me:

  1. there's another object (showing as yellow-ish in the false color image) of what looks like similar brightness nearby with only one set of six-plus-two diffraction spikes
  2. the two pair of diffraction spikes on the blueish object in question look to be the same color (so not coming from different wavelength exposures).

So now I'm thinking multiple, overlapping exposure fields such that the object on the left was exposed at a different time than the one on the right? I can't figure it out.

Question: Why does one object (star?) in this JWST image have two sets of six+plus+two diffraction spikes but another, similar object nearby have only one?


cropped detail from Screenshot from CNN's February 24, 2023 "Astrophysicist says 'there is nothing wrong with not knowing'" https://www.youtube.com/watch?v=qjo7VrR3puA showing one object with two sets of six-plus-two diffraction spikes

Screenshot from CNN's February 24, 2023 "Astrophysicist says 'there is nothing wrong with not knowing'" https://www.youtube.com/watch?v=qjo7VrR3puA showing one object with two sets of six-plus-two diffraction spikes

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if you think that's weird, you should check out this press-release image from ESA -- showing a mosaic of HST and JWST images of a region near the North Ecliptic Pole as part of the PEARLS program -- which has multiple examples of double-diffraction-spike patterns in the same image as single-diffraction-spike patterns.

Here's a cutout from that image, showing normal PSF patterns in the stars to the left and multi-PSF patterns in the stars in the center to the right:

A close-up of the PEARLS North Ecliptic Pole field

The answer is almost certainly that we are seeing the effects of compositing multiple exposures taken at different times and different telescope orientations, which is why you can get the overlapping of diffraction patterns from two different orientations. The key thing to understand is that these are mosaic images built from shifted (offset) pointings, so that some parts contain data from a single exposure and other parts contain data from the overlap of two exposures. (Think of stitching together a panorama taken with an ordinary camera: the result is a mosaic with some regions that come from single exposures and regions in between those which come from the overlapping of adjacent exposures.)

Here's a closeup of a different part of the PEARLS image. Note that the diffraction pattern of the bright star below and to the left of center is rotated with respect to the patterns of the stars in the upper right. This is a clear indication that this is a mosaic of at least two different pointings with different telescope rotations.

enter image description here

And here's a schematic illustration of what's probably going on, showing what might happen if you wanted to combine two exposures from different pointings. The "Exposure 1" pointing (cyan) is at one orientation, while the "Exposure 2" pointing (red) is at a different orientation. In the left part of the figure, the two pointings have been aligned; there is one star in the overlap region, and two stars in the non-overlapping regions. In the right part of the figure, you can see the resulting mosaic image, with the star in the overlap region having a combination of the two diffraction-spike patterns.

Demo of mosaicing two offset and rotated exposures

If you look at the full PEARLS image (bigger than the zoomed-in excerpts I provided above), you can see that the double-diffraction-spike patterns seem to be in the middle parts of the mosaic image, while the single-diffraction-spike stars are in the left or right sides, presumably because only single exposures went into those parts of the mosaic.

And in fact that text accompanying the PEARLS image alludes to this: "Because this image is a combination of multiple exposures, some stars show additional diffraction spikes."

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    $\begingroup$ This is a really nice, clear answer. Thank you! $\endgroup$
    – uhoh
    Feb 25, 2023 at 0:39

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