Starship Asterisk*

johnnydeep wrote: ↑Sun Mar 20, 2022 4:05 pm So, what explains the 8 diffraction spikes we see in this image? There are the six large equal spaced ones, which I assume are due to the hexagonal mirror segments, but there are also two small horizontal ones. Or are those not diffraction spikes at all and just "processing artifacts" as I thought might be being hinted at in some of the posts above? Also, where are the diffraction spikes caused by the three secondary mirror support struts?

Also, I notice that the diffraction spikes exhibit some striated complexity - what's causing that?

Chris Peterson wrote: ↑Sun Mar 20, 2022 4:40 pm

johnnydeep wrote: ↑Sun Mar 20, 2022 4:05 pm So, what explains the 8 diffraction spikes we see in this image? There are the six large equal spaced ones, which I assume are due to the hexagonal mirror segments, but there are also two small horizontal ones. Or are those not diffraction spikes at all and just "processing artifacts" as I thought might be being hinted at in some of the posts above? Also, where are the diffraction spikes caused by the three secondary mirror support struts?

Also, I notice that the diffraction spikes exhibit some striated complexity - what's causing that?

The secondary mirror is supported by two struts that are aligned with the hexagonal mirror vertices, and therefore their diffraction spikes overlap those from the mirrors themselves. The upper support strut, however, is aligned with the mirror edges and its diffraction pattern is seen in the much less prominent horizontal spikes.

Not sure about the striation. Might be related to the massive overexposure of this image and the effects of the internal mirror edges. Might try playing around with the math a little later.

UPDATE: Here's the aperture pattern (mirror segments and support struts), and the Fourier transform of that pattern, which is basically what the diffraction pattern is. (Zoom in for a better view.)
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JWST_optics_fourier.jpg

AVAO wrote: ↑Sat Mar 19, 2022 4:34 pm I'm a bit surprised that the quality isn't better than Hubble when comparing the resolution of details to the nearest Hubble object. The third image in the series shows the nearby Cat's Eye Nebula overlaid at the same size. But of course, the process of optimizing the instruments is not yet complete.


https://live.staticflickr.com/65535/519 ... 2763_k.jpg
https://sky.esa.int/?target=268.9184271 ... ue&lang=en

https://live.staticflickr.com/65535/519 ... 412c_k.jpg

https://live.staticflickr.com/65535/519 ... fc11_k.jpg
Jac Berne (flickr)

Knistian wrote: ↑Mon Apr 25, 2022 5:33 pm

AVAO wrote: ↑Sat Mar 19, 2022 4:34 pm I'm a bit surprised that the quality isn't better than Hubble when comparing the resolution of details to the nearest Hubble object. The third image in the series shows the nearby Cat's Eye Nebula overlaid at the same size. But of course, the process of optimizing the instruments is not yet complete.
...

Regarding the resolution of the engineering-picture from JWST, where several think it's low: It has to do with wave-length. It's unfair to compare a picture taken in the UV-Visible range(where Hubbel opperates), and compare it directly with an image taken in the deeep part of the IR. The diameter of JWST is ca. 2.3 of Hubbel, so wavelengths 2.3 times as long as visible would be at the same resolution (with teoretical best acheivable resulution from both). The Webb has acheived to get a resolution at the wavelength used in the image, that is not limited by missalignement by the optics, but by fundamental limitations given by the diameter of the primaty mirror.

Knistian wrote: ↑Mon Apr 25, 2022 5:33 pm Regarding the resolution of the engineering-picture from JWST, where several think it's low: It has to do with wave-length. It's unfair to compare a picture taken in the UV-Visible range(where Hubbel opperates), and compare it directly with an image taken in the deeep part of the IR. The diameter of JWST is ca. 2.3 of Hubbel, so wavelengths 2.3 times as long as visible would be at the same resolution (with teoretical best acheivable resulution from both). The Webb has acheived to get a resolution at the wavelength used in the image, that is not limited by missalignement by the optics, but by fundamental limitations given by the diameter of the primaty mirror.