In a recent astrometry question in Space SE I needed to check Proxima Centauri's position and the article contained the image below.

I've saturated the colors, cropped it, and added a "retroactive coronagraph" (black dot in the center) to show that the color scheme of the dozens of small radial striations is similar to that of the four major spikes coming from the vanes holding Hubble's secondary mirror.

Hubble's point spread function should contain diffraction from various things including:

  1. the primary aperture
  2. the central occlusion of the secondary
  3. the four vanes holding the secondary
  4. the three holes in the mirror (!!!) used to mount it

but there must certainly be more items in this list.

Since this is an intensity map rather than amplitude, there is no phase information so I can't perform an inverse Fourier transform to see what's causing this, so instead I'll ask here.

Question: What produces all of these small radial striations in this very overexposed image of a star by Hubble's WFC2? (the four big ones are from the vanes)

It seems that this image can be found at https://esahubble.org/images/potw1343a/ which says that this image contains two wavelengths: Optical R 675 nm and Infrared I 814 nm. Since diffraction is purely chromatic we can indeed expect any diffraction effects to demonstrate strong dispersion (i.e. those colorful two-and-not-three-color patterns in this image).

processed from New shot of Proxima Centauri, our nearest neighbour New shot of Proxima Centauri, our nearest neighbour

Source click the right one for larger

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    $\begingroup$ I won’t make it into an answer, but I think these are called speckles. They depend upon the exact wavefront of the imaging system—in other words, they’re always the same for all point-like objects (except for their brightness, of course, which is proportional to the object’s brightness) for a given imaging system. Side note: It’s through the removal of speckles that astronomers are now able to directly image exoplanets, through a process conceived by Canadian astronomer Christian Marois. $\endgroup$ Commented Feb 6, 2021 at 5:44
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    $\begingroup$ @PierrePaquette ah I see what you mean. Because I'm of the right age, I'm more familliar with the term "speckle" from Speckle interferometry which was quite hot when I was an undergraduate studying Astronomy. Those were dynamic and due to the atmosphere, but sure there can be static speckles due to fixed wavefront errors as well. $\endgroup$
    – uhoh
    Commented Feb 6, 2021 at 5:53
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    $\begingroup$ I asked a friend of mine who worked on Hubble: I suspect that it’s in the instrument, rather than the primary [but otherwise I don't know]. This is really high dynamic range and there’s all sorts of bits between the OTA proper and the CCD. One bit of trivia - the three “pads” on the primary are not mounting holes, but a restraining device. Had the spacecraft ever come back down with the shuttle, they were concerned about the primary coming detached and flying up the shroud into the crew compartment. The metal pads held the mirror on its reflective side to prevent that. $\endgroup$ Commented Feb 7, 2021 at 8:24
  • $\begingroup$ @StuartRobbins cool, thank you for all the information! $\endgroup$
    – uhoh
    Commented Feb 7, 2021 at 13:09


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