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Hubble wouldn't have to change focus for Pranksat to work because Pranksat is cleverly designed to present a virtual image with a focus at infinity. But if the diffraction-limited Hubble wanted to bring something much closer than the Moon into focus, at some point it would have to move away from the best focus for celestial objects.

This question was inspired by a discussion under this answer.

Has this ever happened?

Question:

  1. Has Hubble ever focused on something close enough that it had to move away from being focused at infinity?
  2. If so, what's the closest distance that this has happened, or at least would need to if it did?
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    $\begingroup$ Interesting question. Naively, I would have thought that Hubble doesn't even have the capability to change its focus away from infinity! $\endgroup$ Commented Mar 20, 2021 at 13:30
  • $\begingroup$ @user2705196 I'm guessing that it will have some range of focus to accommodate manufacturing errors, thermal and other mechanical variations, etc. During the Shuttle days when they replaced instruments a few times those were not installed to microns of accuracy; there must be some range focusing to insure each instrument on the telescope can achieve best focus. I'm not sure what the full range of focus is though, and if it differs between instruments. That would determine just how close Hubble could focus. $\endgroup$
    – uhoh
    Commented Mar 20, 2021 at 14:38
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    $\begingroup$ See this question: space.stackexchange.com/questions/58765/… $\endgroup$ Commented Jun 8, 2022 at 20:27
  • $\begingroup$ @MarkFoskey I'd already linked the two questions; is there something about Hubble that we should see at that JWST question? $\endgroup$
    – uhoh
    Commented Jun 8, 2022 at 22:15
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    $\begingroup$ Oh, I missed that you had linked it. Should have known you would have, though. My bad. $\endgroup$ Commented Jun 9, 2022 at 0:33

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  1. If so, what's the closest distance that this has happened, or at least would need to if it did?

Hubble's resolution is 0.014 arc seconds (6.8e-8 radian) and its mirror diameter is 2.4 m. So objects closer than (2.4/2)/6.8e-8 m ~= 18,000 km will become detectably blurred and would benefit from refocussing.

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