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?


  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?
  • 1
    $\begingroup$ Interesting question. Naively, I would have thought that Hubble doesn't even have the capability to change its focus away from infinity! $\endgroup$ Mar 20 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
    Mar 20 at 14:38

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