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?
- Has Hubble ever focused on something close enough that it had to move away from being focused at infinity?
- If so, what's the closest distance that this has happened?