One thing I've always wondered about Hubble — if it's orbiting earth, scooping up protons for long periods of time, doesn't that mean it'll create a photo made up of the captures from every part of that small area....in the entire orbit? So two galaxies next to each other on the photo could be on the opposite sides of us really?
Parallax isn't going to cause much error here. We're about 8 light-minutes from the sun, but the telescope was trained on galaxies millions (edit: billions) of light-years away.
Edit: Proportionally, it's about the difference of the view between your two eyes, of the sun.
They keep the telescope pointed at the same area of the sky of course. Stars (except for our sun) are so far away that both their real motion and their parallax motion (apparent motion of the stars due to us moving around the sun) appear very tiny. Hence the term 'fixed stars' from antiquity (as opposed to the 'wandering stars', which turned out to actually be planets one we got telescopes).
I really fail at cosmology…