I'm curious for an estimate of how wide a field the Hubble space telescope has observed. Say, for example, how many times it has changed its pointing, and what fraction of that were revisiting the same field.
25 years of say 16 orbits/day is a (very over-estimated) maximum of 146,000 pointings.
The largest instrument is the various incarnations of the optical imager with a FOV of 6 (WFPC2) to 11 (ACS) square arcminutes. Thus the absolute maximum fraction of the sky (40,000 square degrees) that can have been covered is 1 per cent. And this is a big overestimate, since many parts of the sky have received many repeated pointings (e.g. the Hubble deep field).
I found another discussion on the same topic here:
Which is pretty old, but at least some numbers. According to that I gues its far bellow 10% these days.
It is also interesting to search Hubble Legacy Archive for "footprints" - but this does not cover the whole sky at once, you need to search for an object and see if there are some footprints in its area on a flashplayer-powered map.