How large part of the sky has Hubble observed? 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).