That's the most detailed? Google Sky has about 2x the magnification of a random, boring spot I picked, and much higher on interesting locations. Maybe there's some small spot which goes to 8x, but otherwise I think they've been beat.

(comparison: both scale roughly 2x on each level. linked: 9 levels. Google: 10 levels before informing of no more data, many boring locations go higher. Apparently, I found one of the least interesting sections to try first.)