At the risk of getting into a `PI vs PS' thread, which I know we all want to avoid, I'll point out something in PI's defense:
Although PS is popular with many astro-imagers, it doesn't come with a manual if that's what you're going to do with it. The vast majority of documentation sources for PS are all about terrestrial photography, and don't come with PS by default anyway. So, for astro-imaging with PS, you basically have to buy Ron Wodaski's `New CCD Astronomy' book, Scott Ireland's `Photoshop Astronomy' book, maybe some books or CDs from Jerry Lodriguss, and if you're *very* lucky, you might land a rare copy of Ron's `Astro Zone System' book. (I was one of the lucky ones who got a copy at AIC 2011.) One will probably have bought Berry and Burnell's book, too.
Then, there are other video tutorial series, such as... Warren's, for one. Plus those by Adam Block and others.
In the end, I'd argue that if one doesn't know much about PS, and one starts using it seriously as an astro-imager, it requires as much of an investment in `outside learning materials' as PI.
- Marek