Mick,
If you can't afford Photoshop, another alternative, and quite possibly the best I've seen to date, is, The GIMP (The GNU Image Manipulation Program), found at
www.gimp.org.
It was initially created for UNIX and UNIX-like operating systems, such as Linux and FreeBSD. It did so well that it's been ported to the Windows platform. Click on the Gimp for Windows link in the menu on the left-hand side.
It's pretty much a Photoshop clone *hides from those who swear it isn't*, but, for free.
I'm using Adobe Photoshop CS2 for all my astrophotography editing.
I hope that helps you out.
Regards,
Humayun