First, I need to clarify what I mean by “free”. I do not mean that this is software that you are free to do with as you wish, but it is probably free for personal use. Always check the terms and conditions, usage limitations of any software that you download and respect the authors rights. With that out of the way, let’s look at some of the excellent sofware that is available.
Planetraium software for your computer. I love this bit of software available from http://stellarium.org/. Don’t let the version number of nought point something put you off, this really is a slick piece of software. As it says on the site it is free, open source and gives you a realistic view of the sky, great! Try it now!
Registax is comprehensive image processing software particularly useful for stacking image. It is available from http://www.astronomie.be/registax/. A robust piece of software with a long standing history of development, version 6 looks like it will be released in early 2011.
Windows and linuX Astronomy Capture. A neat piece of software for capturing single or image sequences. Works well with the Philips Toucam and has the facility for capturing long exposure images with a webcam – assuming that you have the right hardware. Available from http://arnholm.org/astro/software/wxAstroCapture/.
The GNU Image Manipulation Program (GIMP for short) is an excellent and free piece of software. OK, it doesn’t (yet) have the fill power of Photoshop, but in terms of value for money it is a winner. Available for download from http://www.gimp.org/ primarily for Linux, executable are also available (follow the links) for MS Windows and elsewhere.
The GIMP is very much a community effort, and you will find a lot of helpful tricks with a quick search on the web. For instance, hot pixel removal is explained very well at http://gimpguru.org/tutorials/removinghotpixels/ and as the URL suggests there are a number of other good tutorials there in addition to the documentation pages at http://www.gimp.org/docs/.
If you are dabbling in astrophotography and are on a budget then this piece of software, perhaps coupled with something like Registax, is a winner.
Not free software
The software above actually covers all of my astronomy needs (so far), however there are obviously a large number of commercially available astronomy-related software packages available too which can, sometimes, give you something more. Obvious ones that I have a little experience of are: