CS2 is free, it was released to the public by Adobe about 3 years ago to get wannabes hooked. But I wasn't referring to that for my 'three'. They were DSS, Autostakkert and Registax respectively. Sorry for the confusion.
Substitute for Photoshop would probably be the latest version of GIMP. Every bit as powerful and comprehensive. And free. I just happen to be more proficient ( read; learnt from more mistakes !! ) with Photoshop, PixInSight just confuses me, sorry afficianados.
