Hi Bert
Your original image was fine, all it needed was a run with Gradient Xterminator, a Photoshop Plugin, which gets rid of most of the vignetting, and also neutralises any colour cast in the background. It even gets rid of most of the amp glow in my 300D camera.
Gradient xterminator site
http://www.rc-astro.com/resources/GradientXTerminator/
Its well worth it. Only time it struggles is if imaging nebulosity that extends off the image, it works best if you can draw round the nebula or object, allowing Gradient Xterminator to smoogh out the area outside that.
So the best thing is to process your images as you normally do, and run Gradient Xterminator once youre at the stage where you are using Photoshop
Scott