This is the image of NGC2903 before applying a gradient removal technique. The way I do it in PhotoShop7:

Duplicate the image.

Use Filter-Noise-Dust&Scratches
Leave threshold at zero, move the radius slider to the right until all detail disappears, all the way to 100 pixels if you want.

Now you're left with a copy of the background gradient.

Click on your original image to make it active, then select Image |Apply image | and apply that
background image of the gradient. "Source" is the gradient image, "Layer" is Background, "Channel"
is RGB, "Blending" is Subract, "Opacity" is 100%, "Scale" 1, "Offset" 0.

Play with Opacity and Offset to taste
Byebye gradient.

Image with gradient removed: http://www.pbase.com/emagowan/image/91630241