Fortunately today in the age of high powered computers the digital darkroom has come of age and gone are the days of smelly chemicals and "darkroom drudgery". The two leading contenders in the astrophotography software market are the old standby "Adobe Photoshop" and the recently released "Picture Window".
I am by no means an expert in the use of Photoshop and it would be a waste of time and unfair to my readers if I undertook an explanation on it's use. Photoshop, in my opinion is an extremely difficult program to use and a substantial amount of practice is necessary to achieve success with it's use. Instead, I would like to refer my readers to astropix.com. This is Jerry Lodriguss's excellent site on the digital enhancement of astrophotos.
Instead, I would like to show you what is possible with digital enhancement and let you make up your own mind whether or not you think digital enhancement is for you.
Below are three stages I employed on repairing a badly scratched negative, cropping out a bad area of the composition, and color correcting it to make a more pleasing image.
Here is an of a photo made from a badly scratched negative. Also note that on the far left is an off-color band that is either an artifact of the developing process or a flaw in the negetive itself (which I doubt since no discolored area is apparent on the negative when examined with a jewelers loop. By the way, this photo was scanned from a print.). Also apparent is that the print was not placed in on the scanner bed perfectly straight and that the image borders are missaligned.
As you can see, the negetive was well exposed with a fairly dense image of M31. Since this image is "repairable" there is no reason to not try to make the necessary corrections digitally.
The technique used here was judicious use of the "clone tool". I have found that magnifying the image enough so that the individual pixels are visible is the best method for scratch repair. Bring up the "brushes" option and select the feathered brush option. Move the mouse just off the scratched area and copy the background using the "Alt" key and the right mouse button. Now move the mouse cursor over the scratch and click the right mouse button. This will copy the background you copied onto the scratched area. Alternately move the copied area both above and below the scratch to prevent repeating the background pattern and making the repair obvious. When I meant "judicious" use, I meant that as long as the backgound being copied contains no stars and is homogenous with the area to be repaired then the "ethics" of photo restoration are not being violated.
Look at the previous image and notice the discolored "band" on the left edge which most likely is a result of contaminated developing chemicals or dirty rollers in the automatic developer. Photoshop experts can probably call on the vast resources of the program and correct this area but I haven't developed the expertise to do so yet. So I took the simple approach and simply "cropped out" this undesired portion of the image. The end result to the right in my opinion doesn't take away from the photo in the least. This operation is performed easily in Photoshop by clicking the crop tool and drawing an outline around the area of the image to be saved. The crop tool is also useful for reframing images that have been made with the use of a focal reducer. With f/10 cassegrains such as mine, an f/6.3 reducer severly vignates the image resulting in the image being within a circular area. To me this is unattractive and is easily remedied with the crop tool.
To the right, see the results of various adjustments to the color balance and levels options. The sky background was darkened using the "Selective Color" option along with the "Brightness and Contrast" options. I can find no regimented procedure to follow when adjusting the color balance, etc. in Photoshop. What works one time does not seem to work in another image.
I save a copy of the original image in bitmap format and then make a .bmp copy for safekeeping in case my adjustments don't go as I would like. This way, I can always start fresh and try something else.