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.