Image Processing Tutorial #2
M31 Andromeda Processed in Photoshop CS.
This tutorial assumes you have a pre-stacked group of raw files. The example uses a stack of M31 images captured thus:
Optics: Celestron C102 refractor, focal length 500mm, f/5.
No filtration.
Mount: Celestron CG5 goto, unguided.
Camera: Canon 400D (XTi), ISO 1600, auto white balance.
Frames captured: 65 at 2 minutes each, 39 at 1 minute each.
Frames stacked in Nebulosity, dark and flat frames applied. Stack saved as 16-bit TIFF file.
There is moderate light pollution at my site (suburban Santa Barbara, CA). Shooting date: July 18 & 19, 2007.
The unaltered stack, composed of 104 linear frames combined in Nebulosity. This stack is available for download on the Galaxy page.
The first of several Curve functions used to bring out shadow detail. The upper area of the curve is moved downward to prevent burning out highlights. Note the histogram has begun to expand.
Now that the fuzzy edges can be seen (a result of aligning so many frames), the image can be cropped.
Soft Color Gradient Removal, a tool in Noel Carboni's Photoshop Action Set, is used to reduce the amount of light pollution present.
Another application of the Curves function and detail can barely be seen in the galaxy's outer edges.
The Levels function was used to set the black point (re-darkening the sky) and to further stretch the histogram. Now the detail is really coming out.
5 of Carboni's Action Tools were applied:
1. Reduce Large Blue Halos
2. Reduce Small Blue Halos
3. Color Blotch Reduction
4. Horizontal Banding Noise Reduction
5. Vertical Banding Noise Reduction
Their effect is subtle, and doesn't show very well in these small images. Much experimentation is required to know which tools to use and when to use them.
Another Curve brings out more outer rim detail, but lightens the sky background too. Note the position of the bottom left corner of the curve line.
Again with Curves, the red and green channels are reduced to restore color balance (at least as much balance as can be had after so much light pollution removal). Note the red is reduced about three times as much as the green.
click here.
There is a better version of a final image made from this stack on the Galaxy page.
Remember that each image, even of the same target, requires its own variations on processing.
The example shown here was the result of trial and error. If I processed this image again
I would try different techniques and would most likely get different results.
