Processing Images with Photoshop  
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This "How-To" was aquired for another source on the internet (I don't remeber where).


Align and Combine Images
I recommend using your astro imaging software (e.g., CCDSoft v5, MaxIm DL, etc.) to do your
alignment and combining. These programs usually support sub-pixel alignment, and Photoshop 
does not.

[Note: You can simulate sub-pixel alignment in Photoshop by enlarging the images 2x, 3x, or 
4x, and then performing the alignment. After alignment, downsize to the original dimensions.]

1.      Perform data reduction (dark subtraction, flat fielding).

2.      Look for flaws (remaining hot/cold pixels; dark spots from cosmic ray hits on the dark 
        frame; bright spots (false stars, arcs, lines) from cosmic ray hits on the images); 
        bright/dark columns and rows. Fix these before you move on.

3.      Align all images with each other. I recommend aligning all individual reduced images 
        before you do any combining. This gets all of the alignment over at one time. Otherwise
        you have to align the individual images you use to build you four master images, and 
        then you have to align your master images. Every alignment step introduces a small 
        amount of softness into your images.

4.      Determine what combination method to use for making the four master images. If there 
        are a lot of hazards (airplanes, meteors, cosmic ray hits, etc.), use a median combine. 
        If the images are clean, or can be cleaned up with pixel editing, sum or average will 
        give you better S/N than median combine.

5.      Create the four master images.

6.      Combine the R, G, and B images into an RGB images (usually in MaxIm DL or Photoshop).

7.      Optional: Remove gradients from the combined images now using automatic tools in Mira, 
        MaxIm DL, etc. If you are using Photoshop for most of your processing, and the 
        automatic tools won't work well enough to suit your requirements, hold off on gradient 
        removal.

8.      RGB combine using MaxIm DL or Photoshop.

Check point: you now have two files for export into Photoshop: an RGB file in 16-bit TIFF 
format, and a Luminance image in 16-bit TIFF or FITS format. [Note: if you are only doing an 
RGB  combine, no L layer, you only have the one file. Skip steps relating to the luminance 
layer below].

Histogram Adjustments in Photoshop
Perform the following histogram adjustment steps on both RGB and L images in Photoshop or your 
favorite image editor.

9.      Adjust Levels by lowering the white point so that you are just able to see the brightest 
        areas of nebulosity or galactic detail. (Note: depending on the image, you may want to 
        lower the white point further. The lower you set the white point in this step, the 
        greater the overall contrast in your image. The higher you leave the white point in 
        this step, the lower the contrast but the greater the range of brightness levels 
        represented in your final image.) While you are lowering the white point, raise the 
        black point as needed if the background gets bright. You may raise the black point at 
        any time during processing if the background seems bright. Just don't go so far as to 
        eliminate any of your dim details! Repeat this step several times if you find yourself 
        lowering the white point by a large amount, to avoid coarse adjustments.

10.     Use Curves to generally brighten the midrange details. You have two basic ways you can 
        use curves at this point (with a few variations that I will ignore until I can write a 
        tutorial on them <g>): drag a point upward from the left half of the curve, which will 
        brighten dim areas at the risk of burning out the bright areas; drag a point upward 
        from the right half of the curve, which will brighten the bright areas while keeping 
        dim areas fairly dim. The first method is good on images that have very good signal to 
        noise ratio, or for the first 1-2 times you apply curves to an image that needs a lot 
        of curves adjustment. If necessary, flatten the right-hand portion of the curve to 
        prevent it from clipping (see text of book re: clipping). This brightens dim details 
        without making stars over-sized or bloated. Following use of this technique, your last 
        Curves adjustment should be used to emphasize the bright areas appropriately, since you
        have been suppressing brightening them. This technique favors bringing out dim details 
        when there is a very bright core to cope with. If necessary, raise the black point as 
        you adjust curves to keep the background fairly dark gray.

[NOTE: Stop short of full application of curves! You want to have room left for additional 
curves adjustments a little later on. But don't leave too much adjustment for later, or the 
following conversion to 8-bits per channel might add objectionable noise.]

Check point: convert to 8-bits per channel.

Image Corrections and Conversions
11.  Gradient removal 

12.  If the image does not have a large object or much nebulosity: Remove gradients by making 
     a duplicate of the image layer, then apply Dust & Scratches filter with a radius of 5-15 
     to remove most stars, then clone to remove remaining stars/nebulosity/galaxies. Set 
     opacity to 75-95%, and blend mode to Difference. Poof! Gradients are gone.

13.  If the image has a large object or extensive nebulosity, you'll need to use one of the 
     manual techniques outlined in the book to remove gradients. 

14.  [Optional] If necessary, using the Channels palette, fine tune the black point for the 
     RGB channels. If you used the 4a gradient removal method, your background should be in 
     excellent shape as a result. If you did manual (or no) gradient removal, then raise the 
     black point of each channel to make it a very dark gray. Use your eye to get all channels 
     as close to the same shade of gray as possible.5. Save these two images.

Check point: You now have two 8-bits per channel images that look pretty good, one RGB and 
one L. 

15.  Convert the L image to RGB.

16.  Flatten the L image, and save it under a new name. I suggest <objectName>_LRGB.PSD, for 
     example, to show that this will be an LRGB image. (Use RGB if no L!)

Create the LRGB Image
17.  Flatten the RGB image, then copy and paste the RGB image into the L
     image. Make sure the color image is above the L image. Close the RGB image without saving 
     the flattening!!! You only flattened it to combine the two layers into a single layer.

Check point: Verify alignment of the two images by temporarily setting the opacity of the RGB 
layer to ~50%

18.  Set the blend mode of the RGB layer to Color. The result is an image that takes all of its 
     color information from the RGB layer, and all of it's luminance detail from the L layer.

Tune the LRGB Image
19.  Observe the quality of the color. If it looks grainy, apply a moderate Gaussian blur to 
     the entire RGB layer. Moderate would be a radius from 0.8 to 2.0. If the image is very 
     noisy, use an even larger radius. Note that the larger your radius, the greater the 
     reduction in color saturation. You are trading color saturation for freedom from color 
     noise.

20.  Temporarily hide the RGB layer.

21.  Examine the L layer to determine which brightness levels contain noise, and which do not. 
     Select the noisy brightness levels (usually the background, but may include a portion of 
     the dim details in nebula and galaxy images) using the Color Range tool, and apply a very 
     light Gaussian blur. I like 0.3 or 0.4 radius for this. For extremely noisy backgrounds, 
     apply a 0.6 radius.

22.  Examine the L layer to identify the brightness levels that contain very good S/N. This is 
     not simply the inverse of the selection in step 11. You may want to omit stars from this 
     selection, as you could wind up creating halos around them when you sharpen. Use Color 
     Range, and select the bright areas but not the white areas; this will help you avoid 
     selecting stars if you choose to do so. Apply an unsharp mask to the selection, usually 
     at 50%, a radius of 1.0 to 2.2, and 0 for threshold. See text for more on use of this 
     tool.

Check point: Show the RGB layer. Look at your image and decide how you might want to adjust 
the histogram.

23.  Make minor histogram adjustments, using levels and curves, to finalize the appearance of 
     the image. Use levels to raise the black point so you have a very dark gray background 
     (or as dark as the S/N permits). Use Curves to brighten the brightest nebula detail to 
     give you satisfactory overall contrast. Use Curves to emphasize any portion of the 
     brightness range that needs it. Use Curves to hide noisy areas that remain after any of 
     the above. Perform a final Levels adjustment to set the black point precisely where you 
     want it.