Image Processing in Photoshop

Above:  RGB image assembled in MaxIm DL compared to final image after Photoshop processing

Standard Workflow

Despite the many features of programs such as MaxIm DL and Photoshop, you will find that you typically use a limited set of tools to enhance astrophotos.  You can always try new things, of course, but in general you will find a routine that works and use it on most of your images.  In Photoshop, my usual workflow goes like this:

• Combine Luminance and RGB images

• Adjust Curves in individual layers

• Adjust Color Balance and Saturation in RGB layer

• Flatten image

• Clean up image with Clone Stamp/Healing Brush

• Apply Zone Mask filter

• Make any final Color Balance and Curves adjustments after filtering

• Save master TIFF file

• Save smaller JPEG web file

Combining Luminance and RGB Images

Photoshop gives more control over combining LRGB images, so I always use it instead of MaxIm for this purpose.  Once the images have been aligned in MaxIm, stacking them in Photoshop is simple.  Copy the Luminance image and paste it on top of the RGB image.  (Note, the Luminance is black and white, so if you do the reverse and paste the RGB onto the Luminance, the color goes away.)  Once the Luminance layer is on top of the RGB, change the layer blend mode to Luminosity.  For more details on this technique, see the LRGB Images with Photoshop page of the Software Instructions section.  The resulting image has the RGB image in the bottom layer and Luminance image in the top layer.

Above:  Two layers stacked to create LRGB composite

Above:  Initial LRGB image

Adjust Curves in Each Layer

The next step is to adjust the individual layers while they are still separate.  I find the best adjustment to be Curves.  The basic procedure that seems to work well for almost all LRGB images is to lower the midpoint of the curve of the Luminance layer and raise the midpoint of the curve of the RGB layer.  The best way to do this is by applying Adjustment Layers.  This lets you go back and play around with the Curves setting later without affecting the original data in case you don't like the results.  Be sure to apply the top layer mask (for the Luminance layer in this example) as a Clipping Mask so that it applies directly to the top layer only.  Otherwise it will affect all the layers below it, which is not what you want.  For details on Clipping Masks, see the tutorial on Using Clipping Masks to Create Color Images in the Software Instructions section.

Above:  Layers window showing Curves adjustment layers.  Note the top adjustment layer is a clipping mask.

Above:  Curves settings for the RGB layer (left) and Luminance layer (right)

Adjust Color Balance and Saturation in RGB Layer

The next step is to make any color balance adjustments necessary and to boost the color saturation, which usually is a little weak after LRGB combining.  Again this is done with Adjustment Layers to allow future changes if necessary.

Above:  Layers window with Color Balance and Hue/Saturation adjustment layers added

Above:  Color Balance and Hue/Saturation adjustments applied to RGB layer

Above:  Image after applying adjustment layers.  Now it's starting to look good!

After all the image adjustments have been made to the individual layers, I save the image in PSD format, just in case I totally botch something later (which I've been known to do).  Then I flatten the image and save it as a 16-bit TIFF.

Cleaning Up the Image

There are almost always some defects in the image to clean up.  There will be some artifacts from the blooming spike removal as well as dead and hot pixels.  These are easily removed using either the Clone Stamp tool or the Healing Brush/Patch Tool.  I find the Clone Stamp best for removing bad individual pixels, especially from the background sky.  The Healing Brush works like magic for pixel defects within the subject itself.  For larger defects, the Patch Tool also works very well, often even better than the Healing Brush.  Removing blooming artifacts can be trickier.  This takes a combination of all three tools.  Artifacts far from stars can be removing using the Healing Brush, but when you get close to a bright star, the healed area gets too bright and the Clone Stamp is a better choice.

Above:  Pixel defects near the core of the galaxy removed using the Patch Tool and about 12 seconds of effort.  I love Photoshop!

Zone Mask Filter

The last major step is to apply a Zone Mask Filter.  The Zone Mask is a plug-in developed by Starizona to provide significant enhancement to astronomical images.  This image was used for the Zone Mask tutorial linked above, so for specific instructions see the tutorial.  The Zone Mask filter is one of the last adjustments to be made to this image.

Above:  Before and after Zone Mask filter.  Note the considerable increase in detail in the right image.

Final Adjustments

The last thing is to tweak the Color Balance, Curves, Levels, etc., if necessary.  Little adjustment was necessary for this picture, just a touch on the Curves.

Above:  Final image

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