UNSHARP MASKS FOR PLANETARY IMAGES
using Adobe PhotoShop
by Jim Ferreira, contributions by Gordon Garcia
using Paint Shop Pro
by David Moore
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The two Saturn images above are made from the same TIFF image file.  The one on the left was sharpened slightly with PhotoShop's unsharp mask command and adjusted for brightness and contrast using the LEVEL settings.  The image at right was processed with a custom unsharp mask intended to improve the dynamic range of the image, another words, show the full range of gray scale available in the image.  In the image at left, the Crepe ring can only be seen where it crosses the planet while in the image at right the Crepe ring can be seen against the black sky.  Also the darker clouds over the polar region are more clearly defined in the image at right.  And there is even some hint of the Encke division.....well, maybe just wishful thinking.

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Saturn imaged with a 25cm f/6 reflector, 3x Barlow lens and PC23C video camera fitted with a #25 red filter.  Video was recorded to a Hi8 camcorder and 8 single frames were captured to computer from the video tape using a Snappy frame grabber.  The 8 frames were then mildly sharpened and stacked using Adobe PhotoShop.  The image is reasonably sharp with Cassini's division and an equatorial cloud belt clearly visible, however the Crepe Ring is only visible where it crosses the planet.
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 		 Increasing contrast and sharpening moderately with the PhotoShop unsharp mask darkens Cassini's division a little and improves the contrast of the cloud tops but further hides the Crepe ring except in front of the planet.
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Increasing brightness until the Crepe ring can be seen against the sky washes out the cloud tops on the planet, lightens Cassini's division and produces a gray rather than black sky.
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APPLYING A CUSTOM UNSHARP MASK IMPROVES THE DYNAMIC RANGE, BRIGHTENING THE CREPE RING WHILE MAINTAINING THE DARKER VALUES. 
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The following is a step by step 'how to' make and use a custom unsharp mask with Adobe PhotoShop.
First open normally an image file, then make a duplicate image [go to IMAGE, scroll down to DUPLICATE].  Next the duplicate image is blurred [go to FILTER, scroll down to BLUR, click GAUSSIAN BLUR, set to 2.0 pixels].
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Now reduce the brightness and contrast of the blurred image [go to IMAGE, scroll down to ADJUST, scroll down to BRIGHTNESS / CONTRAST, set each to minus(-) 30, then click OK.
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Now select the original image and subtract the darkened / blurred image [go to IMAGE, scroll down to APPLY IMAGE > in the APPLY IMAGE box choose the DUPLICATE IMAGE as the source, go down to BLENDING and set it to SUBTRACT, OPACITY should remain at 100%, click OK.]   The original image is now darker but some obvious change has occurred.
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Now use the image level controls to bring up the brightness and contrast [go to IMAGE, scroll down to ADJUST, click LEVELS > bring up the high values (right side of histogram) and the mid range levels until the Crepe Ring can be seen against the dark sky.  Here it is a balance act between darkening Cassini's division and keeping the Crepe Ring visible].
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The suggested adjustments are just that, suggestions.  The amount of blurring needed and subtracted contrast / brightness adjustment will vary from image to image, and of course to individual taste.  Experimentation is the key. 

Thierry Legault and Antonio Cidadao both offer useful information on image processing of planetary images.  Jerry Lodriguss has an excellent collection of articles on CCD imaging, one of which clearly explains the use of unsharp masks for improving the dynamic range in images of deep sky objects and is obviously applicable to planetary images.
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Gordon Garcia used a custom unsharp on a H-alpha solar image.
Active Region 8614 taken on July 10, 1999 at 14:36 UT with my 5.1 f/8 AP refractor, Daystar H-alpha filter and 4X Powrmate.  The unprocessed image above has been cropped, resized and saved as a JPEG, but no unsharp masking, histogram adjustment, etc.  
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.		 An UNSHARP MASK was made and applied  3.0 radius for the blur, and -63 for contrast, -45 for brightness. I adjusted the histogram of the original image before the mask was applied and after. Finally I did a sharpen and then saved the TIFF as a JPEG.  

"The improvement I note over the regular unsharp mask & sharpening is additional detail in the highlight (bright) areas of the image."
 
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