Super-Unsharp Masking
Unsharp masking can be used to significantly enhance the detail visible in certain objects. It can also easily make an image hideous. The process is often overdone for no apparent gain. However, on some objects, the technique of super-unsharp masking can go beyond mere novelty and actually show off an incredible level of structure which is otherwise invisible.
Unsharp masking is generally used to sharpen an image. Properly using the technique in this manner produces a subtle result. In Photoshop, the parameters for the Unsharp Mask filter include Amount, Radius, and Threshold. Running the Amount and Radius sliders way up produces an extreme result, often containing many artifacts such as dark halos around stars and saturated bright regions which no longer contain detail. These artifacts can be avoided to some extent using the "true" unsharp mask technique outlined in the Image Processing Filters section.
Images of large, diffuse nebulae can often be greatly enhanced by running a very strong unsharp mask filter on them. To avoid nasty processing artifacts in the image, a new technique must be used. The super-unsharp mask routine allows for extreme sharpening and contrast enhancement while minimizing the problems of standard unsharp masking.
How It Works
The super-unsharp routine begins by removing the bright stars from the image. This eliminates the source of black halos. The stars are copied to a new layer to be replaced later and removed from the original image. The sharpening filter is run on the original layer (with stars removed). The stars are replaced and are again copied, along with the brighter regions of the nebula. These are placed into another new layer. The contrast is enhanced, allowing structure to become visible in parts of the nebula which were all but invisible in the original image. This saturates the bright regions of the nebula which are then replaced with the copied regions in the new layer.
Procedure
Begin by opening the image you wish to enhance. For this example we will use a black-and-white image of the Rosette Nebula.
Above: Original image of the Rosette Nebula.
To remove the stars from the image, we must select them. The best way to do this is using the Color Range tool (even though there is no color in this image). Color Range works in a black-and-white image by selecting regions of similar brightness. Choose Select > Color Range from the main menu.
Above: The Color Range window shows the selected areas of the image.
To select the stars in the image, use the eye dropper tool in the Color Range window to select one of the fainter stars in the original image. This will choose most of the stars in the image as well as the brightest parts of the nebulosity. Run the Fuzziness slider up to around 100-150 or so to expand the selection to a wider range of brightness. Click OK.
Above: The modified Color Range selector.
Above: The selection after using the Color Range selector.
Next, expand the selection slightly to grab some of the area around the stars. The star images consist not only of the bright cores of the stars but also a dimmer ring around each star. It depends on the size of the image, but typically 1-3 pixels around each selected star should be included in the selection. Go to Select > Modify > Expand and increase the selection by a couple pixels.
Above: Expand selection.
Next, save the selection, as it will need to be used again later. To do this, go to Select > Save Selection. Use the default settings and type in a name for the selection, such as Stars.
Above: Saving the selection for later use.
Press Ctrl-J to paste the selection on top of the original image as a new layer. The appearance of the image will not change, but you should see a new layer containing the selected stars and nebulosity above the original background layer. (Select Window > Show Layers if the Layers window is not open.)
Above: The stars have been pasted into a new layer above the original image. The layer has also been renamed Stars.
Click on the eye icon in the Layers window on the top layer. This will turn off the copied stars for now. We'll add them back in later. Select the background layer by clicking once on it in the Layers window.
The selection has disappeared so you must reselect the stars to remove them from the image. Simply go to Select > Load Selection and choose the Stars selection you saved earlier.
From the main menu select Filter > Noise > Dust & Scratches. It will depend on the image, but select a Radius of about 4-8 and leave theThreshold at 0.
Above: Dust & Scratches settings to remove stars.
It's almost like magic. Click OK then press Ctrl-D to deselect the image. Most of the stars (certainly all the bright ones which cause unsharp-masking problems) have vanished from the image.
Before sharpening, it is recommended that you select all but the brightest parts of the nebula for filtering. Use the Color Range selector again to choose the brightest regions of the nebula. Choose Select > Feather from the main menu and choose a value of around 5-15 (it depends on the image size). Choose Select > Inverse. This will keep the brightest parts of the nebula from being sharpened which tends to cause artifacts in the final image.
Select Filter > Sharpen > Unsharp Mask from the main menu. Play with the settings until you get the appearance you want for your particular image. For this image, the Amount was set at 100% and the Radius at 12.
Above: The background layer after running the strong unsharp mask filter.
Click on the eye icon in the top layer to replace the stars and bright nebulosity. Select Flatten Image from the Layers window or Layer main menu to merge the two layers into one final image.
Above: The final super-unsharp-masked image of the Rosette Nebula. Click image for a higher-resolution version.