Image Sharpening and Blurring

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Bright areas of an image have enough signal to benefit from sharpening. Dark areas of an image have weak signal, giving rise to a grainy appearance that could benefit from blurring. Most astronomical image programs have the filters to both sharpen images and blur images. The disadvantage of these routines is that they usually apply to the entire image. Photoshop has the ability to select a region of the image to apply the filter selectively.

Sharpening is usually reserved for the luminance channel of an image. If you are working with an RGB image, you can "fade" the filter to the luminance by clicking on edit, fade, luminance, 100% (or less) to limit the filter to the luminance portion of an RGB image. This avoids sharpening color, which can create a grainy color texture. If your image has a luminance with a low opacity under 50%, you can first flatten the image before sharpening, and then fade the filter to luminance.

Use the lasso tool to select the brighter areas of the image that you wish to sharpen. Then, use the magic wand tool to deselect (Alt click) the bright stars in your selection. Finally, shrink your selection by 5 pixels (select, modify, contract) and feather your border by 5 pixels (select, feather). Your selection is now ready to apply sharpening.

The easiest sharpening tool is "unsharp masking." Typically, choose an amount between 50 and 100%, a radius of 3-5 pixels, and a threshold of 0 if you have a good image, but a higher threshold if your image is noisy. A more precise sharpening tool is "smart sharpen," that is only available on more recent versions of Photoshop. As with unsharp masking, choose an amount of 50-100% and a radius of about 5 pixels. If your sharpening effect is too harsh, fade the application of sharpening to less than 100%. If your sharpening was on an RGB image, fade the blend mode to luminance.

Some astronomers prefer to use a high-pass filter for sharpening. In Photoshop, this must be applied to a duplicate of the layer that you wish to sharpen. Select the region that you wish to sharpen on the duplicate layer. Apply the high-pass filter to this duplicate layer (filter > other > high pass) with a radius of about 5, or the same as amount of pixels that you used for contract and feather in your selection. Then, change the blend mode for this layer to overlay. If the sharpening effect seems too harsh, you can reduce the opacity of this layer to less than 100%.

Blurring seeks to reduce image noise at the edges of the target and create a smooth background sky. Because noise has both luminance and color components, the image should be flattened before applying noise reduction.

In Photoshop, begin by selecting the background areas that you wish to smooth. This can be accomplished either with the magic wand tool or with the color range tool selecting shadows. If you have a recent version of Photoshop, the "reduce noise" filter works very well. Begin with the default values but adjust the various options depending on the severity of your noise and the amount of color noise. If you have an older version of Photoshop, you can use the Gaussian blur filter, but this can erase smaller stars. To reduce the effect on small stars, contract your background selection by 3 pixels and feather by 2 pixels, then apply a Gaussian blur of not more than 2 pixels.

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