Image Processing with Photoshop

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With Adobe Photoshop you can sharpen the image's lightness channel after wavelet sharpening in RegiStax. Lightness channel sharpening does a good job of sharpening the image without causing color artifacts. There are three steps to this process. First, convert the image to Lab colors. Next, in Channels, select only the lightness channel and do an unsharp mask on it. Finally, convert the image back to RGB colors. The Moon's limb and image noise will determine the strength of lightness channel sharpening.

Levels are used to adjust the image's white and black points. Increase the contrast in the midrange with Curves. Be careful: with a too-strong "S" curve the bright and dark detail areas will be replaced by pure white and black.

Typically, you can improve an image after it is resized by sharpening it again. The brightening of the Moon's limb and dim noisy areas limit the sharpening for the whole image. Try sharpening only selective areas, not the whole image.

Figure 2.13. Sunset image was taken in Oregon, U.S.A., star party August 2003 with same equipment as in Figure 2.1.

Figure 2.13. Sunset image was taken in Oregon, U.S.A., star party August 2003 with same equipment as in Figure 2.1.

When you have color fringing between the Moon's bright white limb and the black sky, select the black sky and expand this selection by a few pixels until just the black sky and the Moon's color fringing is selected. Decrease the color saturation for this selection in the Hue/Saturation menu until the color fringing goes away in the white areas of the limb.

If your image has color blotches, check each of the RGB channels to see if the color blotches are related to a noisy color channel. If so, select only the noisy color channel and Gaussian blur it. This technique was used to remove noisy blue color blotches in the sunset image (Figure 2.13).

Another technique is to oversharpen the image to the point that the dim areas become noticeably noisy, then select only the noisy dim areas and Gaussian blur them to reduce the noise.

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