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Contents:

1. Digital Magic
2. Painting By Numbers
3. A Revealing Image
4. As Easy as Calling Up a Log
5. A Lunar Odyssey
6. Fun with Fuzzy Filters
7. Building Color
8. A Digital Toolbox

Fun with Fuzzy Filters

If there were no atmosphere and every telescope were perfect, there would be less need for image proces-sing. As light from the Moon, planets, or stars passes through the atmosphere and optics of your telescope, it is spread out into a blur. Mathematicians say that the image has been "convolved." Image processing allows you to deconvolve the image and extract much of the original sharpness.

One technique useful to amateur astronomers is unsharp masking, a technique first developed for use in conventional darkrooms. In the digital version of unsharp masking what happens is this: When you give the computer a blurry image for unsharp masking, it builds an even fuzzier image by averaging each pixel with nearby pixels. In the case of a 7 by 7 mask, for each pixel in the original image the computer averages the 48 pixels around it and makes that the new pixel value in the new image. This new image is the unsharp mask. The computer then subtracts this fuzzy image from your original image. Like magic, the image sharpens.

To understand how an unsharp mask works, think about what would happen if you added a fuzzier version of your image to the original. You would get an image that looks even blurrier.

Now imagine subtracting the fuzzy version. The fuzzy parts cancel, leaving only the "unfuzzy" elements. Sharp features remain intact. Lunar craters pop out sharp and clear and Jupiter's cloud belts reveal a wealth of detail. The main trick is getting the mask size right. It should be equal to or a little larger than the blurring effects you're trying to eliminate. Too small and it has little effect; too large and it can produce dark haloes around every bright object.