This chapter covers the heart and soul of image processing for just about any astronomical image. You will learn how to apply the principles of the Zone System using Photoshop® CS (or later). Other image processing programs, such as Picture Window Pro and Paint Shop Pro, include similar tools but the Levels and Curves tools in Photoshop® are powerful and fluid. Photoshop® CS adds a critically important feature: the ability to use layers and other tools with 16-bit images. Since 16-bit images contain up to 256 times as much information as 8-bit images, this is a very important feature for CCD image processing.

The key to success is mastering the somewhat challenging Curves dialog. You will learn the basic shape of a useful curve, as well as how to tweak that basic shape to get specific results. You will also learn how to use Curves to balance the available brightness levels between Zones.

Basic processing involves defining the range of data you want to work with (setting black and white points with Levels), and then expanding the dark areas and compressing the bright areas using Curves. Typically, you will use multiple iterations of Curves to accomplish your goals. You will use Levels several times to keep the black point appropriately controlled.

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