Digital cameras, webcams, and Bayer-matrix astronomical CCD cameras capture color images directly, and after scanning, negatives and photographic prints provide a further source of images that can almost certainly benefit from digital image processing. In this chapter, we examine what to expect in digital color images, how to collect good color images, and how to process color images
Unlike CCD cameras, digital cameras deliver a color image to you as a complete "package." With CCD cameras, a color image begins life as a set of stacked red, green, blue, and luminance exposures that must be assembled into a color image—the previous chapter covers how such color images are "built" from scratch. By putting the color content in a ready-to-use package, digital cameras greatly simplify making astronomical color images. The first really sensitive digital single-lens-reflex (DSLR) cameras (such as Nikon's D70 and Canon's Digital Rebel and 10D) proved capable of exciting wide-field piggyback shots of the Milky Way, and at the focus of a telescope, could make color "snapshots" of the deep-sky favorites such as the Andromeda Galaxy and the Great Nebula in Orion.
In this chapter we explore the processing of astronomical color images. Although we focus our attention on color images from digital cameras, DSLRs, and webcams, we also address the unique technical problems encountered in processing scanned film negatives and prints.
We begin by examining the properties of color images to see how these set them apart from the monochrome FITS images generated by astronomical CCD cameras. Next, we discuss basic calibration techniques (dark subtraction and flat-fielding) that can often improve image quality, and then combine multiple exposures (a process called "stacking") to build an image with a high signal-to-noise ratio. Finally, we cover methods of representing color images inside your computer and how these effect color balance and image enhancements such as sharpening, deconvolution, and wavelet sharpening.
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