Before proceeding I should tell you that you don't have to do all this work. A much simpler procedure is to let the camera do most of it for you. Here's how:
• Turn on long-exposure noise reduction in your camera. That way, whenever you take a celestial photograph, the camera will automatically take a dark frame and subtract it.
• Tell the camera to save the images as JPEG (not raw).
• Open the resulting image files in Photoshop or any photo editor and adjust the brightness, contrast, and color balance to suit you.
Why don't we always take the easy way out? For several reasons.
First, we usually want to combine multiple images. With digital technology, ten 1-minute exposures really are as good as one 10-minute exposure -almost. They're certainly a lot better than one 1-minute exposure. Combining images improves the signal-to-noise ratio because random noise partly cancels out.
Second, having the camera take automatic dark frames is time-consuming; the dark frames will take up half of every observing session. It's more efficient to take, say, 10 images of the sky and then three dark frames which can be averaged together and applied to all of them. Averaging several dark frames together gives more accurate correction than using just one.
Third, we often want to do processing that wouldn't be easy with a photo editor, such as digital development (a combination of curve correction and unsharp masking) or deconvolution (deliberate correction of a known blur). In subsequent chapters I'll tell you more about these operations.
If you don't want to avoid all the work, you can avoid some of it. You don't have to start with camera raw images; JPEGs can be aligned, stacked, and enhanced. Dark-frame subtraction is marginally possible with JPEGs and completely feasible with linear TIFFs. These techniques are discussed at the end of this chapter.
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