Stacking Images

For most digital cameras, the way to increase exposure time to the lengths deep-sky objects need is to stack several images. This increases brightness and contrast and smooths out noise. Astro imagers use a variety of programs for this task, including Adobe PhotoShop, Maxim DL, AstroStack, and ImagesPlus.

Austrian astro imager Johannes Schedler does his stacking in Photoshop by aligning up to five raw images as separate layers. The first raw image is assigned to be the background, the second is layer No. 1, third is No. 2, and so forth. The background is set for normal (100 percent) opacity, while the succeeding layers are set at 50, 33, 25, and 20 percent opacity. The layers are then flattened (combined) to form a single image. "If I have 15 raws, I combine three groups of five images, then combine the resulting three images into one," he says.

Schedler further processes the final image in Photoshop by adjusting its brightness and contrast levels and using unsharp masking. "Usually, the colors need only slight correction, since the camera gives true-color rendition," he adds.

Photoshop's "Curves" function is especially valuable for deep-sky images. It allows you to adjust the brightness and contrast within different brightness ranges separately. In other words, it lets you custom-allocate all parts of the available dynamic range (that is, from 100 percent white to 100 percent black in your final version) so you can reveal everything that's captured in your image to best effect.

Before doing any such processing, of course, be sure to save an unprocessed copy of your image that you can revert to if you mess up. Remember, when touching up your photos, go easy. You are striving for realism, not artificiality. As soon as the photo starts to get the slightest "processed look" to an experienced eye, you have gone too far. Back up, or start over.

For deep-sky use, today’s consumer digital cameras are still limited by noise and short exposure times compared to true astronomical CCD cameras (though the gap is narrowing). But digital cameras do offer most people the easiest and simplest way to venture into the joys of astro imaging.

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A digital-camera convert, Sky & Telescope associate editor Edwin Aguirre has taken more than 4,500 images of astronomical and terrestrial subjects with his trusty old Nikon Coolpix 990.