Printing, pre-press, publishing and photography services – yesterday, today and tomorrow

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

Computers and image-processing programs are providing astrophotographers with powerful new tools for bringing the most out of astronomical images.

It is a dark and stormy night - hardly a time for astronomy. Yet images of the starry universe glow before you on the computer screen. You can hardly wait to see the pictures you took with your friends during that long night of imaging with their CCD camera.

One of your friends now leans toward the computer screen. She checks the logbook and its list of the calibrated images they prepared a few days earlier. After a moment, she proclaims, "Let's start with NGC 7479."

Little is visible as the image scrolls down the screen. A few stars glow dimly, and you see a dark gray patch at the center of the screen. That's the galaxy. Your other friend is already moving a crosshair pointer across the screen to read pixel values. As the glowing pointer moves over the screen they change: over the sky the numbers lie in the low hundreds; as it sweeps over the galaxy the count tops 1,000. "Good numbers," he grunts. She turns and reminds you, "A count of 65,000 shows on the screen as pure white, so the 1,200 reading we have for the nucleus of the galaxy looks awfully dark. But watch this. . .."

The pointer touches a control and the screen swiftly lightens. "I'm setting 1,500 to display as pure white and 200 as black. That'll show us what's in the galaxy." Although you've seen it happen many times before, you still gasp as the glowing image of a galaxy emerges from darkness. "Great shot," you say, breaking into a smile. "Great shot!" Then it dawns on you - "Hey, is this one of the pictures you asked me to guide?"

Digital Magic

Last month's "Digital Darkroom" talked about how to get the best digital "negatives" from a CCD camera. That process, akin to taking and developing a photographic negative, involves taking special dark frames and flat-field frames at the telescope that you then use to electronically subtract noise and imperfections from your hard-won CCD images. The result: a clean, noise-free file called a "calibrated image." But assembling calibrated image files is just the first step to creating stunning images from a CCD camera.

The next step is image processing - the digital equivalent to printing a negative. In the traditional darkroom, you would use different grades of paper and techniques such as dodging and burning to make the best possible print from a negative.

With a digital image, you use a computer and image-processing software. Using simple computer commands, you can uncover faint outer streamers around galaxies, display the thousands of stars in a globular cluster, or reveal fine markings on the surface of a planet.

Computer processing is a powerful and easy-to-use technique. It replaces hours of fastidious darkroom work with quick clicks of a mouse. Image-processing software allows you to display all the details hidden in the original image, from the bright cores of galaxies right out to the faint tendrils of their spiral arms.

Fortunately, computer processing is not restricted to users of CCD cameras. New technologies such as low-cost slide scanners and Kodak's Photo CD process now make it possible for you to convert any photograph, be it slide film or negative, into a digital image. Once it's in electronic form, you can apply the same computer processing magic to a photograph as CCD camera owners can to their frames. The digital darkroom has arrived for all astrophotographers.