July: Just Globular Clusters

Abstract

The Great Hercules Cluster M13 is the colossus of globular clusters visible in the northern hemisphere. M13 can be seen with the unaided eye from a dark sky site. It packs about a half million stars in a region only 150 light-years across, orbiting our galaxy like a giant satellite 24,000 light-years away. The brightest stars in this image are red giants, thousands of times brighter than our sun. At its core, the glare of brilliant suns would obscure the remainder of the universe. Of course, planets are unlikely in the center of a globular cluster, as frequent gravitational interactions with other stars would strip away planets. Imaging. This image of M13 is framed tightly to create high resolution. Innumerable small stars are resolved around the edge of the cluster. Such high-resolution imaging, at less than 1 arcsec/pixel, requires steady skies, accurate focusing, good tracking, and autoguiding. If your skies or mount are not up to the task, then consider a larger field of view as in the image of M5 shown in section “June 26: Globular Cluster M5” in Chap. 6. The bright globular clusters can be imaged successfully with RGB or single-shot color methods. Luminance layering is usually not necessary for globulars. Processing. The best images of globular clusters show color matched precisely to individual resolved stars. This requires precise alignment of your color channels. Inspect your channels after alignment to confirm proper alignment. Although it is best to complete alignment before color combine, a last minute fix in Photoshop is possible. To do this on a single RGB image, go to channels, choose just the color channel that is fringing, click RGB to show all colors, then select all and use either the move tool or transform functions to shift the offending channel (Fig. 7.1).