June: Globular Clusters and More Galaxies
The Globular Cluster M3 lies in the constellation Canes Venatici, on the border with Bootes. Although it may contain as many stars as the Great Hercules Cluster M13, it is one-third farther away at a distance of about 33,000 light-years, and therefore somewhat dimmer and visually smaller. Furthermore, the cluster contains a dense core, with half of its 500,000 stars contained within an 11 light-year radius of its core. For comparison, only a dozen stars reside within 11 light-years of our sun. M3 is also notable for the unusually high number of variable stars, which have been used to calculate the distance to the cluster. Imaging. On a steady night, you can image M3 using a small field of view of about 20 arcmin and a resolution under 1 arcsec/pixel to capture vivid detail in the core. On a less than perfect night, you can image successfully with a larger field of view of 40–50 arcmin and still have an attractive image. Globular cluster M3 is bright enough for routine RGB or single-shot color. Processing. Sharpening methods are often easier to apply to just a luminance channel to avoid the creation of color noise. If you have only color data, one solution is to create a synthetic luminance. A synthetic luminance is formed by the combination of all of the red, green, and blue exposures as if they were monochrome exposures. This new channel has a higher signal-to-noise ratio than any of the individual color channels, and thus can tolerate more aggressive histogram adjustments and sharpening without excessive noise. It is then applied as a layer in Photoshop just like a regular luminance channel. For objects imaged at low altitudes subject to atmospheric refraction, or when imaging with a semiapochromatic refractor, a synthetic luminance may be sharper than a conventional luminance taken with a clear filter, if you refocus with every filter change (Fig. 6.1).