Instrument

12.5" RCOS @  ~f/9 (2880 mm fl) 0.643 arcsec / pixel.  Shown resampled to 2.28 arcsec/pixel.

Mount

Paramount ME

Camera

SBIG STL-11000 w/ internal filter wheel, AstroDon Filters

Acquisition Data

5/4/2008 to 5/26/2008  Chino Valley... with CCDAutoPilot3

Exposure

Lum    165 min.  (33 x 5 min. bin 1x1)

RGB    180 min.  (12 x 5 min. bin 2x2) each

Software

• CCDSoft, CCDStack, PixInsight and Photoshop CS6.

• eXcalibrator v4.25 for (g:r) color balancing, using 68 stars from the SDSS-DR9 database.

• CCDSoft for calibrartion

• CCDStack to register, normalize, data reject, combine the sub exposures and create the RGB image.

• PixInsight for gradient removal and initial non-linear stretching.

• PhotoShop for the LRGB combine and final touch-up.

Comment

North is to the top.
M13, the Great Globular Cluster in Hercules, is a system of stars numbering in the hundreds of thousands.  It is one of the brightest globular star clusters in the northern sky. At a distance of 25,000 light-years, the cluster stars crowd into a region 150 light-years in diameter, but approaching the cluster core over 100 stars would be contained in a cube just 3 light-years on a side. For comparison, the closest star to the Sun is over 4 light-years away. The orange stars are red giants, which are typically 2000 times brighter than our sun.  Also visible are many hot blues stars known as "blue stragglers." The most likely explanation for blue stragglers seems to be that they are the result of stellar collisions or mass transfer from another star.