M13 – Great Globular Cluster in Hercules

 

M13 is a beautiful globular star cluster located in the constellation Hercules. It is about 25,100 light years away, and recent estimates state its age at 14 to 16 billion years. This cluster was discovered by the British astronomer Edmund Halley in 1714, and catalogued by Charles Messier in 1764. M13 contains at least 300,000 stars; some observers believe it contains nearly a million. Notice the faint galaxy (IC4617, magnitude 15.2) near the left side of the photo, toward the bottom.


M13 globular star cluster
Processing this image was a challenge. Exposure was limited to 60 seconds to prevent stars from blooming (a magnitude 7 star did bloom, but I was able to crop it out of the final image). With such a short exposure, I needed many frames - a total of 180 for the luminance, and 270 for the color. When it came time to subtract the dark frames and apply the flats, CCDSoft seemed to give a little sigh but was up to the task - it just took a while.

Combining 180 3MB luminance files at once was pushing things a bit, so I broke them up into six groups of 30 images apiece. After a sigma-reject combine on each group, I averaged the six groups together in CCDSoft. The red, green, and blue sets were combined similarly - three groups of 30 for each color.

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Equipment
Celestron 9¼" at f/5.6 on a Celestron CGE equatorial mount
SBIG ST-8XM camera
Optec IFW filter wheel with Astrodon TruBalance LRGB filters
Optec TCF-S focuser
Optec Pyxis camera rotator
Imaging and autoguiding with MaxIm DL 4.04
Exposure
Luminance   3 hours (180 x 1 min.), unbinned, -5°C
Red   1½ hours (90 x 1 min.) binned 2x2, -5°C
Green   1½ hours (89 x 1 min.) binned 2x2, -5°C
Blue   1½ hours (90 x 1 min.) binned 2x2, -5°C
Processing Dark and flat processing in CCDSoft
Sigma-reject combined (see explanation above)
Levels and curves, Neat Image, unsharp mask in Photoshop CS
Date and Location 2, 3, & 4 August 2005
Montpelier, VA    N 37° 49' 12", W 77° 42' 06"