by: Jerry Lodriguss
M13 is the Great Globular Cluster in the constellation of Hercules. It is one of the largest and most beautiful globular clusters visible from the northern hemisphere.
At magnitude 5.8, M13 is visible to the keen unaided eye at a dark-sky observing site. It is located about 25,000 light years away. It has an apparent angular diameter of 15 to 25 arc minutes, depending on the size of the telescope used to observe it. This corresponds to a real size of about 145 light years. It contains hundreds of thousands of stars.
This advanced Canon EOS 60D DSLR image is a stacked composite of six 6-minute exposures shot at ISO 800 through an apochromatic refractor with a focal length of 1,000mm at f/8. It was calibrated with a master dark frame made of 5 individual darks, and a master bias frame made from 4 individual bias frames. It was stacked with a min-max excluded average combine method. The white point was adjusted for correct color with a G2V color star calibration. A non-linear curve was applied to the linear raw data and the foreground sky color subtracted to give accurate color. The high-dynamic range image was constructed with a shorter exposure with masked highlights to preserve color and detail in the core of the cluster. A slightly blurred color enhancement layer was also added in post processing.