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M22: This globular cluster in Sagittarius is one of the most spectacular ones visible from the mid-northern latitudes. It is visible with the naked eye, but is in a very "busy" part of the sky towards the galactic center, so might not stand out as much as it would elsewhere. The cluster actually extends well beyond the edge of this frame.

Messier: 22
NGC: 6656
Right Ascension: 18h 36m
Declination: -23° 54'
Apparent Magnitude: 5.1

Date: August 2010
Telescope: Meade 16" Schmidt Cassegrain with f6.3 reducer
Camera: SBIG ST-10XE
Guiding: AO-8

Exposure: L: 20x1m, binned 1x1
RGB: 3x3m, binned 2x2
The camera was at -30°C

Processing Notes: Data acquisition with CCDSoft. Reduced and aligned in CCDStack. Subs combined in Sigma Beta. Arcsine stretch import of L into Photoshop. Adjusted curves and levels. Slight blur and noise reduction on the dim areas and then overall sharpening with several degrees of an unsharp mask filter. The stars outside the core received a slight minimum filter. RGB combined in AstroArt at 1:1.07:1.92 ratio, to account for atmospheric extinction, since this was only ~25° above the horizon. Adjusted curves and levels, and reduced noise of RGB. L was combined with RGB using three layers: Luminance on the bottom; desaturated RGB as a multiply(40%) layer; and RGB as a color layer on top. Final stretch and tweak in Photoshop.
Scale: 0.53"/pixel on high resolution

Additional Comments: A target of opportunity. The night had reasonable seeing, and I had just taken a set of Luminance on nearby M16 and wanted to try a nearby target without changing focus. It has been several years since M22 was imaged at the FLC observatory. The color data was the minimum for adequate processing, but the maximum allowed by the time available.

The name "globular" cluster, coined by William Hershel, is very descriptive of their appearance - a glob of stars. There are fewer than 200 globular clusters known around the Milky Way, and unless you are familiar with specific ones, they all look very similar. These clusters condensed while the Milky Way was still forming, and reside in a spherical halo around the disk. Unlike objects in the disk, which have a mostly orderly rotation about the galactic center, the orbits of globular clusters are random. It was the three dimensional distribution of globular clusters that Harlow Shapely used to find the center of the Milky Way. All large galaxies are observed to have globular clusters in a halo. Globular clusters are ancient. Generally comprising what are called Population II stars (old ones with fewer heavy elements), there are no longer any luminous, blue, main sequence stars, and the remaining distribution of stars is noticeably redder than that found in open clusters in the spiral arms. Lots more information can be found here: http://en.wikipedia.org/wiki/Globular_cluster

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