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Messier 23 and Sagittarius Star Field

Messier 23, also known by its New General Catalog number of NGC 6494, is a beautiful open cluster in the constellation of Sagittarius. M23 was first documented by Charles Messier and added to his famous list in June 1764. Although the exact number varies with the source, M23 contains somewhere in the neighborhood of 150 to 180 member stars. It is 2,100 light years distant and has an apparent diameter of 20 light years. The prominent cluster stars seen in this image are of spectral type B9 which sets the age of the cluster at between 220 and 300 million years old.

Messier 23 is beautifully framed in this image by the dense star fields and dust of the central Milky Way Galaxy.

Messier 23 can easily be seen with binoculars or a small telescope under moderately dark skies. The prominent star to the upper-right of the main cluster is a foreground star. This link has some excellent additional information about Messier 23 –

Exposure Information – This is an RGB filtered image. Each separate color channel was created from 19 subexposures of 210 seconds each.

Follow this link to my Astrobin gallery for a 3000 x 2268 pixel version and detailed exposure information.

Processing – All subexposures were calibrated, aligned and stacked using PixInsight. The PixInsight LocalNormalization tool and the PixInsight DrizzleIntegration tool were also used with the integration of each color channel. The master red, green and blue channels were balanced before combining with the LinearFit tool. The color image was created by combining the color channels with the LRGBCombination tool. The AutomaticBackgroundExtraction tool was used to remove a slight light pollution induced green gradient. The image was “stretched” with the HistogramTransformation tool and then the image was saved in 16 bit TIFF format for import to Photoshop. Minor changes to the color balance were performed in Photoshop and then the final images were saved as JPEG images with a sRGB color profile.

Equipment –

Telescope – Stellarvue SV70T with the 0.8x photographic redicer.
Camera – ZWO ASI1600MM-C with Baader 36mm unmounted filters
Mount – Astro-Physics AP900 GTO
Date – May 16, 2018




Posted in Processing Notes, Star Clusters

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