Antilhue - Chile

Messier 4
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Image Data

Designation Messier 4  ( M 4 ),  NGC 6121,  Bennett 75
Object type Globular cluster
Coordinates 16h 24 min    - 26° 32'    Scorpius  (Sco)
Description M 4, size 26',  is a loose globular, classified by Dreyer as an open cluster.   He also described a bar of 8-10 stars across the center of the cluster and called it extremely rich (for an open cluster).  Bennett observed it with a 5-inch short-focus refractor,  including it in his list of cometary objects as number 75.  His coded annotation describes it as a circular or slightly elliptical extended object, fully or partially resolved into stars under a higher magnification, with a larger aperture or in very good conditions.  

This marginal naked eye object ( mag 5.9 )  is located 1.3° west of Antares.   At less than 6.500 light years, Messier 4 may well be the nearest globular cluster to the sun.  Interstellar absorption in this direction is estimated at 1.4 magnitudes and accounts for the yellow-orange colour.

Click on the image for  higher resolution (1600 x 1050).  The field of view is  37 x 25 arcminutes with north towards the top.

Exposure LRGB 50:15:15:15 min @ -20°C ;  all exposures unbinned
Camera SBIG STL-11000 with Astronomik Type 2 filterset  -  selfguided
Optics RCOS 14.5" Ritchey-Chrétien @ f/9 (prime focus)
Mount Astro-Physics AP1200GTO
Software MaxIm DL/CCD,  Sigma Pre Beta,  Registar,  PixInsight V.1,  Adobe Photoshop CS
Location - Date - Time San Esteban (Chile) - 11Apr2005 @ 08:00 UTC
Conditions Transparency 8,  Seeing 8, Temperature  + 7°C