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                                                             NGC5139 - Omega Centauri


      Omega Centauri (ω Cen) or NGC 5139, is a globular cluster in the constellation of Centaurus that was discovered by Edmond Halley in 1677. Located at a distance of 15,800 light-years , it is the largest globular cluster in the Milky Way galaxy. Omega Centauri is so distinctive from the other galactic globular clusters that it is thought to have an alternate origin as the core remnant of a disrupted dwarf galaxy.
      In 150 A.D., Greco-Roman writer and astronomer Ptolemy catalogued this object in his Almagest as a star on the horse's back, "Quae est in principio scapulae". German lawyer and cartographer Johann Bayer used Ptolemy's data to designate this object "Omega Centauri" with his 1603 publication of Uranometria. Using a telescope from the South Atlantic island of Saint Helena, English astronomer Edmond Halley rediscovered this object in 1677, listing it as a non-stellar object. In 1715, it was published by Halley among his list of six "luminous spots or patches" in the Philosophical Transactions of the Royal Society.
      Swiss astronomer Jean-Philippe de Cheseaux included Omega Centauri in his 1746 list of 21 nebulae, as did French astronomer Lacaille in 1755, who gave it the catalogue number L I.5. It was first recognized as a globular cluster by Scottish astronomer James Dunlop in 1826, who described it as a "beautiful globe of stars very gradually and moderately compressed to the centre".
      Omega Centauri is one of the few globular clusters visible to the naked eye—and appears almost as large as the full Moon when seen from a dark, rural area. It is the brightest, largest and at 4 million solar masses the most massive known globular cluster associated with our galaxy. Of all the globular clusters in the Local Group of galaxies, only Mayall II in the Andromeda Galaxy is brighter and more massive. Orbiting through the Milky Way, Omega Centauri contains several million Population II stars and is about 12 billion years old.
      The stars in the core of Omega Centauri are so crowded that they are estimated to average only 0.1 light years away from each other. The internal dynamics have been analyzed using measurements of the radial velocities of 469 stars. The members of this cluster are orbiting the center of mass with a peak velocity dispersion of 7.9 km s−1. The mass distribution inferred from the kinematics is slightly more extended than, though not strongly inconsistent with, the luminosity distribution.


     My first globular cluster and 90 minutes - this is the all imaging time I can devote for this object in one night. From my location the only time to image this cluster is in the spring and even then, the highest point it can reach in the sky, is only 12° above the horizon. Imaging that low is the main challenge, mainly because of the atmospheric turbulence. Fortunately, bright clusters do not require very long exposures, so I decided to take my chances.


      After some calculations, I have found, that it would be possible to image it half an hour before it's crossing meridian and half an hour after that, maybe a bit longer, depending on the weather conditions. This time I was imaging other object and where the right time has come, I slewed to the Omega Centauri and start imaging. After I'm done I slewed back to continue with the main object till the morning.


      I have repeated this process during the second night too and have collected 53 frames in total, each 120 sec of exposure. I knew that I won't be able to stack them all, but I had high hopes that some of it will be usable. And yes, after my demanding selection for the best frames, only 14 of them have survived.


     The processing was fun, because it's my first globular cluster and as for all the new things, it was really exciting.


Technical Info:

Optics :          Meade 10" + CCDT67 @ F8 @ 2016 mm

Camera :        Canon T3i (600D) Baader Mod           

Mount :           NEQ-6 Pro (Self Hypertuned/Belt Mod)​

Guiding:          Telescope Service OAG9 + SX Lodestar            

Acquisition :    BackyardEOS 3.0.3   

Exposure :      14 x 120 sec @ ISO1600 - 28 Minutes

Stacking :       PixInsight 1.8 

​Processing :    PixInsight 1.8 

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