Tightly bound spherical collections of stars known as “globular clusters” exist in most galaxies, orbiting around their cores. Our own Milky Way galaxy has about 150 or so. Most astronomy enthusiasts who live in the Northern Hemisphere are quite familiar with the “Great Hercules Cluster” M13 located in the constellation Hercules. An impressive sight in any telescope, the Hercules cluster has a few hundred thousand stars and measures 145 light years in diameter. It is faintly visible to the naked eye. M13 is by far the largest globular cluster visible from the Northern Hemisphere, but it isn’t the largest in our galaxy. That title belongs to Omega Centauri, or NGC 5139, in the constellation Centaurus. Unfortunately Omega Centauri is visible “mainly” from the Southern Hemisphere. The reason I say “mainly” is that last year I accidentally came across it while casually sweeping the southern horizon with a pair of binoculars from the deck of the house up at Orion’s Belt Observatory! It turns out we actually do have a small 2 hour window of visibility from this location. Omega Centauri is quite visible with the naked eye from a dark site and looks to be about the same diameter as the full Moon! By contrast to M13, Omega has several million stars packed so densely they are on average only 0.1 light years apart from each other! At 180 light year diameter it is only slightly larger than M13 but has many times the number of stars.
While working on the “pier 2” equipment, I decided to take a couple of test images of the cluster during the brief window of visibility just to see what I could see! Of course this is not an easy target from this location, with a maximum elevation of 7-8 degrees above the horizon! The observatory has panels on the south wall that can be dropped down to access the horizon. This is exactly the situation they were designed for! I pointed the Tak 180 to Omega and tried a few 3 minute exposures. The results were much better than anticipated. The dark skies certainly help. On the screen was the glorious Omega Centauri cluster! The image I thought was pretty clean and stars well resolved for just a single exposure. The Tak 180 is a known bear to collimate and I did the best I could just for this test but it will need some work..a subject for a future post. Anyway I felt this was going to be a definite legitimate target in the future based on this preliminary test!
Screen shot from the “Stellarium” program showing the location currently of Omega Centauri (broken square icon due South) at its maximum elevation during transit of a wopping 7 degrees above the horizon! Some of the brighter stars in the constellation Centaurus are seen to the left
NGC 5139 or Omega Centauri is the largest globular cluster in our galaxy! This is a single raw image taken from Orion’s Belt Remote Observatory using a Tak 180ED on a Paramount MX+. 3 minute exposure. Canon 60Da camera. I did a quick collimation with CCD inspector and it’s actually pretty decent but if you look to the left side of the image the stars are a little elongated. I was quite thrilled with this quick test and can certainly add the cluster to the list of obtainable targets from this location!
Thanks for reading!
At last first light for the Astrotech 12″ RC Truss! Finally after collimating, establishing a new pointing model with the new payload and working out some glitches with the new image scale in the various control programs a successful 2 hour test run was accomplished. For our test I chose a globular cluster. This is an excellent way to assess the general alignment and collimation of your optics. Stars are unforgiving especially at the full resolution of your set up. How many times have you seen images posted of nebulae etc which look great at fractions of the full image scale only to show the egg-shaped stars when the true full resolution version is revealed! Don’t be like those guys!
Stars are either round or they are not. A star cluster especially a globular has lots of stars so they make an excellent test for your system. Now there is a quantitative way to determine “roundness”. It’s called aspect ratio. This is the ratio of the width to the height of the star image in the case of stars. The program CCD Inspector (CCDWare.com) is able to determine that for us. The values in the program are read as percentages. For example if there is a 5% difference between width and height the aspect ratio is 5. In this calculation therefore lower numbers are better. My own experience tells me that when you go much above 15 you start to see the stars becoming egg-shaped. So as a matter of convention I tend to throw out any subs with aspect ratios over 15 or so. CCD Inspector is very handy in that you can simply place your cursor over the star and it automatically reads the aspect ratio and full width at half maximum. Last night the seeing was excellent so most of the subs registered in the low 2’s for FWHM
Now about our target! M15 is well positioned right now in Pegasus almost directly above us so this is great for our test object. It is one of the most densely packed globulars in the Milky Way with an enormous number of stars in the center. It has a number of variable stars and pulsars in it as well, AND the first planetary nebula discovered inside a globular!
I think overall I am satisfied with the test results and we can now move forward with our imaging projects!
This is a raw 6 minute luminance image of M15. Yes there are significant star blooms (vertical streaks) however this is the trade off with a highly sensitive CCD and these can be processed out
At full resolution the central stars are most definitely round! At the periphery (not shown) there is a subtle decrease in roundness due to the field curvature from the optics but this is expected
This is the CCD Inspector Viewer pane which shows the very nice feature where you can display the object parameters by just moving your cursor over it. Shown is a core star with aspect ratio and Full Width at Half Maximum values. I think this is a keeper!