Old 16-08-2013, 03:11 AM
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Weltevreden SA (Dana)
Dana in SA

Weltevreden SA is offline
Join Date: Apr 2012
Location: Nieu Bethesda, Karoo, South Africa
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NGC 6749, the most difficult NGC globular?

NGC 6749 Aquila has a reputation as the most difficult NGC object in the sky. The observing reports one finds on CN involve 300mm and upward (way upward) scopes. I wondered about its visibility in my 150mm to 200mm medium-aperture scopes. Now I've observed its field across the past three nights for more than an hour each night under excellent skies using 100mm to the 200mm scopes. There are two issues in positively distinguishing N6749 from adjacent field contamination. First, listed globular visual magnitudes in most charts are misleading. Globular light falls off at a specific exponential rate from core to halo which depends on the cluster's density, from Class I highly concentrated, through Class XII, very loose. A better measure is the half-light radius which is a measure of core concentration. You can look up this data and all the other relevant data in the Harris Catalog. After checking what I can actually see against what's listed in Harris, the half-light radius is the only visibility criteria I use now .

N6749's half-light radius is 1.10 arcmin across a total diameter of 6.1 arcmin. That indicates a very weak halo concentration and low core visibility. The visual mag in N6749's half-light radius is 11.9, which compares with 11.0 for NGC 7009 at its 1 arcmin half-light radius. N6749 is the loosest globular in the sky at Cl XII, which explains its mean surf.brightness of 21.8 (compar. w. N7009 at 19.0). Both N7009 and Pal 12 are convenient nearby reference points when getting a handle on 6749's observability under a specific session's sky.

Problem #2 is that when using smaller scopes there is a sprinkle of seven mag 11.8 to 14.2 stars in a roughly trapezoidal shape directly adjacent to N6749. It stands out in WikiSky much less clearly as a distinct group than it does at the eyepiece. This group abuts N6749 out to 5 arcmin W and 2 arcmin SW. At lower apertures and magnifications this group blends into a patch which can easily give the impression of a low-contrast extended object. It shows up that way in my 100mm refractor at 60x and I know there's no way I can ever see a mag 12 GC one arcmin in dia. in a 100mm scope. My 200mm Mak is mounted alongside the 100mm refractor. Switching to it at 169x quickly resolves the patch into distinct stars. Only then does N6749 become visible as a very faint glow just touching the NE corner. It took nearly an hour to confirm the cluster on first night out under approx. mag 7 seeing, and that was only three fleeting glimpses. This was nearly the same visual impression the 100mm refractor gives of the starry patch. The next two nights I could see N6749 as a 1.5 arcmin very faint glow in a 180mm Mak, and more cleanly distinct in the 200mm. As is often the case, once having cleanly ID'd the object, it is much easier on subsequent tries.

Further research reveals N6749 to be a distant fossil halo cluster undergoing what may be a final disruptive pass through a turbulent interarm region near the point where the MW bar turns sharply to the left to become the Sagittarius Arm. The cluster's light path to us traverses 25,750 light years, of which nearly half is through a dense, turbulent star-forming region where the Sagg Arm joins the MW bar at 90 perpendicular. N6749 has only 1975 solar luminosities of stars left and is a case study in what happens when a globular is absorbed into our galaxy. The Milky Way disc has disposed of roughly 250 globulars in the last 11 billion years; the present population is approx 23% of the calculated original populace. Many of those disruptees ended in the outer halo. N6749 may soon join them.

I'd love to read other IIC field reports on this globular.
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Old 28-08-2013, 12:34 PM
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Paddy (Patrick)
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Join Date: Jun 2007
Location: Strangways, Vic
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Thanks for a very informative post Dana!

My only notes for this object are from Sept 2010 uisng my 16" dob at 250X "250x A barely perceptible mist against the background stars"

Hopefully will get a chance to look again this weekend!
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