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Old 04-09-2013, 06:02 PM
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Globular cluster NGC 6717

I was quite entranced by Dana de Zoysa's post here that became a great article on hard-to-spot globular NGC 6749. I have spent the last two nights hunting down unusual globulars.

I'm observing from the suburbs with a 10" SCT, so I'm not going to break any magnitude records. But I had fun, for instance, with NGC 7006 in Delphinus - at 2' and Mag c.11, it's not exactly going to pop like Omega Centauri. My scope was over in the vicinity looking at the Nova.

But my favourite unusual globular was NGC 6717 in Sagittarius. At Mag 9.2 and 3' diameter it shouldn't be a difficult object, and near enough to zenith at the moment. It's a pretty sparse cluster, type X.

The problem is it's only 2' from the Mag 4 star Nu Sagittarii. The glare from the star is quite overpowering. Looking directly, I couldn't see it at all. The two objects actually overlap, so if you move the star, and its glare, out of the field you lose almost all of the cluster too.

The only way I caught it was with averted vision, when it popped into view quite clearly. This puzzled me; I understand (or thought I understood) the principles of averted vision, and certainly use it as a matter of course to bring out detail in faint objects. But I hadn't heard of averted vision being used to capture objects that were drowning in bright glare. But there it was: stare directly at Nu and the glob vanished; move my vision to the side of the field, and out it pops.

I'd love someone else to give it a try and see if you have the same experience.
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Old 04-09-2013, 06:25 PM
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Nice work, Jon
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Old 05-09-2013, 08:20 PM
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Weltevreden SA (Dana)
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NGC 6717 is also Pal 9

Good catch on NGC 6717, Jon! Never heard of it before and it's always nice to learn about something new up there. There's no note on how 6717 got that number by Draper, but it was thought an OC till identified as a GC by Per Collinder in 1931. In 1955 George Abell classified it as a GC, too, and the NGC name is cited more frequently than the Pal 9 appellation. It became fully understood as identical to Pal 9 by Alter only in 1958. I've attached a couple of context images from the always-handy (and free) Where is M13, plus an image from a fellow who used as 305mm Newt to get it. You can also gauge how the cluster will look on first glimpse from these links: 1 and 2. Professionally 6717 is amazingly little studied. A quick search shows the first paper of note about its RR Lyraes done back in 1988 (but pretty low-rez stuff by today's standards), and the first real study by the prolific team of Ortolani, Barbuy, & Bica who pioneered systematic GC studies in the 1990s and early 2000s. N6717 is a very old, low-metals bulge globular, which inevitably means it has, like 30-odd other bulge GCs, frittered away its youth living in a rough neighbourhood. (More fussily, it suffered considerable mass lose through thermal evaporation during a core-collapse lasting 5 billion years and core re-expansion across another 5 billion years. My hair is thinning, too, but not because I live in such dissolute company as the bulge globulars.

Thanks to your tip, this GC will be top of the list tonight as soon as it gets dark.

=Dana in SA
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Old 06-09-2013, 07:46 AM
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Weltevreden SA (Dana)
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Sure doesn't look like a GC

Jon, I checked out 6717 tonight under clear but not wonderful skies. It sure doesn't look like a GC. More like a low-concentration OC with a handful of bright giants. Nu Sagg is distracting enough, but two tiny groups of unrelated field stars >1 arcmin away on the W & E sides don't help either. The cluster itself is tough to spot at all. If it wasn't for those adjacent sets of 10-11 mag field stars, I would have seen nothing special at that position. I have a feeling the Harris vis mag of 9.28 includes those field stars, since his data likely originated with the 1998 Ortolani paper. In that study, the resolution of the equipment used was less than the cluster's core concentration of 2.9 arcsecs. The half-light radius (which is what our eyes interpret as a GC's visual diameter) is only about 35 arcsecs. It's a tiny little thing, to the eye and in physical reality. How the dickens did you ever learn of this curious little gem?

NGC 6717's color-mag diagram shows a blueish main sequence, very steep red giant branch, and a horizontal branch well to the blue side of the RR Lyrae Gap. There's no sign of blue stragglers or a red clump. All this points to a very early two-generation (and possibly primordial single generation) origin about 12.5 billion years ago, with a slow evolution rate since then. At that age, it was here before the Galaxy was. Most of its current main sequence stars are well under a solar mass, perhaps 0.7 to 0.8 solar masses. There's a discernable asymptotic giant branch, which is usually the repository of the few remaining ~2 solar mass stars in a cluster of this age. Put more prosaically, it's an aging spinster who's been living off a tiny pension saved in her youth.

The problem with this cluster is the same as so many bulge globulars: resolving the properties of its members requires the very keenest of modern equipment, and that means very expensive telescope time. These days, who wants to open their wallet for a lonely old globular? Bulge globulars are sort of the floppy shirt-tail of the glamour scene when it comes to globular studies—there's a lot of them, most are alike, and none are especially notable to look at. The remote, exotic globular systems of Virgo Cluster galaxies and photogenic lovelies like M104 the Sombrero Galaxy attract all the high-budget attention these days. Poor little 6717 is a lonely little wallflower out there. Thanks for sending a little IIS Valentine to her.

=Dana
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Old 06-09-2013, 10:55 AM
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Hi Dana,

Thanks for all that background information. Fascinating! Both the scientific and observational detail. Your point about the small groups of field stars explains what was troubling me about how the cluster appeared at the EP.

I must confess that globulars are a bit of a passion of mine. I think the notion that they are all the same overlooks fascinating subtle visual differences (and of course a wealth of differences in terms of metallicity, composition, age, and all the rest).

I have quite a limited field of view of the sky from my balcony where I do a lot of observing. I can see the Eastern horizon from N to S, up to about 70 degrees altitude; and a small patch about 40 degrees diameter directly at zenith. The upshot is I have to plan my observing very carefully, around a quite small area of sky (but I get almost all of it over the course of a year).

I found 6717 because I would have done a search in SkyTools for globulars within 20 degrees of Nunki (or similar) that night, and chased the ones that looked interesting.
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Old 06-09-2013, 04:56 PM
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Interesting isn't it, I've seen both NGC 7006 & 6717 through a 4.5" f8 reflector at low powers (43x) several times in excellent dark skies. To me they don't seem terribly challenging objects compared to some of the much fainter ones I've chased down with that set-up.

I'm not a great one for observing notes but I did find these sparse entries from 2007:

'NGC 6717 (Sagittarius): said to be “challenging object even for large aperture scope” – relatively easy in 4˝”. Very tiny fuzzy glow beside bright star, averted vision not needed. Seen 16 June 2007.'

'NGC 7006 (Delphinus): very remote GC. Easy to see in 4.5”. Dim, very small, centrally brightened glow. Seen 5 September 2007.'

With 6717, maybe the larger apertures turn Nu Sag into a blinding glare, inhibiting the view of such a small subtle object.

Cheers -
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Old 10-09-2013, 04:08 PM
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Thanks for another great thread - alas in spite of clear dark skies this week I was harbouring a foul virus and unable to follow up, but soon...

Great discussion!

Oh, and Rob, relatively easy in 4 1/2" scope attached to your particular eyes and brain!
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Old 25-09-2013, 11:57 AM
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As a postscript to this discussion, I thought I'd post an image of the cluster I captured last night. 12 x 2 min plus 3 x 3 min, EOS 60Da with the 10" SCT and f/6.3 reducer.
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Old 26-09-2013, 09:28 AM
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Thanks for posting your image, Jon. It helps clear up the actual globular versus the confusing field stars. I checked several other images to confirm (Google NGC 6717 for the list), and all point to the same thing: the globular is the fuzzy spot in the middle of the concentration in your image, while the surrounding brighter stars above, below, & to the left, are field stars. This stands to reason, because if cluster members they would be on the outskirts of the globular. Globular outskirts are populated by the cluster's faintest stars—many to sooner or later escape if they near the L1 or L2 Lagrange points moving at above the escape velocity of the cluster. The field stars in your image are too bright to be outliers. I don't know of any GC whose outer members outshine its core population. These field stars have apparently tripped up a number of observers. This Aug 2006 thread on Cloudy Nights reports several resolutions, but the cluster's brightest red giants are only mag 14 and brightest horizontal branch stars are 15.6, which are below the limits given the apertures quoted. Also, Sagg is pretty far south for N American observers. (One more feather in the cap of we 'Southies'.)

Thanks again for observing & imaging. 6717 is a very tricky cluster, and therefore lots of fun. If not for you, I would have gone through life in clueless bliss. =Dana in SA
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Old 26-09-2013, 02:45 PM
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Dana,I observed NGC 6717 last night and was surprised at how easy it was.
Under High mag was able to resolve quite a few of the field stars around it.
16" F 4.5 Dob,6mm Radian, 300xmag.
When first observed with the 17mm type 4 nagler 107xmag it was so sharp and with no interference from Mag 4 star Nu Sagittarii.
It is a nice sight in a wide field eyepiece,and a beautiful part of the Milky Way.
I enjoy your posts and challenges.
Cheers
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