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Old 05-05-2012, 02:04 AM
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FJA (Faith)
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Talking Some southern globulars from Texas

I spent the week of 15-22nd April this year at the Texas Star Party in south-west Texas which, at 30°N is 20° further south than my home latitude of 50°N. I took the opportunity to observe those objects that don't rise above the horizon at home or are on the horizon (in Corona Australis, Lupus, Norma and parts of Scorpius and Sagittarius) plus the ones that don't rise very high here (Hydra, northern Sgr, northern Scorpius).

These observations were made on the nights of 19/20 and 20/21 April. The seeing was poor to good while the transparency was pretty good. The naked eye limiting magnitude was 6.5 to 7.0. I used a (borrowed) 25cm dob with magnifications of 48x and 139x.


Corona Australis:

NGC 6541 – This was almost in the trees. Very bright, quite small and round. At 139x it is lovely, with a compact centre and totally resolved outer halo. A foreground star touches the cluster’s eastern edge.

Hydra:

NGC 4590 (=M68) – Large and bright at 48x. Round. At 139x it brightens a little towards the centre, which is not dense. Totally resolved.

NGC 5694 – Extremely easy to find this is a moderately bright, tiny globular which lies on the northern end of a line of two stars. Star like but fuzzy at 48x.
At 139x it is a not-quite-round glow with a slightly brighter middle. Dense, compact core. Unresolved.

Lupus:

NGC 5824 – This is very small but extremely bright. It has a very bright, dense core – one of the densest I’d yet observed. Not resolved at 139x but there are hints of granularity with averted vision.

NGC 5927 – Bright, despite its low altitude. At 48x it is a round, fairly even glow that brightens a little towards the centre. At 139x it shows a slightly more irregular shape, with chains coming off from the centre. The centre is brighter than the outlying areas but is not compact or dense.

NGC 5986 – This is large and bright, even at low power, with a granular appearance. It is evenly bright across the face of the cluster and there is a foreground(?) star just touching the eastern side.
At 139x the cluster is fully resolved to the centre but there is also a background glow hinting at unresolved stars. The cluster is uniformly bright.

Norma:

NGC 5946 – This is small and faint and quite hard to find, probably due mainly to its low altitude from my observing site.
At 139x it appears as an unresolved but granular evenly bright glow. There is only very slight concentration towards the centre. There is a foreground star on the SW edge.

Sagittarius:

NGC 6522 – This forms a very nice pair with fellow globular NGC 6528, in the same field of view at 48x and located adjacent to γ Sgr. At 48x, they are both of similar brightness. However, 6822 does look slightly brighter at 130x. 6522 is triangular at 48x at low power. Brighter in the middle but core not dense.

NGC 6528 – Fainter than 6522 at 139x. Direct vision shows uniform brightness across the cluster but averted vision shows moderate brightening towards the core. Partly resolved with averted vision.

NGC 6558 – Easy to locate, between ɛ and δ Sgr, and lies in a rich field. At 48x it is moderately bright and round. 139x shows a partly-resolved glow which appears granular across its face. The cluster brightens towards its centre but the core is not dense.

NGC 6624 – A very bright and small globular which lies in a nice rich field. Two stars lie immediately to the east. At 139x, this shows a very bright compact core. With averted vision it is partly resolved, especially the outer areas. A nice object.

NGC 6723 – Very bright, despite its low altitude. At 48x and 139x, it appears uniformly bright with no brightening towards the core at all. At 139x, it appears more irregular, due mainly to a dark area cutting into its north west side. It also appears to have dark areas in the eastern side. Resolved, particularly with averted vision.

Scorpius:

NGC 6139 – Obvious at 31x as a bright, circular object. Round, with a brighter core. 139x shows a round object with a bright, dense core. The outer edges are only partly resolved with averted vision.

NGC 6144 – Very close to Antares (and M4) this was easy to find, even at 139x. It is mostly resolved especially with averted vision. There is no concentration towards the core and the cluster appears evenly bright throughout. There is a star on the edge of the western side of the cluster.

NGC 6388 – Very bright and small. Very obvious and easy to see at 31x. At high power (139x) there is a dense bright core surrounded by a halo which looks significantly larger with averted vision. A faint star almost touches the northern side.

NGC 6441 – This is very bright, small and round with a faint star to the south-west. At 139x it is round and shows some brightening towards the centre. Unresolved but granular.

NGC 6453 – This is faint and lies on the NW border of M7. Visible at 31x as a brightening of the background sky. At 139x it is irregular and shows only slight brightening towards the core, which is not dense. Partly resolved with averted vision.

NGC 6496 – Much fainter than near neighbour NGC 6541 this is well defined, irregular, smudge against the sky at 48x. At 139x, it is uniformly bright, unresolved and slightly elongated east-west. There are four foreground stars superimposed on it.

And, yes, I did look at the great southern globular itself, NGC 5139, Omega Centauri. Through a 122cm (48") it knocks your socks off (and blinds you for about 10 minutes - don't use your observing eye! But I couldn't resist it)
In the far more modest 25cm at 104x, it appears very rich but not concentrated in the middle. It is totally resolved and the curious 'footprint' feature is obvious on the western side. Fabulous.

Hopefully, my next view of southern objects will be from Australia next year...

Here are a few random photos from TSP - the telescope areas shown are just the Upper Telescope Field, there were two more fields. Numbers were down from previous years, with just over 400 attendees, but it is still a big event.
Attached Thumbnails
Click for full-size image (tsp2012-3.jpg)
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Click for full-size image (tsp2012-46.jpg)
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Click for full-size image (tsp2012-11.jpg)
198.9 KB29 views
Click for full-size image (tsp2012-6.jpg)
166.7 KB35 views
Click for full-size image (tsp2012-1.jpg)
157.3 KB32 views
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  #2  
Old 05-05-2012, 06:39 AM
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peter_4059 (Peter)
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Looks like a pretty amazing event - maybe a bit dusty though! I bet you see some impressive gear there.
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Old 05-05-2012, 11:38 PM
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FJA (Faith)
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It was very dusty and I expect most people were cleaning their optics when they got home. The gear is pretty impressive, the largest of the 'portable' scopes is a 36 inch Obsession and there are a lot of scopes of 18 inches and larger (18 inches is a popular size). There's also a lot of high-end imaging gear, too, although I didn't get a close look at any of that, as imaging isn't my thing.

I mentioned a 48 inch scope, that belongs to a guy who lives a few miles up the road from the TSP site. I've been fortunate enough to do quite a lot of observing with that scope over the past few years. The scope is as big as it sounds, and it's necessary to climb to the top of a 5 metre ladder to look through the eyepiece at the zenith!
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Old 06-05-2012, 04:22 AM
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GrampianStars (Rob)
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Cool Texas Star Party

"The naked eye limiting magnitude was 6.5 to 7.0"
Not 2 shabby eh! You'd have to be happy with that...
Great write-up on visual Southern DSO's
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Old 06-05-2012, 04:54 AM
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Thank you Rob.
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Old 06-05-2012, 05:49 AM
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mozzie (Peter)
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great report faith,and some nice photo's.are they dark skies or is the light pollution everywhere.how far away from the nearest town or city is it?
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Old 06-05-2012, 08:49 AM
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kustard (Simon)
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I love looking at a field of telescopes
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Old 06-05-2012, 01:49 PM
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It is always good to get reports from one of our Northern "astro cousins". Your observations are detailed and accurate.

N6522 and N6528 are in one of those rare "low extinction windows" which enable the viewer to see right into the bulge component of our own Galaxy. They are only about 2000 light years from the very center of our own galaxy.

I reprint, here, some of my posts regarding these clusters:


For information about all of our own Galaxy's globular star clusters, I refer all of you to the catalog of Milky Way globular star clusters by William E. Harris at:

http://www.physics.mcmaster.ca/Globular

Harris has specialized in globular clusters for all of his professional life, and I believe that he is still keeping this list of globulars (and their properties) updated.

In this catalog you will find many nuggets of information;
for instance, the fact that NGC 6522 and NGC 6528 are almost the very nearest known globular clusters to the very centre of our galaxy. They sit well within the central bulge component of our galaxy at only 0.6 kiloparsecs (1950 light years) from the very centre of the Milky Way galaxy.

When we view this pair of globulars, we have a "porthole view" into the centralmost part of our galaxy, untroubled by the heavy dimming of light (from interstellar dust) that afflicts many fields in the direction of the centre of our galaxy.



An interesting question is:
"are there other fields (apart from NGC 6522/6528) where foreground extinction is so low that we can see all the way in to near the centre of our own galaxy?".

The low extinction (little foreground dust) region at and around NGC 6522 corresponds to the famous Baade's Window.
(see this preprint: astro-ph 9512137 at http//arxiv.org
and/or this reference : (1996), ApJ, 460, L37 (letters, page 37)

The stars in this low extinction window have been repeatedly studied, in order to characterize the stellar population that is found in the bulge component of the Milky Way galaxy.
(e.g. stellar ages & masses & metallicities & temperatures & colours)

It is often said that Baade's Window is one of the few places where we can view inwards to near the centre of our own Galaxy, but I wonder if this isn't just the sort of myth that becomes "truth" through constant repetition.
My own impression, from those long ago days when I was a regular night watchman of the Milky Way, was that there should be other places where we can see bulge stars.

It is not uncommon for the same incorrect information to be repeated again and again in astronomy textbooks, for instance the myth of the Milky Way being 100,000 light years across.
In fact, the region of spiral structure is only 70,000 to 80,000 light years across.
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Old 06-05-2012, 09:28 PM
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FJA (Faith)
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Quote:
great report faith,and some nice photo's.are they dark skies or is the light pollution everywhere.how far away from the nearest town or city is it?
Thanks Peter. The nearest town is the small town of Fort Davis, which is about 8km south east of the ranch where the star party is held. Light pollution is neglible, although there is some visible from Ft Davis when there is haze or cloud in the sky. The skies are very dark, and the limiting magnitude (naked eye) often gets better than 7.0.

Quote:
I love looking at a field of telescopes
I do too, Simon, especially when you get such a contrast of sizes and types. The big Dobs dominate but there are plenty of smaller Dobs, as well as refractors and SCTs.

Robert - thank you.

There are six areas through which bulge stars can be seen, Baade's Window is the largest. I don't know how often the other five are studied or whether, beyond these six, astronomers have looked for others.
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Old 12-05-2012, 09:38 PM
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Paddy (Patrick)
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Great report Faith. Your descriptions are very vivid. I am trying to imagine looking at NGC 5139 with a 48" scope. The mind boggles. What a sight that must have been.
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Old 13-05-2012, 01:04 AM
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FJA (Faith)
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It was, Patrick. It was indescribable. It was totally resolved and it appeared as if I could actually see through the centre. I think that, if I had got my cell phone out and held it up to the eyepiece, I would have got a decent photo...I wish I'd tried that, come to think of it!

I also looked at NGC 5128 through the 48", that was equally as speccy as Omega Centauri was.
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