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RA: 16h 53m 25.36s
Dec: −22° 10′ 38.8″
Ch: MSA:1396, U2:337, SA:22
Ref: SIMBAD, Archinal&Hynes (2003), SEDS
Type: globular cluster
Mag: B=?, V=8.9
Synonyms: H II-584
Discovered in 1786 by William Herschel with an 18.7-inch f/13 speculum telescope. He called it "pB, cL, gbM, easily resolvable, undoubtedly stellar."
Observed by Sir John Herschel at the Cape of Good Hope with an 18-inch f/13 speculum telescope. He recorded it as "pretty compressed, small, 2', rather triangular than round, mbM, resolved into stars 14..16th mag."
The NGC description reads: "pretty bright, considerably large, irregularly round, clearly resolved into stars of magnitudes 14..16".
A catalogue of star clusters shown on Franklin-Adams chart plates. Mem.R.A.S., 60(5), 175-186.
RA 16 53 25.4 (2000) Dec -22 10 38 Integrated V magnitude 9.97 Central surface brightness, V magnitudes per square arcsecond 18.98 Integrated spectral type F9: Central concentration, c = log(r_total/r_core); a 'c' denotes a core-collapsed cluster 1.33 Core radius in arcmin .36. ["Catalog Of Parameters For Milky Way Globular Clusters", compiled by William E. Harris, McMaster University. (Revised: May 15, 1997; from http://www.physics.mcmaster.ca/Globular.html; Harris, W.E. 1996, AJ, 112, 1487) ]
Cluster, scarecely globular, but condensed in the centre, and with outer stars in streams.
The RNGC (Sulentic and Tifft 1973) notes that this is a 11.0 mag globular cluster.
Uncloaking Globular Clusters in the Inner Galaxy. [2012AJ....143...70A]
The moderately high values of extinction are a consequence of the cluster's projected position, close to the Ophiuchus dust complex.
Tom Lorenzin, in the e-version of "1000+ The Amateur Astronomers' Field Guide to Deep Sky Observing", notes: "10M; 2' diameter; faint, diffuse, small and round; unresolved at any power; possibly not a GLOB, but a dense cluster; the company it keeps and its position relative to the galactic plane does not reduce the ambiguity; 12.5M star 2.5' NNW; tough in any but darkest skies."
Hartung notes that "this is one of the less-concentrated type of globular cluster, a hazy luminous body about 1.5' across full of tiny gleaming points with no conspicuous outliers. An 8-inch telescope shows some of these stars but a 6-inch failed, and a 4-inch gives only a small hazy spot in the field."
This magnitude 10.2 cluster measures 5' across and has a concentration rating of 10.
Steve Coe, using a 13" f/5.6, notes: "Bright, pretty large, irregularly round and somewhat brighter in the middle at 165X. It is easily resolved in the 13", this nice globular has a central bar that is brighter than the other parts of the cluster."
In a 15.5-inch telescope at 220x, this interesting cluster, although it is small to medium in size, is quite faint. The individual stars are of 11th mag and smaller. The cluster appears as a faint, extended patch of light. Careful examination with averted vision shows a few stars sparkling out within the globular, whilst the outer circumfrance appears irregular with ragged edges. The cluster lies inside a pretty large right-angled triangle of 9th magnitude stars.
16-inch f/10 SCT (127x 290x)
Relatively bright, but small in size, bath in a hazy cocoon. The shape is elongated in a north-east to south-west direction. Although a few faint stars embedded into the hazy north-eastern part could well be responsible for this impression. High power bring out a granular effect with various faint stars surrounded with haze. A magnitude 11 star is situated only 2 arc minutes to the north shine with a strong yellow colour.
12-inch f/10 SCT (EP: 2-inch 32mm SW 95x 42' fov; 2-inch 14mm UW 218x 23' fov; 2-inch 8.8mm UW 346x 15' fov)
Very faint and small with no stars visible, but the edges seem frayed (346x). It brightens slowly towards the core, which is moderately bright. The core has an uneven angular shape. This globular is situated between a 12th magnitude triangle-shape of stars. (Mag 10.0; size 5.0'; brightest stars 14.0 mag. )
Observing site: Little Tycho Observatory
[16h 53m 24s, -22° 11' 0"] A faint smudge, on the edge of visibility.
Sky Conditions:The fainter parts of the Milky Way are barely visible.Haziness only visible on the horizon.Atmosphere stable with little interference.
The shape of this globular cluster looks like a faint snowflake of light and that no stars are resolved in this cluster at all. In overall the nucleus of this globular cluster is slightly condensed and that the central core of this cluster looks like a halo.The nucleus of this globular cluster grows slightly brighter compared to the far outskirts of this cluster.Chart No.292,NSOG Vol.2.
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