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RA: 18h 31m 54.23s
Dec: −23° 28′ 34.1″
Ch: MSA:1391, U2:340, SA:22
Ref: SIMBAD, Collinder (1931), SEDS
Type: globular cluster
Mag: B=11.3, V=10.24
Synonyms: H II-205
Discovered in 1784 by William Herschel with an 18.7-inch f/13 speculum telescope. He called it "pB, cL, iE, bM."
Observed by Sir John Herschel at the Cape of Good Hope with an 18-inch f/13 speculum telescope. He recorded it as "globular, pB, R, gpmbM, 2', resolved into visible but vS stars 15..16m."
The NGC describes it as "pretty bright, pretty large, irregularly round, gradually pretty much brighter towards the middle, well resolved into stars of 16 mag."
A catalogue of star clusters shown on Franklin-Adams chart plates. Mem.R.A.S., 60(5), 175-186.
RA 18 31 54.3 (2000) Dec -23 28 35 Integrated V magnitude 9.13 Central surface brightness, V magnitudes per square arcsecond 16.68 Integrated spectral type F8 Central concentration, c = log(r_total/r_core); a 'c' denotes a core-collapsed cluster 1.99 Core radius in arcmin .10. ["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) ]
The RNGC (Sulentic and Tifft 1973) notes that this is a 10.5 mag globular cluster.
Vorontsov-Velyaminov, B. "Integral magnitudes of south star clusters", Astron. Nach. 228, 325. Comparing the brightness of the cluster with the extrafocal images of stars, he estimates the magnitudes as 7.92
"globular cluster, few stars, condensed"
Bailey, S.I. (1908) A catalogue of bright stars and nebulae. Ann.Harv.Coll.Obs., 60(8), 199.
This small and faint globular cluster lies about a degree west-northwest of M22, NGC 6656. According to Hartung, "it is a compact irregularly round mass of faint stars about 1' across, concentrated centrally with scattered outliers. An 8-inch is just able to show a few of these stars, and the object is a plain hazy spot in a star-strewn field with a 4-inch".
Tom Lorenzin, in the e-version of "1000+ The Amateur Astronomers' Field Guide to Deep Sky Observing", notes: "8M; 2' diameter; bright and small; 1 degree WNW of GLOB M-22."
Steve Coe, observing with a 13" f/5.6, notes: "Pretty bright, pretty small globular that is somewhat elongated and very compressed with a bright middle. I was able to resolve 5 stars at 135X and 10 stars at 220X."
A 15.5-inch reflector shows it as a very small patch of light. The centre of the globular suddenly grows brighter, and is surrounded by a nebulous haze. The pinpoint nucleus is quite prominent.
30/04/93: Observing with a 6-inch f/8.6 Newtonian, this cluster appears vF, pS, reasonably easy at 52x.
1997 October 27: Jonkershoek, seeing 3, transparency 3, sky darkness 4, lim.mag. at south pole 6.0 (naked eye), 10.7 (binoculars). 11x80 tripod-mounted. "With averted vision, a very faint starlike glow"
12-inch Meade, 40mm eyepiece, 53' fov. 1997-08-05, fair sky conditions "This is by far the baby of the globulars. Just a little trace of light. Could not make out any detail,just a little haziness around." [Magda Streicher, Pietersburg 23-53S, 29-28E]
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)
This is by far the baby in the globular family. Could not discern any detail, just a little hazy puff. This emphasized the feeling of distance in space. I estimate that NGC 6642 is fainter than the appointed 9.4 magnitude. Clumps of tightly packed faint stars can be seen to the south and west in the field of view (95x). Very rich star-field.
Location:Night Sky Caravan Park,Bonnievale.
Sky Conditions:Whole Milky Way is visible.The sky is clean.
Atmosphere stable with little interference.
This globular cluster has the shape of a round soft halo of light and that this cluster's stars are not resolvable.The central core of this cluster is strongly condensed towards the center.The central core of this cluster is slightly brighter compared to the stars on the far outskirts of this cluster.This globular cluster measures 3'*3'.Chart:No.328,NSOG Vol.2.
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