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RA: 18h 06m 8.6s
Dec: −27° 45′ 55″
Ch: MSA:1415, U2:339, SA:22
Ref: SIMBAD, Collinder (1931), SEDS
Type: globular cluster
Mag: B=?, V=14.6
Synonyms: H II-198
Discovered in 1784 by William Herschel with an 18.7-inch f/13 speculum telescope. He called it "Pretty faint, not large, crookedly extended, easily resolvable."
The RNGC (Sulentic and Tifft 1973) notes that this is a 14.5 mag open cluster.
RA 18 06 08.6 (2000) Dec -27 45 55 Integrated V magnitude 9.30 Central surface brightness, V magnitudes per square arcsecond 16.40 Integrated spectral type Central concentration, c = log(r_total/r_core); a 'c' denotes a core-collapsed cluster 2.50c: Core radius in arcmin .03. ["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) ]
Tom Lorenzin, in the e-version of "1000+ The Amateur Astronomers' Field Guide to Deep Sky Observing", notes: "11M; 0.7' diameter! 10-plus 14M and dimmer members; another toughie! great in larger scopes; 30' due W is cluster N6520."
Harrington notes that this open cluster is "all but ignored by casual deepsky observers. You'll probably need an 8-inch telescope just to find it, and larger instruments are required to see any inner detail. My 13.1-inch f/4.5 Newtonian at 214x discloses only an arc of a half dozen faint stars amid the subtle glow of unresolved suns - not exactly an outstanding sight!"
Date: Mon, 28 Aug 1995 12:16:14 -0600
Location: Elizabeth Ridge, 110 deg. 40 min 6 sec W, 40 deg 56 min 26
Time: 0200 to 0645 UT
Telescope: 10" f10 Newtonian
NGC 6540: This tiny cluster was not hard to find. There were several (about 7 of 8) stars outlining the Northern edge with a somewhat defined glow to the south. The description of Luginbuhl and Skiff seemed accurate; the cluster seemed nearly linear except for a southerly deviation at the west end of the cluster. I was definitely concave southward. This was an interesting sight at 276X.
Crute, C. (2000) The obscure Sagittarius globulars: The Djorgovski & easy Palomar globulars. The Deep Sky, vol.1, winter, p12-13.
"On the opposite side of NGC 6520and B86 from DJ2, is DJ3, also know as NGC 6540. It was first seen by William Herschel, but like another Herschel discovery, Palomar 9, it became obscure and poorly studied. In fact when Djorgovski first discovered it in 1987, it was first thought to be a globular cluster in the vicinity of NGC 6520. Not until 1993 when astronomers Bica, Ortonlani and Burbay studied thearea and compared NGC description of NGC 6540 with DJ3 was it realized that the two were one and the same – and that NGC 6540 had been misidentified as an open cluster all along!
Not only was NGC 6540s (a.k.a. DJ3) class off, so was its overall magnitude and number of stars. (Both the Observer's Guide and "NGC 2000" give its magnitude as 14.6, with the Guide saying it has only 10 stars!). In fact the Harris catalogue gives its overall magniude as 9.3, which is closer (but no cigar) to the magnitude of the object seen in my 10-inch reflector, which was also slightly larger than its originally given size of 0.9 minute. Tom Lorenzin in his 1000+ database gives NGC 6540s magnitude as 11, while Skiff and Luginbuhl in their book, while listing the magnitude as 14.6, gives its size as 1.5 arc minutes – both given before it was known than 6540 was a globular. In his current globlar cluster list, however, Skiff leaves the size, magnitude etc. vacant. I have not resolved it so far, but if the Harris estimate of the Horizontal Branch Level midpoint stars at 15.3 is accurate, ti should not be difficult for an 8-inch or larger to do so. Smaller and nearer than DJ2, it is only 25,000 light years away."
Steve Coe, observing with a 13" f/5.6, notes: "Pretty faint, small, round, not rich, very compressed at 165X. Three stars are resolved constantly and another 3 or 4 come and go with the seeing at 220X. This very compact cluster demands high power."
The Messier objects
The Bennett objects
The Caldwell list
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