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RA: 18h 04m 49.74s
Dec: −07° 35′ 9.1″
Ch: MSA:1320, U2:294, SA:15
Ref: SIMBAD, SEDS
Type: globular cluster
Mag: B=?, V=9.33
Discovered by the Danish astronomer Theodor Brorsen in 1856.
The RNGC (Sulentic and Tifft 1973) notes that this is a 12.5 mag globular cluster.
RA 18 04 49.8 (2000) Dec -07 35 09 Integrated V magnitude 9.33 Central surface brightness, V magnitudes per square arcsecond 19.31 Integrated spectral type G4: Central concentration, c = log(r_total/r_core); a 'c' denotes a core-collapsed cluster 1.60 Core radius in arcmin .54. ["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) ]
Included in a list of doubtful objects;. A very faint cluster in a large obscured area.
vF, 5'x5', globular cluster.
A catalogue of star clusters shown on Franklin-Adams chart plates. Mem.R.A.S., 60(5), 175-186.
Tom Lorenzin, in the e-version of "1000+ The Amateur Astronomers' Field Guide to Deep Sky Observing", notes: "12M; 2.5' diameter; small, round, faint and unresolved; 14M star 2' NW of core; DIFFICULT! 50' to SW is DBL ST Tau OPH."
Houston calls it a difficult object, 2' across and of 12th magnitude.
Hartung describes this globular as a faint round haze nearly 3' wide, rising a little and broadly to the centre with no resolution apparent with a 12-inch, although a few faint nearby stars near may be outliers.
Steve Coe, using a 13" f/5.6, notes: " Faint, pretty large, round, little brighter in the middle at 165X. This globular is in a very dark, star poor field."
Steve Coe, observing with a 13" f/5.6, notes: "Faint, pretty large, round, not brighter in the middle at 135X. This globular is somewhat mushy, and no resolution can be seen at powers up to 250X."
Observer: Steve Coe
Your skills: Advanced (many years)
Date/time of observation: 25 July 98
Location of site: Strawberry, Arizona USA (Lat +34, Elev 7000 ft)
Site classification: Rural
Sky darkness: 8 1-10 Scale (10 best)
Seeing: 7 1-10 Seeing Scale (10 best)
Moon presence: None - moon not in sky
Instrument: 13" f/5.6 Newtonian on Bigfoot German EQ
Object(s): NGC 6539
Category: Globular cluster.
100X--faint, pretty large, brighter middle, irregularly round, resolved even at 100X, about 8 very faint stars seen, better resolved with averted vision. There is an 11th mag star at the edge of this globular, it is in a PA of 330 degrees. This low surface brightness cluster is not helped with higher powers.
11x80.Technopark. 23:00 SAST. Hazy sky. Not found.
11x80. Kelsey Farm. 23:00 SAST. Not visible, neither is the IC globular nearby.
1997 July 7, Monday, 21:00 - 24:00 Jonkershoek. 11x80's tripod-mounted. Most extremely faint globular cluster to date. At first, not seen. Then, seen. At a third look, not seen! Certainty: 40% seen. Stars of 9.5 well seen.
Observing site: Little Tycho Observatory
[18h 4m 48s, -7° 35' 0"] Field found, not found.
Observing site: Pinnacles overlook
[18h 4m 48s, -7° 35' 0"] An obvious, faint smudge. No resolution.
The Messier objects
The Bennett objects
The Caldwell list
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