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RA: 17h 23m 58.65s
Dec: −26° 21′ 12.3″
Ch: MSA:1394, U2:338, SA:22
Ref: SIMBAD, Collinder (1931), SEDS
Type: globular cluster
Mag: B=12.46, V=11.05
Synonyms: H I-046
Discovered in 1784 by William Herschel with an 18.7-inch f/13 speculum telescope. He called it "pB cL R BM resolvable."
The NGC calls it "considerably faint, large, round, gradually brighter in the middle, well resolved into stars."
The RNGC (Sulentic and Tifft 1973) notes that this is a 9.5 mag globular cluster.
RA 17 23 58.6 (2000) Dec -26 21 13 Integrated V magnitude 9.14 Central surface brightness, V magnitudes per square arcsecond 18.05 Integrated spectral type G0- Central concentration, c = log(r_total/r_core); a 'c' denotes a core-collapsed cluster 2.50c: Core radius in arcmin .05. ["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) ]
prob. a globular cluster (vis. obs)
S globular cluster, alm. spiral in structure. See HOB 15.
Tom Lorenzin, in the e-version of "1000+ The Amateur Astronomers' Field Guide to Deep Sky Observing", notes: "10M; 3' diameter; bright and small; unresolved; looks extremely distant."
Hartung writes: "A few very faint stars may be made out with a 12inch telescope scattered through this irregularly round haze about 1' across, and signs of resolution are also apparent with an 8-inch. A four-inch shows a faint but quite clear hazy spot with a group of scattered stars north and following." The cluster has a diameter of 5' and an integrated magnitdue of 9.6.
Observer: John Callender
Instrument: 8-inch Dobsonian reflector Location: Carpinteria, CA, USA
Light pollution: light Transparency: fair Seeing: good
Time: Sat Jul 5 08:05:00 1997 UT Obs. no.: 201
Unable to spot it at 49x.
Steve Coe, using a 13" f/5.6, notes: " Pretty faint, pretty large, round, somewhat brighter in the middle, three stars resolved at 165X."
A 2-inch refractor shows this elusive cometary cluster in a rich starfield of faint stars. No irregular shape or definite concentration seen, it appears as an uniform glow. I found the cluster pretty elusive, but it was actually found whilst sweeping for the field. The slight movement seems to have increased its visibility. Just east of the cluster is a beautiful isoceles triangle of 10th magnitude stars, with a fainter companion turning the grouping into an irregular kite. Due north of the cluster, in the same low-power field, lies a 9th magnitude slightly red star with a companion pointing toward the globular. The cluster can be found easily by locating the bright Theta Oph, which is the first in a chain of stars leading south and slightly east towards the globular.
Observing site: Little Bennett Regional Park
[17h 24m 0s, -26° 21' 0"] A faint smudge, like an E0 galaxy.
The Messier objects
The Bennett objects
The Caldwell list
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