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RA: 19h 37m 10s
Dec: +46° 22′ 30″
Ch: MSA:1109, U2:84, SA:8
Ref: SIMBAD, Collinder (1931), DAML02, Archinal&Hynes (2003)
Type: open cluster, 31r
Mag: B=7.47, V=6.8
NGC 6811. JH has two observations of this, separated by nearly a minute of time in RA and 6 arcmin in Dec. The RA of the first observation is correct, while the declination of the second is correct. Unfortunately, the position JH adopted for the GC carries the RA of the second, and a Dec 10 arcmin further on north. I think he meant to use only the second observation (he notes that the first observation refers to "A double star in the southern part..."), so the incorrect Dec must be a transcription or typographical error.
Once these errors are corrected, though, N6811 turns out to be quite a nice cluster, ten or twelve arcmin across, with perhaps a hundred stars, many of the 10th and 11th magnitudes.
Trumpler (Lick Obs Bul, Vol 14, No. 420) gives the diameter as 13' and the class as 3 1 p.
Raab, S. (1922) A research on open clusters. Lund Medd. Astron. Obs. Ser. II, 28, 1.
Discussed, based of F-A plates.
The RNGC (Sulentic and Tifft 1973) notes that this is a 9.0 mag open cluster.
Bailey, S.I. (1908) A catalogue of bright stars and nebulae. Ann.Harv.Coll.Obs., 60(8), 199.
Photo Index by Jim Lucyk: Sky&Tel. 10/86 p427, The Astrograph 8-9/81 p4.
A catalogue of star clusters shown on Franklin-Adams chart plates. Mem.R.A.S., 60(5), 175-186.
Houston (1973) notes: "A 6-inch instrument will show some 75 faint stars shimmering inside an area 15' across. It reminds some amateurs of an opera-glass view of the Beehive."
Houston notes that this small group of stars shines with a total light of magnitude 6.8. He reports a comment by Tommy Christensen, who observed it with a 3.5-inch refractor and saw a distinct "smoke ring" of stars with a centre that was conspicuously dark though not completely starless. "Since I remembered the cluster as an unimpressive, nondescript group, clearly another look was in order. At the next opportunity I examined the cluster with an 8-inch reflector. No smoke ring! I tried a 10-inch with the same result. Nevertheless, I tossed the problem out to readers ... scores of observers turned their scopes on the cluster, and responded with all kinds of descriptions - bells, butterflies, dinosaurs, a pair of fighting peacocks .. But no smoke rings .. I tried stopping down my Clark refractor from 4 to 2 inches. Lo and behold, there was the smoke ring." Houston also reports an observer in Czechoslovakia, using a Zeiss 2.5-inch refractor from 14x to 140x who saw it as "a clear-cut ring of stars with no hint of the butterfly pattern I suggested in an earlier column."
From a collection of observations reported by Houston, there is "a dark band about 5' thick running through the middle of the cluster" This impression was retained with apertures up to 11-inch, but with large telescopes the dark centre did not show.
Tom Lorenzin, in the e-version of "1000+ The Amateur Astronomers' Field Guide to Deep Sky Observing", notes: "9M; 15' diameter; large and sparse with stars in two curving chains; 30' to SE is a smaller (7' diameter) fairly rich, faint cluster (70-plus members 12 thru 14M) with a central void; which is N6811? I don't know; do you?."
7x35mm - mod f at diffuse at first glance. attention shows a few of the brtst *s. unconcen. to immed W is br trapezoid of *s that could be mistaken for cl. BS, 28Jun1992, Hutch Mtn.
8cm - polygonal, nicely res @ 20 into 20 *s, no haze. BS, 15Sep1982, Anderson Mesa.
15cm - avg cl, 20' diam, rich in f *s m10+. 50 *s w/a bit of haze. rich bkgrnd. HM/BS, 28Jun1971, FtL.
- mod br, mod rich, annular cl fully res @ 80x. annulus 10' diam w/3' hole, outliers to 15' diam. 140x: 75 *s m10+ pretty uniformly scattered around ring. m11.5 pair SE side. m13 * in N side of annulus that seems dbl. brtr loose circlet about 1/2 deg WNW, some *s of which are plotted on U2000. BS, 17June1991, Anderson Mesa.
25cm - lg, 20' diam w/*s m9+. much seen at lox. 90x: some haze and grpings throughout cl. 60 *s.
30cm - 149x: 75 *s in 11' fld. loose. elong E-W. *s m9.5-10.5.
Alldays (22.50S, 20.12E, 770m).
12-inch f/10 SCT (95x, 218x)
It is a very faint spacious grouping of stars with some open areas towards the middle. (With imagination if forms the letter 83 with a asterism of five stars going out from the letter 8, or may be a bicycle). Slightly elongated in a N-S direction.
The Messier objects
The Bennett objects
The Caldwell list
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