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RA: 19h 08m 42.5s
Dec: +04° 42′ 21″
Ch: MSA:1269, U2:251, SA:16
Ref: SIMBAD, Collinder (1931), NGC/IC, Archinal&Hynes (2003)
Type: open cluster, 11m
Mag: B=?, V=10.6
Synonyms: H VII-062
Discovered in 1791 by William Herschel with an 18.7-inch f/13 speculum telescope. He called it "a small pretty compressed cluster of stars not very rich."
Burnham calls this a small, rich, little compressed 10.5 magnitude open cluster which has 25 stars 11th magnitude and fainter in an area 3' across.
Trumpler (Lick Obs Bul, Vol 14, No. 420) gives the diameter as 4' and the class as 1 2 p.
The RNGC (Sulentic and Tifft 1973) notes that this is a 10.5 mag open cluster.
Steve Coe, in "SACNEWS On-line for September 1996", observing with a 13-inch, notes: NGC 6756 is pretty small, much compressed, elongated, a compact cluster at 165X. I counted 15 members resolved.
Steve Coe, using a 13" f/5.6, notes: "Pretty small, much compressed, elongated, compact cluster at 165X. I counted 15 members resolved."
AJ Crayon, using an 8" f/6 Newtonian, notes: "is an open cluster with NGC 6755 50' to the southeast. It is 11m and 3' at 60x.
Subject: [amastro] Dwarfs in a dwarf telescope
I spent a bit more time last night (5 Sep UT) casting about with the Pronto at Anderson Mesa on another excellent post-monsoonal night. Among the targets were some low-surface-brightness Local Group dwarf galaxies: in decreasing order of brightness NGC 6822, IC 1613, and the Wolf-Lundmark-Melotte galaxy. At about mag. 9 and mean surface brightness about mag. 14.5/square arcmin, NGC 6822 was readily spotted at 30x. IC 1613, a magnitude fainter, was a marginal object, and WLM, somewhat fainter still by the specs, was apparently a bit too faint and I couldn't reliably see it. I'd like to try WLM again perhaps from a more southerly location.
In the field with NGC 6822 is the bright planetary NGC 6818. Since this was substellar at 30x, I estimated its brightness (without filters) with respect to the mag. 8 star west (HD 186107) as about 0.6 mag. fainter. This star has V=8.1, but is quite red, B-V=1.7, so assuming the 0.2(B-V) factor to get visual magnitudes, the planetary comes out right at mv=9.0, perhaps slightly brighter than Marling's mv=9.3. One might also make an estimate wrt the fainter star east (HD 186368, V=9.5/B-V=0.3), since its color is more neutral.
I also looked at NGC 6760 in Aquila. This was comfortably faint for the Pronto. The halo reaches roughly to a relatively bright star on the NE side of this cluster. This is mentioned by Luginbuhl & Skiff as mag. 10.5, but is actually mag. 12 or 12.5 (Kepple & Sanner also wrong.) This star was used by several 19th century observers (Winnecke, Lord Rosse, Bigourdan) as an astrometric reference; Bigourdan also calls it mag. 12 or 12.2 in two of his four observations.
While poking around near NGC 6760, I came across what appeared to be a large "absorption hole" starcloud and a tiny fuzzy knot half a degree north of it. After refinding it from 6760 with atlas in hand, I was surprised to learn that these were NGC 6755 (starcloud) and NGC 6756 (knot). NGC 6755, although a nice object at 30x, has what is to me a distinctly non-open-cluster appearance (luminosity function not right)---I'd be willing to bet this is nothing more than a starcloud. NGC 6756 was inscrutable, simply too small and faint for the Pronto at 60x.
15cm - faint, no *s @ 120x. HM, Roof.
- partially res, 3' diam. brtr *s m12, brtst nr central clot is m11.5. about ten *s res, too f for 190x. BS, 3Sep1981, Anderson Mesa.
- in same 50x/1.2 deg fld with -55. pretty sm, quite concen w/hisfcbr unres center. 140x: 4' diam, 20 *s, brtst is m12.5 * on S. cl reaches somewhat beyond radius of this *. tight clump nr center is just res but can't say much about components. rel poor 50' surrounding fld. BS, 12Oct1990, Anderson Mesa.
25cm - sm, like core of -55 @ 180x. sm chain extends S&W. 2'.5 diam, 10 *s, the brtst m11.5 on N edge. BS, Roof.
- cen grp still unres, 20 others. core E of geom center. interesting. BS, 3Sep1981, Anderson Mesa.
30cm - sm, core 45", elong ESE-WNW by 30%. 15 *s m11+, m11 on N. CBL, Roof.
- knot in center is res, but *s vclose and hard to distinguish w/averted vis. about 10 *s in knot, one on N end, then pair (w/third comp) then a triplet (at least). pair and triplet aligned E-W. CBL, 12Aug1983, USNO.
16-inch f/10 SCT (127x 290x)
At first I thought it might be a planetary nebula but high power reveal a roundish haze became faint light specks, barely seen. The grouping is more concentrated towards the south-western side or so it looks to me.
The Messier objects
The Bennett objects
The Caldwell list
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