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RA: 19h 07m 49s
Dec: +04° 15′ 59″
Ch: MSA:1269, U2:251, SA:16
Ref: SIMBAD, Collinder (1931), NGC/IC, Archinal&Hynes (2003)
Type: open cluster, 22r
Mag: B=8.6, V=7.5
Synonyms: H VII-019
Discovered in 1785 by William Herschel with an 18.7-inch f/13 speculum telescope. He called it "a pretty compressed cluster of prettyscattered stars of various sizes, magnitudes and colours. iF, and unequally compressed, 12' or 15' diameter."
Trumpler (Lick Obs Bul, Vol 14, No. 420) gives the diameter as 15' and the class as 4 2 m.
Burnham calls this a very large, very rich, little compressed open cluster of about 50 stars spread out over 10' of sky, stars from 12th magnitude and fainter.
The RNGC (Sulentic and Tifft 1973) notes that this is a 9.0 mag open cluster.
Steve Coe, in "SACNEWS On-line for September 1996", observing with a 13-inch, notes: NGC 6755 is an open cluster at 19 hr 07.8 min and +04 14. It is large, not compressed, irregular in shape and has nice chains of stars at 100X in the 13". 50 members were counted. This object and NGC 6756 both fit in the one degree field of my giant 38mm Erfle eyepiece. I called it the Double Cluster in Aquila. Both clusters are just visible in the 11 X 80 finder.
Steve Coe, using a 13" f/5.6, notes: "Large, not compressed, irregular shape and has nice chains of stars at 100X. 50 members counted. This object and 6756 both fit in the one degree field of my giant 38mm Erfle eyepiece. I called it the Double Cluster in Aquila. Both clusters are just visible in the 11 X 80 finder."
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.
Hoag: (15cm) m10.5 *: V=10.35/0.66. * in pair close to br *: V=12.57/0.81.
Smost * in circlet: V=12.95/0.78.
15cm - easier @ 60x than -56, somewhat brtr. speckled @ 120x. HM, Roof.
- fairly consp and hazy, best @ 125x. split in two w/dk lane running NE-SW. brtr *s on SE side, m10+. 40 *s. BS, 3Sep1981, Anderson Mesa.
- pretty br but hazy cl of f *s @ 50x nr E edge of Great Rift. circ, 15' diam, but few outliers on W twd Rift. core 8' across and annular w/consp clumps N&S of center, both of which are oval and elong ~E-W. brtr *s in Srn clump, which also has m12 pair just SE of m10.5 * at W end, and tiny circlet of m13 *s at center. still hazy @ 140x, but 75 *s easy to count; many more at threshold. BS, 12Oct1990, Anderson Mesa.
25cm - 90x/180x: cen core 3' across containing a doz *s. long spiral chains extend from it to 30' diam. clusterings 15' SE and 8' NW. BS, Roof.
- 125x: bifurcation clr, more *s on SE side. 75 *s total. BS, 3Sep1981, Anderson Mesa.
30cm - 220x: 30 *s in 7' area. four *s in core. to 10' are strings in curved lines. CBL, Roof.
On this same night, I was surveying some objects in Aquila, including a couple that Brian mentioned. I noted that NGC 6760 (which Brian described using the 70mm Pronto) was easily visible in my 11x80mm finderscope. Through the 20-inch at 250x from a not-so-dark site near San Diego, it appeared 4' across with very broad concentration. I only noted several dozen stars resolved across the face. The brightest stars in NGC 6760 (from http://www.ngcic.com/papers/gcdata.htm) are tabulated as V=15.6. I had the same impression of NGC 6755 as a non-cluster. I saw about 60 stars mostly in two groups of widely varying brigtness between 10th and 14th magnitude. The northern group may be Cz 39. Centered in the southern grouping is a tiny clump of five 12th-magnitude stars about in an area 30" across.
Aquila as a whole is interesting. The western half of the constellation is nothing but Milky Way planetaries and clusters, some of which are impressive. East of a line that connects Altair to Lambda Aquilae, these give way to very distant galaxies. In fact, there is nothing in this half of Aquila that would qualify as a showpiece on anybody's list.
Arizona Sky Pages
AJ Crayon, using an 8" f/6 Newtonian, notes: "is an open cluster with 5 stars of 10m in a 10' area at 60x. It has a large glow of unresolved stars and NGC 6756 is 50' to the northwest.
16-inch f/10 SCT (127x 290x)
NGC 6755 displays a nice large and rich cluster with star string that concentrate more to the centre of the group. Czernik 39 a smaller group situated on the north-western edge of NGC 6755 displays a double cluster impression although half the size of NGC 6755.
Location: Campsite (23 16 South - 29 26 East)
Date: 15th November 1998
Telescope: 8" Meade telescope & 18mm eyepiece.
Field of view: 36.2 arc minutes
Mixed magnitude stars in-groups randomly take places along a very large S shape cluster formation (east to west), with open spaces in between. To the north a small grouping of faint stars share the field of view.
The Messier objects
The Bennett objects
The Caldwell list
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