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RA: 18h 24m 32.89s
Dec: −24° 52′ 11.4″
Ch: MSA:1391, U2:340, SA:22
Ref: SIMBAD, SEDS
Type: globular cluster
Mag: B=8.73, V=7.66
Discovered by Messier in July 1764, he described it as a "nebula containing no star... round, seen with difficulty in 3.5-foot telescope, diam 2'"
In the Appendix to the 1912 'Scientific Papers of Sir William Herschel' this object is described as "1799, August 1, 20 feet telescope, It may be called insulated though situated in a part of the heavens that is very rich in stars. It may have a nucleus, for it is much compressed towards the centre, and the situations is too low for seeing it well. The stars of the cluster are pretty numerous."
Observed by Sir John Herschel at the Cape of Good Hope with an 18-inch f/13 speculum telescope. He recorded it as "globular, vB, R, vm vomp, gbM, but not to a nipple; diam in RA = 12 seconds, resolved into stars 14..16m, fine object. Occurs in the milky way, of which the stars here are barely visible and immensely numerous."
The NGC calls it "very bright, round, very much compressed, resolved into stars 14..15 mag."
Doig, P. (1925) Notes on the nebulae and clusters in Webb's 'Celestial Objects for Common Telescopes' (Sixth edition, Vol.ii). Part IV. M.N.R.A.S., 36(2), 58.
Burnham notes that "it appears as a round fuzzy spot requiring considerable aperture for real resolution."
Shapley found a somewhat elliptical outline for the cluster in PA 50 .
"!! globlar cluster, condensed"
Bailey, S.I. (1908) A catalogue of bright stars and nebulae. Ann.Harv.Coll.Obs., 60(8), 199.
Remarks, p.218: "an interesting globular cluster containing 9 known variable stars"
The RNGC (Sulentic and Tifft 1973) notes that this is a 8.5 mag globular cluster.
RA 18 24 32.9 (2000) Dec -24 52 12 Integrated V magnitude 6.79 Central surface brightness, V magnitudes per square arcsecond 16.08 Integrated spectral type F8 Central concentration, c = log(r_total/r_core); a 'c' denotes a core-collapsed cluster 1.67 Core radius in arcmin .24. ["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) ]
A study by W. W. Morgan of Yerkes Observatory indicates this globular cluster to have a spectral type of F9.
Vorontsov-Velyaminov, B. "Integral magnitudes of south star clusters", Astron. Nach. 228, 325. Comparing the brightness of the cluster with the extrafocal images of stars, he estimates the magnitudes as 7.23
A catalogue of star clusters shown on Franklin-Adams chart plates. Mem.R.A.S., 60(5), 175-186.
Mallas calls the centre "intense" and finds it "oddly shaped" in small telescopes.
Hartung notes that it is easy to find in 3-inch, and "a fine object with 30cm [12-inch], bright round and symmetrical and well resolved into stars, the outliers extending to 3'.5 15cm [6-inch] shows this resolution by mottling the haze with faint stars; it lies in a well-scattered star field."
Stewart Moore (Fleet, Hampshire, UK), observing with a 12-inch f/5, writes in the The Webb Society Nebulae and Clusters Section Report No. 10, July 1992: "A small, woolly-looking cluster. Stars easily resolved at the edge."
John Bortle (Webb Society Quarterly Journal, January 1976) using 10x50 binoculars, estimates the visual magnitude as 7.4.
Observer: Lew Gramer; Your skills: Intermediate; Date and UT of Observation: 1997-07-4/5, 03:40 UT; Location: Savoy, MA, USA (42N, elev 700m); Site classification: rural; Limiting magnitude: 7.1 (zenith); Seeing: 5 of 10 - mediocre, increasing cumulus; Moon up: no; Instrument: Naked eye, 50mm Simmons binoculars; Magnification: 1x, 7x; Filters used: None; Object: M22, M28; Category: Globular clusters; Constellation: Sgr; Data: mags 5.2, 6.9 sizes 24', 11'; RA/DE: 18h30m -24o;
Description: M22 was barely spotted with the naked eye, in spite of its low altitude, 2o NE of lambda Sgr amid the background blur. M22 even in binoculars was a suprisingly LARGE, glittering ball of haze, with stellarings all the way around its edges, especially to the S and SW. Haze seemed to spray out from it in cluttered streamers to N, S, and W, merging with a pretty background of field stars & Milky Way haze. A yellowish star of about mag 8 was noted on its NE edge. Visible in the same binocular field with M22 were the much fainter globular M28, and the bright yellow star lambda Sgr. Intermediate between M22 and lambda was an apparent open cluster (no catalog #), a pretty clustering of stars mags 6 to 10. Closer still to lambda, the field stars appeared to attenuate somewhat. Then about 1/4 field NW of lambda, M28 was just visible as a spot of haze, showing loose concentration and no other detail.
Steve Coe, observing with a 13" f/5.6, notes: "(M 28) Bright, pretty large, round and has a very bright middle. It is easily resolved at 100X."
Tom Lorenzin, in the e-version of "1000+ The Amateur Astronomers' Field Guide to Deep Sky Observing", notes: "8M; 6' diameter; small, diffuse blob barely resolved just NW of the Teapot's crown star, 3M Lambda SGR; includes a few 13.5M thru 14M stars in SE reaches; as with all GLOBs, high-x (>200) shows best!."
As seen with 11x80 binoculars, the cluster shows as a soft glow of light, lying in the same field as, and to the northwest of, the reddish Lambda Sagittarius. It is pretty bright, and appears quite small, like a nebulous out of focus star.
A 15.5-inch reflector at 220x reveals the cluster as having a regular condensation towards the centre, with no marked irregularities.
30/04/93: Observing with a 6-inch f/8.6 Newtonian, this cluster is pB, pL, easy at 52x.
23 August 1993, 00:25 Observing with 11x80's from Stellenbosch, NGC 6626 appears very small when compared with its symbol on the Uranometria chart (339). The cluster appears all nucleus -- no outer parts seen, just the central blaze.
1997 October 27: Jonkershoek, seeing 3, transparency 3, sky darkness 4, lim.mag. at south pole 6.0 (naked eye), 10.7 (binoculars). 11x80 tripod-mounted. "Round glow, easily spotted, about 90arcsec across."
Location: Jonkershoek Nature Reserve, Assegaaibosch Station
Date: 1998 July 31 / August 01, 01:00-02:40 SAST
11x80 tripod-mounted binoculars (9.5 mag stars at times not easy)
Sky conditions: Mediocre (transp. low, seeing average, dew) The skies are showing the effects of the combination of pollution (mainly from a nearby wood-processing plant) and a stable inversion layer, turning daytime skies grey-blue, and night skies ashen.
Moderately bright fuzzy patch, readily seen, broad nucleus.
12-inch Meade, 40mm eyepiece, 53' fov. 1997-08-05, fair sky conditions "Medium bright globular cluster with a core very much smaller than the halo. Stars reveal themselves towards the fringy edges from which startrails are seen running out everywehere. Some outliners visible towards the end of this busy starfield." [Magda Streicher, Pietersburg 23-53S, 29-28E]
12-inch f/10 SCT (EP: 2-inch 40mm SW 76x 53' fov; 2-inch 14mm UW 218x 23' fov)
Medium, bright, globular cluster with a bright core far smaller than the halo. Stars reveal themselves towards the fringy edges from which dainty star-trails are seen running out everywhere in a beautiful busy star-field.
Sky Conditions:The fainter parts of the Milky Way are barely visible.Haziness only visible on the horizon.Atmosphere stable with little interference.
This globular cluster has the shape of a round irregular tennis ball and that the stars in this cluster is strongly concentrated towards each other and that the nucleus of this globular cluster is very strongly concentrated and dense towards the center of this cluster.The nucleus of this cluster is brighter than the stars surrounding this globular cluster.This globular cluster measures 5.7'x 4.3'.Chart No.329,NSOG Vol.2.
Instrument:12-inch Dobsonian Reflector Telescope.
Sky Conditions:The fainter parts of the Milky Way are barely visible.
Transparency of the Sky:Haziness only visible on the horizon.
Seeing:Atmosphere stable with little interference.
Object Type:Globular Cluster.
First Impression:This object looks like a globular cluster.
Chart Number:No.18(Extract taken out of "Atlas of the Night Sky").
Size:26mm Eyepiece:Field Of View:57'/14=4.0'.
20mm Eyepiece:Field Of View:50'/12.5=4.0'.
Size in Arc Minutes:4.0'(Nucleus).
Globular Cluster is 4.0'*1.3'.
Brightness Profile:From the far outskirts of this globular cluster the nucleus is brighter.
Challenge Rating:Fantastic Sight.
This globular cluster is well resolved into a multitude of bright stars.The stars are concentrated like a snowball.The central nucleus of this cluster is that the stars are slightly compact.Around the outskirts of this globular cluster the stars have a granular appearance.
The Messier objects
The Bennett objects
The Caldwell list
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