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NGC 6864
Bennett 124
Messier 75, GCL-116
RA 20:06:04
Dec -21°55.3'
Globular
cluster 
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The globular cluster near the border of
Sagittarius and Capricorn was probably first seen by Mechain in August 1780 and
was confirmed by Messier within two months, describing at as “nebula
without stars... possibly composed of very small stars and containing
nebulosity.”
William Herschel, in 1784, described it as “a
miniature of M3 and pale to the gaze.”
In the Philosophical Transactions, 1818,
William Herschel wrote: “1799, 7 feet finder. It is but just visible. 1799,
7 feet telescope. There is not the least appearance of its consisting of stars,
but it resembles other clusters of this kind, when they are seen with low ..
magnifying powers. 1810, 10 feet telescope. With 71 it is small and cometic.
1784, 1785, 20 feet Newtonian. Easily resolvable, some of the stars are visible.
1810, 20 feet, front view. It is a globular cluster. 1799, 1810, large 10 feet.
Its diameter with 171 power is 1' 48 seconds; with 220 it is 2'.”
Admiral Smythe described the cluster as “a
lucid white mass among some glimpse stars” and thought Messier's claim of
suspected resolution was distinctly “bold”. Burnham notes, however,
that there are four or five 12-14th magnitude stars in the field, surrounding
the cluster in a neat semi-circle on the N, E and S; the brightest star of this
arc is 1.5' out from the cluster on the SE side. Very possibly these stars were
glimpsed by Messier and gave the impression of partial resolution.
Walter Scott Houston calls it a “hazy
spot about 2' in diameter” in his 4” Clark, and says that an excellent
night is needed to detect any trace of stars. K.G. Jones, observing with an
8-inch at 120x noted only slight mottling at the edge of the cluster, whereas
Houston, using a 10-inch reflector could begin to resolve only the outermost
part.
Jack Bennett, observing with a 5”
Moonwatch telescope at 21x, classified this globular as C1, which means the
cluster is bright and well seen, but is angularly small, almost stellar and can
easily be missed when sweeping.
Burnham calls it a small but very rich
cluster, and notes that it is one of the more compact globulars. It is claimed
by some observers to be the equal of the better known M80 in Scorpius, which was
listed by Herschel as the most compressed cluster he had ever observed.
William P. Clarke (San Diego, California, USA)
writes in the The Webb Society Nebulae and Clusters Section Report No. 10, July
1992: “Bright and concentrated towards the centre. Only partial resolution
achieved. (21-inch f/20, x140, x350).”
Steve Coe, observing with a 13” f/5.6,
notes: “(M 75) Bright, pretty large, round, much, much brighter in the
middle at 165X. This globular has a blazing core and is very mottled, but I
could not resolve a star in it on a 7/10 night, even with powers up to 270X.”
Tom Lorenzin: “8M; 3' diameter;
pearl-like! small and beautiful with much brighter center; appears as core of a
face-on spiral galaxy! not resolved.”
8-inch Meade, 18mm eyepiece, 36.2’
fov: Bright, medium size, round compressed globular cluster with a
bright core. Faint stars just want to become visible, with bright stars just
outside the cluster in the field. [MS]
8-inch Newtonian, 66x: “Like
a point; small; from the inner point outwards the brightness falls abruptly.”
(suburban skies) [GG] |
