Sky and Telescope - September 2018 - 59

chains of stars at right angles to each
other. At lower magnifications (about
50× for my 18-inch), the chains blur
into seemingly solid lines. Knowing
where to stare, I can follow them in
my 10-inch, too.
The lines, approximately 5′ to 6′ in
length and not equally obvious, meet
at a pinpoint vertex - the cluster's
11.4-magnitude lucida. The stronger
line comprises 12th- and 13th-magnitude stars slanting southeastward from
the vertex, roughly in the direction of
NGC 6946. The fainter line is delineated by 13th- and 14th-magnitude
stars trending northeastward across

the cluster's moderate central condensation. If I double the power, the
many extremely dim stars in that mild
concentration tend to break up the
straight-line illusion.
Remember my kite asterism? The
9.9-magnitude star marking the kite's
east corner is accompanied by an easily
spotted 13.5-magnitude companion
20″ eastward. High-power scrutiny of
the 10.5-magnitude star at the top of
the kite shows it harboring a 14.6-magnitude attendant 16″ east, plus a wee
dot 14″ north. Indeed, I count eight or
nine 13th- to 15th-mag stars within a
90″-wide area encircling the 10.5-mag-

nitude star - a teensy cluster aside
"gigantic" NGC 6939.

"Arming" the Galaxy
It was on a perfect September night in
2011 that I truly got to know NGC 6946.
The backwards S effect was sublime at
78× in a friend's 20-inch f/4 Dobsonian
(effectively f/4.6 using a Tele Vue Paracorr coma corrector). Upping the power
to 137× revealed the most prominent
arm and its lesser inboard companion
curling counterclockwise from the
northeast end. At the southwest end, a
fan-shaped arm angled sharply northward. South of center, a stubby fila-

EN

NGC 6939

"Anonymous"

t PINWHEEL AND KITE The impact of
PGC 64824

HD 196085
HD 195607

NGC 6939, a loose open cluster, and NGC
6946, a face-on spiral galaxy, is quite different in photographs than in telescopes. Unlike
the impression given here, the cluster is easier
to see in the eyepiece than the low surface
brightness galaxy. Tracing the pinwheel shape
of NGC 6946 in large optics is a challenging
but rewarding exercise. Wide-field scopes can
trace the delightful "kite" asterism that captures NGC 6939. The field of view is approximately 60′ × 60′.

tt PALE COMPLEXION The pallid face of the
spiral galaxy NGC 6946 is enlivened by densely
packed star clusters and immense H II regions.
The nebulous knots labeled 1 and 2 are visible
in mid-size reflectors and correspond to the
same knots in S&T: July 2013, p. 60.

NGC 6946
HD 196053

p SEEING THINGS In the open cluster
NGC 6939, two linear chains of stars form
a right angle whose vertex is marked by the
cluster's 11.4-magnitude lucida. The chains can
blur into solid lines at low magnification. Note
the galaxy PGC 166193 (marked with an arrow)
a little more than 3′′ northeast of the topmost
star of the kite. The author has not been able to
detect this minute mist, likely because of interference from an overlaying 16th-magnitude star.
sk yandtelescope.com * S E P TE M B E R 2 018

59


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Table of Contents for the Digital Edition of Sky and Telescope - September 2018

Contents
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