Sky and Telescope - July 2017 - 59
TOP: POSS-I / N ATION A L G EOG R A PHIC SOCIE T Y / PA LO M A R OBSERVATORY / CA LTECH; BOT TO M: DENNIS DI CICCO
cluster would be hopeless as it's dimmed
by nearly two magnitudes of dust.
Globular cluster NGC 6304 may
take the cake for the most interesting
shape it conjures up in my mind. The
central portion of its 8′ extent features
an oval collection of stars containing a
straight line of ﬁve brighter stars crossing it about 1/3 of the way down. To me
this resembles an acorn in the sky with
the line of stars deﬁning the top of the
acorn from its main body.
The next cluster has no central
concentration at all. NGC 6539 has a
7′ extent that can only elicit the classic
description of diamond dust on black
velvet. Three magnitudes of interstellar dust enshroud this cluster, making
it an unresolvable target for all but the
largest amateur telescopes. Interestingly, NGC 6539 lies at almost the same
distance and is of the same absolute
magnitude as M92, a common target
for the backyard enthusiast. The fact
that it's virtually unknown to many
highlights the difﬁculty that dust introduces to visual observations.
NGC 6287 is a well-resolved 5′ cluster. The arrangement of brighter stars on
the globular's periphery led me to calling
this the "Eiffel Tower" cluster. The top
star of the tower is to the southwest,
and the heart of the cluster occupies
DEEPER LOOK Image-intensiﬁed eyepieces are ideal for viewing star clusters.
Although your view will be tinted, color
photography falsely boosts the screen's
green hue in this image.
the "observation deck." In contrast to
this circular cluster, NGC 6401 forms
a distinct arrowhead shape composed
of similar magnitude stars in a 2′ array.
Over two magnitudes of obscuration and
a distance approaching 35,000 lightyears make these stars faint but still
observable with my equipment.
As far as distance is concerned, NGC
6426 is the hands-down winner in my
tabulation. It's a faint, fully resolved
loose collection of stars covering a section of sky approximately 4′ or 5′ across.
I observed these clusters from numeric
lists with no preconceived notions of
what I would see. I was quite surprised
to ﬁnd out later that this cluster was
67,000 light-years away with a magnitude of dust-dimming to boot.
I was even more amazed to see NGC
6256 as a faint but well-resolved cluster
of 7′ diameter. It's very loose, and my
notes at the eyepiece are "class X-XI?"
(the Shapley-Sawyer classiﬁcation class,
categorizing this globular as loose and
homogenous at the center). This cluster
is so obscure that it's neither described
in any of my reference books nor listed
with any of the professional classiﬁcations that I use to compare my own
Our ﬁnal target, NGC 6749, would
be an average cluster for amateurs if
not for the whopping 4½ magnitudes of
dimming by interstellar dust. I'm able
to see a patch of light approximately
4′ in diameter with about half a dozen
stars making a faint appearance. These
stars are barely brighter than 17th
In an article published in 1900,
George W. Ritchey wrote, "The visual
limit . . . of the 40-inch [great refractor
at Yerkes] is at about 16.5 to 17 magnitude." He was talking about globular
clusters. This comparison provides a
measure of aperture increase that I'm
achieving with the electronic eyepiece
coupled to my 10-inch reﬂector.
I've been able to make these observations for two simple reasons. A globular
star cluster is the ideal target for night
vision technology like that originally
produced by Collins. The brightest stars
of a globular are swollen red giants with
p WELL RESOLVED The author estimates
the resolution of globulars viewed through
assistive technology matches that using the
red channel of the Digitized Sky Survey with
a 15′ × 15′ field of view.
peak luminosities in the near-infrared
at a wavelength that exactly corresponds to the maximum sensitivity of
the eyepiece. This longer wavelength,
invisible to the human eye, has a second
beneﬁt especially germane to assisted
observing of globulars. As wavelength
increases, interstellar dust is less able to
hinder its travel. This means the electronic eyepiece is seeing more starlight
exactly at the sweet spot of its sensitivity. The result is a ﬁeld of pinpoint stars
of varying magnitude with the brightest
stars of somewhat larger diameter.
My particular eyepiece is dated and
no longer commercially available, but
Night Vision Devices sells a wide range
of monoculars with speciﬁcations that
meet or exceed the Collins design. These
devices are useful for a wide range
of deep-sky objects, not just globular
clusters. You may be able to ﬁnd imageintensiﬁed eyepieces on the second-hand
market as well. I'm convinced this technology is under-utilized by amateurs and
deserves serious consideration, especially
at light-polluted observing sites.
I'm amazed by the number of clusters that I've been able to resolve and
the cluster diameters that I've recorded.
The most satisfying feature of all is the
fact that every cluster is different and
visually enthralling. I'll never grow tired
of viewing them.
¢ ED MIHELICH has resolved 80 globular clusters from mid-northern latitudes.
s k y a n d t e l e s c o p e .c o m
* J U LY 2 017