Sky and Telescope - July 2017 - 58

JULY 2017 OBSERVING
Going Deep

GREEN SCREEN The amplifying element in the author's image-intensiﬁed eyepiece is
the microchannel plate, which is perforated with millions of tunnels with coated walls.
When light strikes the intensiﬁer's photocathode, the emitted electrons enter the perforations, ricochet off the tunnel walls, and create more electrons. The electrons pass
through the tunnels, hit a phosphor screen, and create an enhanced image.

my Collins I3 image-intensiﬁed eyepiece
not long after it was reviewed in this
publication (S&T: February 1999, p. 63).
After many years of looking at the
brighter globulars with truly remarkable
results, I decided to pursue an observing program of fainter globular clusters
that are largely ignored by most amateurs. Here, I describe a sampling of my
observations using the I3 eyepiece ﬁrmly
seated in the focuser of my 10-inch
Ritchey-Chrétien Cassegrain reﬂector. I

chose to make my observations "blind."
That is, I entered every cluster into my
Go-To mount and described the view
through the eyepiece. The true ﬁeldof-view of my setup is 25′, and my site
conditions are relatively good. I observe
from a site above 7,000 feet in elevation
with a naked-eye limiting magnitude of
6.0. Good to excellent transparency is
key to resolving these targets.
NGC 6626 (Messier 28), relatively
big and bright, is obscured by a little

more than one magnitude of dust. That's
enough dimming to cause some amateurs to bypass this gem as they head
over to magniﬁcent M22. I ﬁnd the
cluster fully resolved with a diameter of
8′. The nucleus consists of a 1′ crowding
of ﬁve large, closely packed stars surrounded by a heavily populated core of
stars extending out to a 5′ diameter. As
with most globulars, the outliers of the
cluster are more thinly distributed, but
here the pattern is a bit more irregular
than most. The three levels of star density make this a worthy target that I've
enjoyed on many nights.
Not far away is the gorgeous cluster
NGC 6624. I call it a "starburst" globular
in that it has a very intense 1′ nucleus
with the rest of the stars radiating out
in all directions to a total extent of 5′.
If you've ever had a well-resolved view
of M80 you'll probably recognize this
description. NGC 6624 stands out well
from the surrounding star ﬁeld that adds
to the attractiveness of the scene.
Our next target is big and beautiful
NGC 6553. The total extent is 8′ with
innumerable pinpoint stars deﬁning the
highly circular cluster. A multitude of
bright individuals deﬁne a 4′ core that
suggests to my mind the outline of a ﬁvepointed star. Without electronic ampliﬁcation resolution, the resolution of this

Object
NGC 6626

Class

Mag(v)

Size

Dec

Dimming

Distance (l-y)

6.8

11′

18h 24.5m

-24° 52′

1.2

17,900

23.7m

-30° 22′

0.84

25,800

NGC 6624

7.9

5.9′

18h

NGC 6553

8.1

8.1′

18h 09.3m

-25° 55′

1.9

19,600

14.5m

-29° 28′

1.6

19,200

NGC 6304

8.2

8.0′

17h

NGC 6539

9.3

7.9′

18h 04.8m

-07° 35′

25,400

05.2m

-22° 42′

1.8

30,300

NGC 6287

VII

9.4

5.1′

17h

NGC 6401

VIII

9.5

5.6′

17h 38.6m

-23° 55′

2.2

34,600

44.9m

+03° 10′

1.1

67,100

NGC 6426

4.2′

17h

NGC 6256

11.3

6.6′

16h 59.5m

-37° 07′

3.1

27,400

6.3′

19h

+01° 54′

4.5

25,800

NGC 6749

12.4

05.3m

Class information is from Strong & Sinnott, Sky Atlas 2000.0 Companion. Angular sizes are from recent catalogs. Visually, an object's size is often smaller than the
cataloged value and varies according to the aperture and magnification of the viewing instrument. Right ascension and declination are for equinox 2000.0.

J U L Y 2 0 1 7 * SK Y & TELESCOPE

G REGG DINDER M A N / S&T ARCHIVES

Dust-obscured Globular Clusters

Sky and Telescope - July 2017

Table of Contents for the Digital Edition of Sky and Telescope - July 2017