Sky & Telescope - December 2019 - 68

S&T Test Report
t Far left: The mounting
base for the focuser has
tip/tilt adjustments to aid
with precision collimation
and squaring equipment
to the scope's optical
axis. A trio of small fans
powered by a supplied
battery holder for eight AA
batteries helps acclimate
the primary mirror to ambient air temperature. Left:
Heavy-duty, tight-fitting
plastic dust covers for the
primary and secondary
mirrors protect the optics
when the scope is stored.

split doubles at its Dawes limit of 0.46
arcsecond even though poor seeing and
clouds foiled my attempts to prove that
by observing 51 Aquarii.
The basic scope weighs 38 pounds
(17 kg), and a ﬁnder, star diagonal, and
eyepiece add a couple more. As such, it
needs a solid mount, and I found it a
good match for my 12-year-old AstroPhysics Mach 1 German equatorial. But
here's the rub. For last January's lunar
eclipse I put that mount on a temporary
aluminum pier made from 5-inch electrical conduit. It was ﬁne for the small
scopes used for the eclipse but totally
inadequate for the 10-inch Cassegrain.
High-magniﬁcation views jiggled like a
bowl of Jell-O. I had to switch the Mach

1 to a permanent pier made from a
scavenged steel streetlight pole 8 inches
in diameter. It simply drives home the
point that you need a very solid support to enjoy using this scope at high
magniﬁcation.
There are two Losmandy-style
dovetail mounting bars on the GSO
scope. Equipped for typical observing or
photography, the scope's balance point
falls near the center of these dovetails,
but be aware that the bars are only 10
inches long. When the scope was balanced in the long saddle of my Mach
1 mount, neither of the saddle's two
clamps would engage the scope's dovetail bar. This might not be a problem for
short saddles or ones with three or more
clamps, but I had to mount my own
long dovetail bar to the scope to make it
work with my setup.
The scope's focus falls a generous 9½
inches (about 240 mm) outside the rear
cell. The tip/tilt adjustable mounting
base for the focuser consumes 1½ inches
of this back focus, and the focuser
another 3½ inches, but that still leaves
plenty of space for a complex camera
setup with a ﬁlter wheel and off-axis
guider. The scope comes with three
extension tubes (a pair 1 inch long and
one 2 inches long), giving you a huge
amount of ﬂexibility in positioning
the focuser the right distance from the
rear cell for various visual and imaging
setups. The downside is that the focuser
t The first quarter Moon nearly fills the long
dimension of a full-frame DSLR thanks to the
scope's relatively long, 3,048-mm focal length.

D EC E M B E R 2 019 * SK Y & TELESCOPE

has only 2 inches of travel. This was
adequate for most of my eyepieces and
a star diagonal without using extension
tubes. But adding a Barlow lens often
meant having to add one of the extensions - a bit of an inconvenience.
For much of my observing the dualspeed focuser was ﬁne. But some of my
heavier eyepieces required putting a fair
amount of tension on the focuser's drive
shaft to keep it from slipping, and that
made the ﬁne-focus knob feel spongy
and even slip at times. It wasn't a showstopper, but for critical high-magniﬁcation observing with heavy eyepieces
it would be nice to have the precision
control that a quality rack-and-pinion
focuser offers. The same goes for imaging with heavy camera setups.

Photographic Performance
Speaking of imaging, this classical
Cassegrain is superb for lunar and
planetary work, but for wide-ﬁeld
deep-sky photography, it has limitations that go beyond its relatively slow
f/number. Even at the edge of the
40-arcminute-wide ﬁeld captured by a
full-frame DSLR camera, stars showed
the noticeable effects of a Cassegrain's
inherent coma. There are more reasons
than just faster f/numbers that make
modern optical designs preferred by
today's deep-sky photographers working with large-format cameras. Even
so, as the images here prove, for some
of the smaller and brighter deep-sky
objects, especially open and globular
star clusters, the 10-inch GSO Cassegrain does a great job. Had I shown up at

Sky & Telescope - December 2019

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