Sky & Telescope - December 2019 - 67

As the years passed, I learned a lot
more about telescopes and observing
than I knew then, but that didn't stop
those early memories from surfacing as
I unpacked the 10-inch GSO Cassegrain. And the best part? It was a fantasy
fulﬁlled - the views through the scope
surpassed what I'm sure I was imagining back in the '60s, even with the nowcritical eye of an experienced observer.

What's Special About a
Classical Cassegrain?
Given the plethora of modern telescope
designs, it's reasonable to ask if an optical conﬁguration from the 17th century
has anything special to offer today. The
short answer is that for many visual
observers and some types of astrophotography it offers a lot. Like a Newtonian, it has only two mirrors so it is
highly efﬁcient at transmitting light,
and it's 100% achromatic, focusing all
wavelengths from ultraviolet through
infrared to a single point. There's no
need to tweak the focus when using
different color ﬁlters, either for visual
observing or astrophotography. There's
also no front corrector plate that can
dew up, and for many of us that's a
huge plus.
The scope's f/12 focal ratio is very
forgiving of eyepiece designs. One stop
on my trip down memory lane involved

digging out a couple of 1963-vintage
Edscorp Orthoscopic eyepieces from
Edmund Scientiﬁc. They performed
beautifully with the scope even though
they can't come close to competing with
modern designs that have larger apparent ﬁelds, which translate into larger
true ﬁelds of view for a given magniﬁcation on any telescope.
It shouldn't be necessary to remind
readers today, but it's still worth
repeating that a telescope's f/number
only affects the brightness of extended
objects (think anything other than
stars) focused onto an imaging detector.
Stars appear the same brightness visually in any 10-inch telescope regardless
of its f/number. And likewise, extended
objects appear the same brightness for
any 10-inch telescope regardless of its f/
number as long as you are viewing with
the same magniﬁcation.

Notes on Using the Scope
The GSO 10-inch classical Cassegrain
is a ﬁrst-class instrument. The optics
proved to be excellent. They were
perfectly collimated out of the box
(Agena AstroProducts says it checks
every telescope optically before it's
shipped.) Images would snap into focus
the moment you hit the sweet spot
when focusing an eyepiece. The scope's
mechanics are well designed and manu-

factured. But there are some things to
consider when it comes to using this
scope. Foremost is its relatively long
focal length of 120 inches (3,048 millimeters), which, for visual use, makes
it a high-magniﬁcation telescope. For
example, I typically do low-power
observing with a Tele Vue 35-mm
Panoptic eyepiece, which on this scope
yields a substantial 87× and a true ﬁeld
about ¾° wide - big enough to show
the entire Moon, but still too small for
some splashy open star clusters and
showpiece nebulae and galaxies. The
maximum ﬁeld possible with a 2-inch
eyepiece (limited by its barrel diameter)
is only 53 arcminutes.
For the other end of the magniﬁcation spectrum, I used a Tele Vue 3.7-mm
Ethos eyepiece yielding 824× and an
8-arcminute ﬁeld. That's beyond the
oft-recommended top magniﬁcation
of 75× per inch of telescope aperture,
and it's a testament to the scope's ﬁne
optics. I resorted to this seemingly
ridiculous magniﬁcation when splitting some of the challenging double
stars listed in Phillip Kane's article on
ﬁnding a telescope's Dawes limit (S&T:
September 2016, p.30). For the record,
the scope easily split SST 359 in Hercules with a separation of 0.75 arcsecond, so I have little doubt that under
good observing conditions it would

q Left: More so than with some optical designs, a Cassegrain's performance is sensitive to optical collimation. The scope was perfectly collimated
out of the box, in part thanks to its rigid secondary-mirror assembly. Right: Well-made light baffles keep extraneous sky light from the scope's focal
plane on axis, but a small annulus of light around the secondary mirror can reach the edge of some 2-inch eyepieces. It did not present a problem
during the author's visual and photographic testing.

sk yandtele scope.com * D EC E M B E R 2 019

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Sky & Telescope - December 2019

Table of Contents for the Digital Edition of Sky & Telescope - December 2019