Sky and Telescope - June 2015 - 67

With a full-frame DSLR camera it delivers a ﬁeld of
view of 3.3° × 2.2° but with a high-resolution image scale
°
of 2.1 arcseconds per pixel, at least with the Canon EOS
5D Mark II camera I used for most of my testing. Using
an image scale in that league, detail in deep-sky objects
is likely to be limited by the seeing conditions at your
site and not by the camera or optics. And yet the ﬁeld of
view is wide enough to encompass most large deep-sky
objects, delivering the best of both worlds: high resolution and wide ﬁeld.
The standout feature of Celestron's astrograph that
attracts the most attention is its sheer speed. At f/2.2
it's possible to shoot deep images in just 3 to 5 minutes,
great for assuring smooth, noise-free images. There is a
price to pay, of course. Indeed, several. There's the actual
cost. At $3,500 the Rowe-Ackermann is a serious ﬁnancial commitment to deep-sky imaging. It is a dedicated
photo instrument; it cannot be used visually.
The Rowe-Ackermann is also quite heavy. It's an
11-inch (28-cm) telescope, with a tube assembly 30.5
inches long that weighs 43 pounds (19.5 kg). That's a
big tube assembly to heft onto a mount. Additionally its
mount must be at least a middleweight model such as
Celestron's CGEM DX or, better yet, their heavier CGE
Pro. Along with suﬃcient counterweights to balance it
all, you're looking at a total system weighing at least 150
pounds (68 kg). Consider that carefully if you don't have
an observatory or other permanent setup.

A Diﬀerent Kind of Astrograph
The Rowe-Ackermann Schmidt Astrograph is a modern reworking of the classic 5.5- and 8-inch Schmidt
cameras Celestron produced in the 1970s and '80s. The
original cameras' f/1.65 to f/1.5 focal ratios made them
extremely attractive though diﬃcult to use. You had to
load individual chips of 35mm ﬁ lm inserted into custom
ﬁlm holders, all completely in the dark. Such was life in
the dark ages of ﬁ lm.
The new Rowe-Ackermann does away with all that
barbaric inconvenience. Optical designer David Rowe,
who also helped develop astrographs for PlaneWave
Instruments, created the initial design. Celestron then
worked with optical designer Mark Ackermann to reﬁne
the ﬁnal conﬁguration.
In addition to its f/2.2 spherical primary mirror,
the design incorporates a Schmidt corrector plate that
eliminates image-blurring spherical aberration. At the
prime focus is an additional four-element lens that corrects ﬁeld curvature, critical for sharp images across the
entire imaging plane.
The design certainly delivers as promised. When
using the Canon DSLR with a 24 × 36-mm detector,
stars were sharply focused pinpoints from the center
out to the extreme corners of the frame. Indeed, I was
impressed with just how tiny the star images were with

Key to the RoweAckermann's optical
performance is
the four-element
ﬁeld-ﬂattener lens
mounted on the front
corrector plate. Its
cell is threaded for
use with either of the
two included camera
adapter plates.
Each conical camera
adapter plate threads
onto the corrector lens
cell and is itself threaded
with either a 42-mm
T-thread or a wider
M48 thread for special
wide T-rings. Screws for
adjusting collimation
are accessible through
the slots, though some
cameras may inhibit their
accessibility.
Users will need to
purchase the proper
T-ring for their particular brand of DSLR
camera. Full-frame
cameras (those with a
24 × 36-mm detector)
should be used with a
wide M48 T-ring recommended to reduce
vignetting as much as
possible.

The body of most DSLR cameras inevitably juts into the light path, causing
diﬀraction spikes on bright stars. Rotating the camera requires careful ﬁnger
work to get at the lock ring on the adapter plate to loosen then retighten it.

Sk yandTelescope.com June 2015

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Sky and Telescope - June 2015

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