Sky and Telescope - June 2015 - 68

S&T Test Report

The back cell includes the dual-speed Feather Touch focuser,
fan vents for cooling the tube (and battery pack input), and two
knobs for locking down the mirror once it is focused.

the Schmidt Astrograph, a function of its large 11-inch
aperture yet relatively short focal length.
As such, precise focus is critical. The Astrograph
comes with a custom Starlight Instruments 10:1 Feather
Touch focuser that moves the primary mirror. The
mechanism produced image shift of only a few arcseconds as I racked the focus in and out. The focusing
movement is extremely ﬁne, important for such a fast
instrument with a shallow depth of ﬁeld.
The Astrograph comes with a small battery pack for
eight AA batteries that plugs into a jack on the back of
the instrument. This powers an internal fan that draws
in outside air through ﬁltered vents. This helps the tube
cool down to ambient air temperature.
The tube is ﬁtted with dovetail plates on both top
and bottom using the Losmandy D-System standard.

An unprocessed raw image recorded with a Canon EOS 5D Mark
II full-frame DSLR displays the level of vignetting introduced
mostly by shadowing from the camera adapters and DSLR's mirror box. Note that the image is also ﬂipped mirror-image.

June 2015 sky & telescope

These make it easy to attach the tube to most heavy-duty
mounts, as well as a guidescope to the top of the tube
using accessories from Celestron or third-party suppliers. The top of the tube also has two pairs of screw holes
for attaching a bracket for a ﬁnderscope.
Also included with the Rowe-Ackermann are two
camera adapter plates: one threaded to accept standard
42-mm T-rings, and the other with a wider 48-mm
thread for special wide-aperture T-rings. The T-rings
(not supplied) connect your brand of DSLR camera to the
Celestron adapter plates.
A smaller 42-mm T-ring (the most common type
everyone sells, including Celestron) will work ﬁne with
APS-format DSLRs with sensors 23 × 15 mm across. But
to minimize vignetting with full-frame DSLRs, it's best
to use a 48-mm T-ring. It is an essential piece of the system that can be hard to ﬁnd. One was not included with
the Astrograph, nor does Celestron oﬀer one as an accessory. I ordered mine from Sky-Watcher USA. They oﬀer
"M48" T-rings for Canon, Nikon, and Sony cameras.
Options for shooting through a ﬁlter using a DSLR
are limited. Doing so requires removing the ﬂat optical
window in front of the ﬁeld ﬂattener and replacing it
with Celestron's 72-mm light pollution rejection ﬁlter
($499) made for the Rowe-Ackermann.
The included adapter plates are designed to connect
a DSLR camera. Imagers with CCD cameras will need
to devise extension rings to place the sensor 55 mm
from the adapter plate ﬂange, matching the distance to a
DSLR sensor that the ﬁeld ﬂattener lens is optimized for.

Shooting at Prime Focus
Whatever camera you choose, it sits at the prime focus,
at the front of the instrument. This introduces a few
caveats. For one, most DSLR camera bodies extend
beyond the 4.5-inch-wide corrector lens and cell, intruding into the light path. This introduces asymmetric
diﬀraction spikes around brighter stars.
Any CCD camera used with the Astrograph should
ideally be lightweight and symmetrical, and no wider
than 4.5 inches (11 cm). One-shot color cameras like
Celestron's own Nightscape units would work well.
Monochrome cameras with a ﬁ lter wheel jutting out of
the side of the camera will intrude into the light path.
Second, the presence of the camera extending several
inches from the front of the instrument makes it impossible to use any manual focusing aids. In addition, the
glowing LCD panel and lights on the back of a DSLR
will illuminate any screen or white panel used to take
ﬂat-ﬁeld frames, ruining these important calibration
frames. Any lights on your camera need to be turned oﬀ
or covered when imaging.
Another issue is that because the camera is looking
at an image that has undergone just one reﬂection (oﬀ
the primary mirror) all photos will be mirror-reversed.

Sky and Telescope - June 2015

Table of Contents for the Digital Edition of Sky and Telescope - June 2015