Sky and Telescope - June 2015 - 69

A processed image of the California Nebula taken with the
Canon 5D shows the generous 3.3° × 2.2° ﬁeld. Bright stars, like
°
°
Xi Persei at bottom, exhibit some diﬀraction spikes. This is a
stack of ﬁve 5-minute exposures at ISO 400.

While this is easy to ﬁ x later in processing, the ﬂipped
images can be disorienting when you're trying to frame
faint or extended targets. Rotating the camera to frame a
ﬁeld requires swinging the tube down to access the camera. Loosening the large lock ring allows you to rotate
the entire camera and adapter plate assembly. Like most
ﬁnely threaded lock rings, It could occasionally bind up,
i found, risking turning the entire lens assembly on the
corrector plate. Making the adjustment is also unnerving
as your ﬁngers are millimeters away from the corrector
plate. It was easiest to set the camera at a certain angle
for the night and be done with it. That would certainly
have to be the case for any remote imaging operation.
However, the main caveat to keep in mind is that
the camera's position at the top of the tube necessitates
operating the camera from a laptop running control software. You won't be able to look though a DSLR camera's
optical viewﬁnder. Even if you could reach the camera,
your head would get in the way!
As with any Schmidt telescope with its exposed corrector plate, the astrograph is prone to dewing. Plan to
buy or build a dewcap.

Imaging Results
The Rowe-Ackermann's fast focal ratio makes it tempting to shoot loads of short exposures at high ISOs and

Zooming in on the
extreme corner of
a single raw frame
used in the previous
image reveals tiny
star images right
to the edges of the
frame. The RoweAckermann Schmidt
Astrograph design
succeeds in providing
an extremely sharp
starﬁeld across a
36-mm-wide sensor.

not even bother with autoguiding. I did that on one test
night, shooting a couple of dozen 60-second exposures
at ISO 1600 with the mount just freewheeling. It worked
ﬁne. Stacking the results produced a smooth, relatively
noise-free image.
I prefer to shoot deep-sky images at as low an ISO setting as is practical to minimize noise. Fortunately, even
using ISO 400 required sub-frame exposures of no more
than about ﬁve minutes for most targets. Stacking these
produces a ﬁnal image with little perceptible noise.
For exposures longer than a minute, most mounts
will require autoguiding. Considering the investment in
eﬀort it takes to set up 150 pounds of mount, counterweights, and optical tube to get the Rowe-Ackermann
going, attaching a small autoguider (it need only be a
small CCD camera looking through a 50-mm ﬁnderSk yandTelescope.com June 2015

69

http://www.SkyandTelescope.com

Sky and Telescope - June 2015

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

Contents
Sky and Telescope - June 2015 - Cover1
Sky and Telescope - June 2015 - Cover2
Sky and Telescope - June 2015 - 3
Sky and Telescope - June 2015 - Contents
Sky and Telescope - June 2015 - 5
Sky and Telescope - June 2015 - A
Sky and Telescope - June 2015 - B
Sky and Telescope - June 2015 - 6
Sky and Telescope - June 2015 - 7
Sky and Telescope - June 2015 - 8
Sky and Telescope - June 2015 - 9
Sky and Telescope - June 2015 - 10
Sky and Telescope - June 2015 - 11
Sky and Telescope - June 2015 - 12
Sky and Telescope - June 2015 - 13
Sky and Telescope - June 2015 - 14
Sky and Telescope - June 2015 - 15
Sky and Telescope - June 2015 - 16
Sky and Telescope - June 2015 - 17
Sky and Telescope - June 2015 - 18
Sky and Telescope - June 2015 - 19
Sky and Telescope - June 2015 - 20
Sky and Telescope - June 2015 - 21
Sky and Telescope - June 2015 - 22
Sky and Telescope - June 2015 - 23
Sky and Telescope - June 2015 - 24
Sky and Telescope - June 2015 - 25
Sky and Telescope - June 2015 - 26
Sky and Telescope - June 2015 - 27
Sky and Telescope - June 2015 - 28
Sky and Telescope - June 2015 - 29
Sky and Telescope - June 2015 - 30
Sky and Telescope - June 2015 - 31
Sky and Telescope - June 2015 - 32
Sky and Telescope - June 2015 - 33
Sky and Telescope - June 2015 - 34
Sky and Telescope - June 2015 - 35
Sky and Telescope - June 2015 - 36
Sky and Telescope - June 2015 - 37
Sky and Telescope - June 2015 - 38
Sky and Telescope - June 2015 - 39
Sky and Telescope - June 2015 - 40
Sky and Telescope - June 2015 - 41
Sky and Telescope - June 2015 - 42
Sky and Telescope - June 2015 - 43
Sky and Telescope - June 2015 - 44
Sky and Telescope - June 2015 - 45
Sky and Telescope - June 2015 - 46
Sky and Telescope - June 2015 - 47
Sky and Telescope - June 2015 - 48
Sky and Telescope - June 2015 - 49
Sky and Telescope - June 2015 - 50
Sky and Telescope - June 2015 - 51
Sky and Telescope - June 2015 - 52
Sky and Telescope - June 2015 - 53
Sky and Telescope - June 2015 - 54
Sky and Telescope - June 2015 - 55
Sky and Telescope - June 2015 - 56
Sky and Telescope - June 2015 - 57
Sky and Telescope - June 2015 - 58
Sky and Telescope - June 2015 - 59
Sky and Telescope - June 2015 - 60
Sky and Telescope - June 2015 - 61
Sky and Telescope - June 2015 - 62
Sky and Telescope - June 2015 - 63
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Sky and Telescope - June 2015 - 65
Sky and Telescope - June 2015 - 66
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Sky and Telescope - June 2015 - 68
Sky and Telescope - June 2015 - 69
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Sky and Telescope - June 2015 - 71
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Sky and Telescope - June 2015 - 73
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Sky and Telescope - June 2015 - 79
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Sky and Telescope - June 2015 - 86
Sky and Telescope - June 2015 - Cover3
Sky and Telescope - June 2015 - Cover4
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