Sky & Telescope - July 2021 - 69

S Left: The CEM70G's 9½-kg counterweight can collide with the right ascension drive housing when balancing light loads; an optional smaller weight
is recommended for such payloads. Right: The built-in iGuider is permanently installed within the saddle. Its objective is recessed about 50 mm inside
the housing to prevent dew formation on the lens. A threaded aluminum cap covers the objective when not in use. Two slightly differing 12-volt DC
power ports are seen to the left of the objective. The iPORT socket at left is used to connect other proprietary iOptron accessories.

side of the saddle plate locks the focus
position. The objective lens is recessed 2
inches within the saddle, protecting it
from most dewing.
The iGuider is controlled using PHD2
guiding software. Both PHD2 and the
iPolar software for PC and Mac computers are available at iOptron's website.
No additional guide cable is necessary
when using the iGuider system, though
iOptron includes an ST-4-style guiding
port when imaging at very long focal
lengths with external guide cameras.
Downloading and installing both
software packages and drivers to my
Windows 10 computer went without a
hitch. Users acquire, focus, and calibrate the iGuider device the same way
that is so familiar with veteran PHD2
users. A tutorial of its use appears in the
December 2017 issue on page 64.
In use, you'll only need to adjust the

focus of the iGuider once and lock it
down. That's a good thing, as it proved
a bit of a challenge to insert and turn
the hex wrench into the hole on the
underside of the saddle. Furthermore,
the fast f/4 optics of the guidescope
makes adjustment very sensitive, and
focus shift would occur when locking
down focus. Success came when I had
one hand on the hex wrench and the
other on the focus knob as I zeroed
in on sharpest focus. Fortunately, the
system held the focus securely, even
after multiple setups and breakdowns
of the mount.
The relatively small aperture of the
iGuider worked best on bright guide
stars, and PHD2 lets you increase the
camera gain to make fainter stars visible, though at the expense of increased
image noise. Creating a good library of
dark frames in PHD2 is key to going

deeper with guide stars and keeping
noise levels low.
I found the iGuider to be extremely
accurate for guiding telescopes with
short focal lengths. Not surprisingly,
iGuider produced round stars on all the
images I took with my 8-inch Celestron
RASA with its 400-mm focal length.
Even without guiding this scope produced very few poorly tracked exposures
on the CEM70G. On other nights, I
obtained a solid set of well-tracked
exposures using a 102-mm refractor
operating at f/7. Of the 30, 120-second exposures I took of M42 with the
refractor at its native focal ratio of f/6.9,
21 of them showed no appreciable star
elongation.
Based on my tests, I could get round
stars in unguided, 2-minute exposures
captured with telescopes with less
than about 500 mm focal length, and
perfectly tracked images of 10 minutes
or more with iGuider operating. Moving
up to my 8-inch f/5 Newtonian also
guided generally well with iGuider in
control, though the weight and length
of the scope together with the slightest
breeze introduced some errors.
Stepping up to an 8-inch f/8 RitcheyChrétien began to show the limits of the
iGuider system. The combination produced slightly elongated stars in about
W A 3-port USB 3.0 hub is located on the rear
of the saddle plate, along with two 12-volt DC
power jacks providing a total of 3 amps of
current.
s k y a n d t e l e s c o p e . o r g * J U LY 2 0 2 1

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Sky & Telescope - July 2021

Table of Contents for the Digital Edition of Sky & Telescope - July 2021