Sky & Telescope - January 2023 - 65

impressive load capacity of 22 kg without
using the supplied 5-kg counterweight,
and 28 kg with the counterweight.
There was a sense of the Mark III's
rigidity even as I lifted it from its shipping
box. It's intuitive that a German
equatorial mount has two moving axes,
but there was no hint that anything
about the Mark III was movable. The
reason being that, rather than the
worm-gear drives typically used in
today's telescope mounts, the Mark
III's axes are driven with strain-wave
gears, often called harmonic drives.
(Harmonic Drive is actually a registered
brand name that's worked its way into
the world of geek speak the same way
that Band-Aid has become synonymous
with any adhesive bandage.)
Strain-wave gears are noted for their
zero backlash and extremely high torque
(the force that turns a shaft) - both
features that are very desirable for a telescope
drive. Instead of wasting a lot of
words explaining how strain-wave gears
work, it's better that I point you to the
internet where there are scores of videos.
One I especially like is on the Harmonic
Drive website (harmonicdrive.net). Click
on the Technology pulldown menu at
the top of the page and select HarmonicDrive®.
There you'll find nice descriptions,
diagrams, and a very cool video.
There is a price to pay, however,
for the advantages of the strain-wave
drives in the Mark III mount. One is
the relatively slow default slewing speed
of 2° per second (it can be increased
to a maximum of 4°
per second, but only
for lightweight, compact
telescopes). There
are also no clutches or
manual controls of any
kind - the only way you
can move the mount is
to have it powered up
and running. Lastly,
there are some important
considerations
for imaging, but more
about this later.
The Mark III's hand control
includes the mount's power
input port (lower left) as
well as the USB 2.0 input
(lower right) for controlling
the mount with planetarium
and autoguiding software
using an external computer.
Because it lacks a number
keypad, data entry is done
with an unusual scrolling procedure
described in the text.
Initial Setup
The Mark III ships
fully assembled, and if
you order the optional tripod ($499)
with the mount you need only set the
equatorial head on it and tighten two
bolts. Nevertheless, there are a few
things that should be done first indoors
with good lighting. One is setting the
altitude adjustment bar for your latitude
range. The setscrews that secure this bar
(as well as its mating hex wrench) are
extremely small and would be easily lost
if dropped on a grassy surface. There are
also very small 10-pin connectors for
the hand control and a cable that goes
between the right-ascension and declination
housings. While both connectors
are keyed and can only be inserted
one way into their respective sockets,
it takes a bit of gentle finessing to get
them properly seated.
While on the topic of cables, the
ones supplied for the
hand control and various
power supplies are
rather short, all being
well less than a meter
in length. Finding
extension cables for the
power supplies is easy, but certainly not
so for the 10-pin cable on the hand control.
I should also point out that there
is no guiding port on the equatorial
head. All computer connections to the
mount have to be made via either an
ad hoc Wi-Fi connection or a USB 2.0
cable that plugs into the hand control.
I tested both and had no problems
operating the Mark III as an ASCOM
mount with Software Bisque's TheSkyX
planetarium program and Stark Labs's
inspired PHD2 autoguiding software,
even with both running simultaneously
over the single USB cable.
The optional carbon-fiber tripod
weighs barely 5 kg and is notably rigid.
It stands 81 cm tall, but the legs are not
adjustable beyond the threaded feet,
which can extend up to about 25 mm
Small things matter. Several aspects of the Mark III's initial setup are best done indoors with adequate lighting. The tiny setscrews that secure the
latitude adjusting bar (left) could be easily lost if dropped in the grass. The small 10-pin connector for the hand control (center) required a bit of gentle
finessing to insure it was properly seated in its socket on the mount. The internal battery holder for the hand control's clock (right) has a small on/off
switch, which is easily seen in this photo but which the author missed when first setting up the mount.
sk yand tele scope .o r g * JANUARY 2023 65

http://www.harmonicdrive.net

Sky & Telescope - January 2023

Table of Contents for the Digital Edition of Sky & Telescope - January 2023