Sky & Telescope - April 2023 - 66

S&T Test Report
control. This makes accurate balance of
the attached gear difficult.
Still, I determined the balance point
on each of my OTA setups by placing
them on a pencil on a flat surface. I
would then slide the mounting bar and
its balance point to the middle of the
AM5 saddle.
An optional 23-cm steel counterweight
shaft is recommended for larger
payloads, though at this time ZWO
doesn't offer any counterweights. However,
you can use any counterweight
with a central hole large enough to fit
over the 20-mm diameter shaft.
I used my own 5-kg counterweight
with my heavier telescopes, including
a Celestron RASA 8 and an Astro-Tech
8-inch Ritchey-Chrétien (RC). To test
stability, I set up the AM5 with the RC
both with and without a counterweight
- there was no change in tracking or
guiding performance. The strain-wave
drives didn't seem to care.
To really give the mount a workout,
I loaded up my Celestron 11-inch SCT
with its imaging accessories (seen on the
facing page). Despite approaching the
mount's recommended weight capacity,
it still tracked quite well, though it was
very sensitive to wind, and the single
counterweight was inadequate to properly
counterbalance the telescope. ZWO
doesn't recommend using any more than
a single 5-kg counterweight.
In practice, loads of around 10 kg or
more without counterweights caused
the tripod to flex, particularly when
pointing near the meridian. And a flexing
tripod can lead to changes in polar
alignment, which could affect imaging
results. The mount's base includes a
central, 3
axes gives the mount a solid feel, with
no backlash on either axis whatsoever.
That same gear system produces some
periodic error - a recurring inconsistency
in tracking speed that makes
active guiding essential.
I measured the periodic error (PE)
on the AM5 to be about 20 arcseconds,
which matched the graph provided by
the manufacturer with the mount. This
considerable PE was easy for all of my
autoguiders to correct. Regardless of
which telescopes I used, autoguiding
with guidescopes or an off-axis guider
consistently averaged about 1 arcsecond
in total error - smaller than what
would show in my images.
Nearly every unguided exposure
/8-inch threaded hole in the bottom
- an industry standard that allows
the AM5 to connect to a wide range of
heavy-duty tripods. Flexure wasn't a
problem when the AM5 was mounted on
a permanent pier in my yard.
Periodic Error
The AM5's strain-wave gearing on both
up to one minute with my 8-inch
Celestron Rowe-Ackermann SchmidtAstrograph
and its 400-mm focal
length showed round stars. Anything
longer required additional autoguiding.
Unconventional Control
The AM5 hand controller is unusual, to
say the least. It lacks a keypad, screen,
or readout panel of any kind - it's just
a small, palm-sized device with a pair of
buttons and a thumb-operated joystick.
Accessing the target database is the job
Above: The ASI Mount control app displays a realistic view of the Milky Way, with good detail as you zoom closer in. Above left: The app provides
a customizable field-of-view indicator to help users frame their targets, like this example showing M31 framed and oriented for the author's
ASI2600MC-PRO camera and 8-inch RC telescope.
66 APRIL 2023 * SKY & TELESCOPE

Sky & Telescope - April 2023

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