Sky & Telescope - June 2024 - 70

S&T Test Report
Polar alignment with the StarSense
and a Celestron NexStar hand control
is an easy process. First, select Polar
Align in the menu. The scope then
slews to several locations around the
sky (not directly towards the pole) and
in a few moments displays how far
your alignment is off in azimuth and
altitude. As you adjust the azimuth
knob on the mount base, the device
continues imaging the sky and updating
the numbers until you're within
2 arcminutes of the celestial pole.
Repeat the process for altitude, then
you're done.
13-mm eyepiece in the diagonal, and
hit the Enter button. The slew stopped
with the scope pointing in the correct
direction. But the hand control
indicated we weren't done yet - it was
plate-solving to improve accuracy. In
10 seconds, the mount moved slightly
a few times, and when it finished, M15
was dead-center at 110×. No doubt I
could have inserted a higher-power
eyepiece, and the story would have been
the same. While I've used plate-solving
for polar alignment before, I hadn't
used it as a Go To aid. For me, the accuracy
was frankly uncanny.
On night two, I planned to test the
StarSense with the company's free Windows-only
Celestron PlaneWave Instruments
Telescope Control Software (CPWI).
This is a full-featured program that displays
a planetarium sky chart indicating
the telescope's position and interfaces
with many of Celestron's accessories,
including the StarSense Autoguider.
There were no surprises when I set up
After polar alignment is complete,
it's time to begin the automatic Go To
alignment. All that's necessary is to
activate the AutoAlign feature and the
Starsense does the rest. After completion,
there is an option to further refine
its centering of targets.
the camera to work with the software.
With CPWI connected to the telescope
via a USB-C cable, clicking Alignment
near the top of the screen offered a
menu with the choice StarSense Alignment.
Selecting that option resulted in
the software asking - as the hand control
had - whether I wanted to perform
a polar alignment. The mount hadn't
been moved, but I chose Yes anyway.
The polar-alignment routine with
the CPWI software is similar to that
with the hand control but with one big
difference. Instead of numbers on the
hand control's dim red display, I could
watch the software's graphic altitude
and azimuth indicators showing the
current alignment errors.
In every other respect, the CPWI
Top: Activating the autoguiding
feature in the hand controller simply
requires selecting SSAG Guiding and
hitting the Enter button. The device
automatically chooses multiple guide
stars and begins its work without additional
user input. Bottom: After each
guiding exposure, the hand control
displays the movement of the guide
star measured in arcseconds. The Q
value indicates the guiding quality.
program functioned just like the hand
control. When I finished the automatic
alignment, it enquired as to whether I
would like to do a Center Calibration.
When using the software, it bypasses
the hand control, and calibration values
are stored on my computer. Out of curiosity,
I selected No. Despite skipping the
Center Calibration routine, anything I
requested was centered in the eyepiece
after each slew was completed.
70 JUNE 2024 * SKY & TELESCOPE
I wondered, though, what would
happen if the plate-solve couldn't be
completed satisfactorily due to a passing
cloud or some other obstruction? I
sent the telescope to Jupiter, which was
low in the east and partially obscured by
tree branches. When the scope slewed to
the planet, it stopped, started its platesolve,
and then CPWI reported " Failed
to plate-solve for the target. " But that
wasn't the end of the story. The program
then announced it was falling back
to the pointing model that the hand
control contained. After completing this
slew, Jupiter wasn't quite centered but
easily in the ½° field with the same good
accuracy as the original StarSense.
Autoguiding Performance
Before calling it a night, I wanted to get
an idea of how autoguiding worked even
though I hadn't attached a camera to
the scope. Activating Guiding Enabled
in the software immediately opened a
guiding error graph. The plot showed
an average error of between 1 and 2
arcseconds on this night of poor seeing,
which was consistent with the performance
with my own Advanced VX
mount under similar seeing conditions.
However, what was surprising was that
I had simply activated the guiding - the
device had already picked a guide star
and immediately went to work.
I didn't have to do any of the things
typical when starting other autoguiding
systems. There were no movement
calibrations or any backlash settings to
adjust. You can set the guide rate for
each axis, though its default settings
worked as expected. With CPWI you can
also have the computer move the scope a
small amount between each exposure, a
technique known as dithering that helps
to reduce noise in the stacked result.
Dithering is not an option available in
the hand controller menus.
The following evening was clear,
so I attempted a rather stringent test
while imaging through my 8-inch
SCT. The seeing hadn't improved since
the previous night, so I used a focal
reducer to bring the focal length of
the scope down to 1,400 mm from its
native 2,000 mm. Even so, this meant
Sky & Telescope - June 2024

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