Sky & Telescope - February 2022 - 56

FEBRUARY 2022 OBSERVING
First Exposure
 Left: This image of Comet NEOWISE gracing the northern horizon is a composite of nine, 40-second exposures captured at ISO 800 with a fullframe
Canon 5D camera and an Askar FMA180 f/4.5 astrographic telescope carried on a Sky-Watcher Star Adventurer Mini tracking mount.
Right: The author captured this portrait of the Southern Cross (Crux) and the dark Coalsack Nebula in a 3-minute exposure made at ISO 1600 with a
Samyang 85-mm lens at f/2.8 and a Canon 1100D camera on a Sky-Watcher Star Adventurer mount. Such a setup is both powerful and portable - a
dream rig for those contemplating a trip to photograph southern treasures.
focal lengths reach 85 mm. And with
telephotos of 135-mm or 200-mm focal
length and small refractors, the margin
for error becomes very small.
No tracker on the market possesses
perfectly meshed gears, motors, or
bearings, so the drive rates vary slightly.
For this reason, more advanced units
usually feature an autoguider port. This
allows the mount to receive drive corrections
from an autoguider - a setup
often consisting of a small telescope,
a specialized camera, and a computer.
The iOptron SkyGuiderTM Pro and the
Sky-Watcher Star Adventurer Pro have
this capability. But if you're just looking
to shoot wide-angle astrophotos, this
capability might be overkill.
A common feature of star trackers
is a choice of drive rates that usually
include sidereal, lunar, and half speed
(0.5×). Most of the time you'll opt
for the sidereal rate for photographing
constellations and the Milky Way.
However, if your target is the Moon,
the lunar rate might give better results
because it compensates for the Moon's
eastward drift relative to the stars. This
would be the speed to select if you plan
on shooting a lunar eclipse sequence,
for example. If you track stars near
the horizon with trees or buildings in
the foreground, the terrestrial features
will blur because they are stationary.
This is the situation that the half-speed
56 FEBRUARY 2022 * SKY & TELESCOPE
rate was designed for. It's a tradeoff
between slightly trailed stars and a
sharper foreground. Your choice of lens
focal length and exposure duration
will determine just how noticeable that
tradeoff is.
Star Tracker Tips
Once you've shot a few frames under
the night sky, you'll quickly discover
that things don't always go according
to plan. Don't be surprised if your
first attempts yield imperfect results.
Here are a few tips I've learned through
experience:
* If your pictures show trailing, the
most likely cause is imprecise polar
alignment. Double-check that
you've performed this correctly, or
that you haven't accidently kicked
your tripod during the night. Also,
make sure that you are set up on
solid ground and your tripod isn't
slowly sinking into the dirt.
* When it comes to tripods, select a
model that's a good match for the
weight of your tracker and camera
gear. An imaging chain is only as
strong as its weakest link!
* All motorized star trackers require
electrical power. The iOptron SkyTracker
Pro has a built-in rechargeable
battery, while the other models
available require AA batteries or can
be powered from an external battery
pack via a USB cable. The latter is
preferred if you're planning a long
imaging session, so that the tracker
doesn't stop working due to batteries.
If you use AA cells, choose heavyduty
alkalines and carry spares in a
warm pocket - batteries quickly die
in cold conditions.
* The gear train of any mount has a
certain amount of slack (termed
backlash). You need to ensure that
this is driven out before you start
taking pictures. One way to do this
is to set your payload so that it's
slightly " east heavy " - the camera
wants to drift eastward when the
drive axis is unlocked. This makes
the drive constantly push the load.
The limits of what you can achieve
with a camera and star tracker are
bounded only by your imagination, the
capabilities of your camera, and the
quality of your skies. If you're already
familiar with the basics of astrophotography,
you can quickly acquire the
additional skills required to operate a
tracker and capture celestial scenes of
outstanding beauty that will wow your
family and friends.
¢ ADE ASHFORD has photographed the
night sky from New Zealand, the United
States, and his native England, where
he currently resides.

Sky & Telescope - February 2022

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