Sky & Telescope - February 2022 - 54

ALL IMAGES COURTESY OF THE AUTHOR
FEBRUARY 2022 OBSERVING
First Exposure by Ade Ashford
Choosing
and Using a
Star Tracker
A basic and (relatively)
inexpensive accessory can
take your astrophotography
to the next level.
T
he awe-inspiring sight of a starfilled
rural night sky with the
misty glow of the Milky Way arching
overhead is something that you might
wish to photograph and share. While a
tripod-mounted camera and standard
lens can record simple constellation
portraits in a few tens of seconds, the
stars will be rendered as unnatural little
streaks and the Milky Way will look
washed out and dim. But by mounting
your camera on a device known
as a star tracker, you can create rich,
detailed pictures of the Milky Way with
pinpoint stars - and so much more!
Star Tracker Basics
If you're wondering if and when a star
tracker is necessary, all you have to do is
consider the so-called 500 Rule. It states
that for sharp stars, the longest exposure
you can make with a stationary,
full-frame camera is 500 divided by the
focal length of the lens. In other words,
if you want to avoid stars turning into
little streaks, a camera fitted with a
24-mm lens is limited to exposures of
around 21 seconds (500/24 = 20.8).
With a 14-mm lens you're limited to
around 36 seconds - good enough to
record the brightest parts of the Milky
Way. If you have a crop-sensor camera
model, you'll have to divide your results
by your camera's crop factor. For Nikon
54 FEBRUARY 2022 * SKY & TELESCOPE
The author captured this photo showing the heart of the Milky Way with the constellations Scorpius
and Sagittarius in a 3-minute exposure at ISO 800 using a 14-mm f/2.8 Samyang lens and a
Canon 1100D camera on a Sky-Watcher Star Adventurer star-tracking mount. Despite the use of
a wide-angle lens, stars in an exposure of this duration would show up as little streaks without the
use of a motorized star tracker.
DSLRs, that's 1.5, meaning you're limited
to 14-second exposures (20.8/1.5)
with a 24-mm lens.
Even allowing for the incredible sensitivity
of modern cameras, most 20- to
30-second exposures will look dim and
noisy simply because you're not capturing
enough light. A fast (f/2 or f/2.8)
lens helps, but satisfactory Milky Way
shots typically require a minute or two
of exposure or more. And the only way
to get exposures that long while retaining
pinpoint stars is with a motorized
tracking mount.
A star tracker not only enables long
exposures, it also permits the use of longer-focal-length
lenses to capture some
of the larger deep-sky objects or comet
close-ups. And because most camera
lenses deliver improved optical performance
when stopped down to f/4 or
f/5.6, a tracker allows for longer exposures
to compensate for the reduced
light reaching your camera's sensor.
How a Star Tracker Works
Stars trail in long-exposure photographs
because Earth is rotating west to east,

Sky & Telescope - February 2022

Table of Contents for the Digital Edition of Sky & Telescope - February 2022