Sky & Telescope - April 2023 - 54

ALL IMAGES COURTESY OF THE AUTHOR
APRIL 2023 OBSERVING
First Exposure by Tony Puerzer
Creating Sharp Astrophotos
A
poorly focused lens can ruin an
otherwise perfect astrophoto. Stars
become bloated blobs (often surrounded
by unsightly color fringes) and fine
details are lost. Imprecise focus also
means longer exposures are required
to record faint objects. And none of
these problems can be easily fixed during
post-processing - that's why it's
important to get everything right under
the night sky.
Your camera's autofocus system
works well in bright daylight but will
struggle to lock in focus in dim conditions.
In fact, some excellent lenses
are fully manual and lack autofocus
capabilities. For best results, you need
to manually focus with the aid of a
feature called Live View. It's included
in most every digital camera sold these
days. This option allows you to zoom
in and examine a highly magnified
image of a bright star or planet on your
camera's rear display. This technique is
54 APRIL 2023 * SKY & TELESCOPE
the only way to guarantee consistent
results at night.
To Infinity . . . and Beyond
To begin, try to get close to infinity
focus by rotating your lens's focus ring
to its most distant setting. If it features
a printed scale, you can use the infinity
symbol (∞) as a handy reference;
however, autofocus models go slightly
past this point. If you have such a lens,
turn the focus ring back a little bit from
its maximum-distance setting - often
there's an index mark to align with the
infinity symbol. This preliminary step
ensures the stars are sharp enough to
show up when you engage Live View.
Now, set your camera to Manual
mode with an exposure time of 30
seconds and set the lens aperture
wide open (the lowest f/stop number).
Increase the ISO setting to 1600 or
more, and turn up the brightness of
your camera's viewscreen. While this
Taking well-focused images of the night sky is easy if you follow these simple tips.
SHARP LAGOON Your time under a clear,
dark sky is always limited, but it's important
to take as long as you need to accurately
focus your camera's lens. This crisply focused,
6-minute exposure of the Lagoon and Trifid
Nebulae (M8 and M20) in Sagittarius was
captured from Mount Kobau, British Columbia,
with an astro-modified Canon 60D DSLR camera
set to ISO 1600, and a 300-mm f/4 lens.
last step will ruin your night vision, it'll
make seeing dim objects much easier.
To use Live View, center a bright star
or planet in the field of view. Gently
nudge the focus ring back and forth,
stopping when the blur circle is smallest.
Recenter your target and increase
the magnification to 5× or 10×. Tweak
the focus ring again slightly until the
object appears as small as possible. You
may notice faint stars suddenly appearing,
just visible through the haze of
sensor noise. This is a good indication
that focus is nearly perfect.
And here's your pro tip for this
Sky & Telescope - April 2023

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