Sky & Telescope - April 2023 QR - 56

APRIL 2023 OBSERVING
First Exposure
devices. Some third-party applications,
such as Cascable (iOS only) support
multiple camera brands. Check your
manual to see whether your camera
is equipped with Wi-Fi and can take
advantage of this handy feature.
Most digital cameras can connect
to a laptop computer using a USB cable
(also known as tethering). This allows
you to use your computer's generous
screen for focusing. Many manufacturers
offer software specifically for
this task. For example, Canon supplies
a free application called EOS Utility,
which allows you to display the camera's
Live View feed on your computer
screen. The software also lets you adjust
autofocus lenses remotely, eliminating
the need to touch your camera and
introduce vibration.
The Bat Signal
Judging precise focus is much easier
with a simple accessory called a Bahtinov
mask, which attaches to the front
of your lens. The mask is a thin, plastic
disk that has a series of slots cut into it
designed to produce a specific pattern of
diffraction spikes around bright stars.
By observing the position of the overlapping
spikes, it's easy to determine the
point of perfect focus.
The mask works extremely well with
telephoto lenses but isn't very effective
SUMMER SWAN Taking the time to properly focus the lens yielded pinpoint stars in this 2-minute,
tracked exposure at ISO 3200 showing the Milky Way around the constellations Cygnus and
Lyra. The author used his Canon 70D DSLR with a 22-mm lens at f/4.5.
with wide-angle models owing to the
tiny size of the resulting diffraction
patterns. Bahtinov masks are available
in various sizes from commercial
sources, and you can even make your
own if you're so inclined. They can be
3D-printed or simply cut from a sheet
of black plastic. If you search online for
" Bahtinov mask template " you'll find
several options.
Finally, achieving sharp focus is one
thing, but maintaining it throughout
an entire imaging session is another. As
your lens cools during the night, thermal
contraction may cause the focus
point to shift slightly. Also, some inexpensive
lenses exhibit focus creep arising
from mechanical components shifting
slightly when the camera is pointed
skyward. Even aiming at a different part
of the sky runs the risk of introducing
focus creep. That's why it's important to
recheck focus occasionally throughout
an imaging session.
As with so many aspects of nightsky
photography, producing attractive
images with pinpoint stars requires
attention to detail. Don't be discouraged
if you initially find yourself spending a
lot of time focusing and less time shooting.
The process will become faster and
easier over time. And the reward is an
image you'll be proud to call your own.
¢ TONY PUERZER is a retired professional
photographer focusing on leading
a simple life on Vancouver Island in
British Columbia, Canada.
MASKING FOR FOCUS Left: The author uses this Bahtinov mask from Farpoint Astro with his 300-mm telephoto lens. The mask installs much like
a standard lens filter and allows for precise focusing. Right: This pair of images illustrates how to focus a lens fitted with a Bahtinov mask. When the
central diffraction spike is positioned exactly halfway between the two diagonal spikes (far right image), the lens is perfectly focused.
56 APRIL 2023 * SKY & TELESCOPE

Sky & Telescope - April 2023 QR

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