Sky & Telescope - December 2023 - 55

gets thicker, dim stars will disappear
and the overall image will become hazy,
as though clouds were passing overhead.
And it can get worse. If it's cold enough,
you could end up with a layer of frost
on your optics similar to what you
would find on a car windshield.
To fix this, don't wipe (or scrape!)
your lens. Instead, you have to gradually
bring the temperature of your equipment
back above the dew point. But
don't make the common mistake of
simply taking everything indoors - this
only makes matters worse. The warm,
moist air inside will instantly condense
on all the cold surfaces of your camera
and lens. A better approach is to seal
your camera in a plastic bag while it's
still outside. This way when you bring it
indoors, the moisture condenses on the
plastic bag instead of your gear. Once
everything has warmed up a bit you can
remove your camera from the bag and
let it dry out completely.
As with many things, an ounce of
prevention is worth a pound of cure.
Using a lens hood lessens the rate at
which your lens radiates away heat,
which will in turn slow the process of
dew formation. These accessories are
critical on humid evenings. If your lens
didn't come with one, you can purchase
an aftermarket version or even make
one yourself out of waterproof material.
Unfortunately, lens hoods don't
completely prevent dewing - usually
they just buy you a bit more time. To
keep your lens from fogging up it needs
to maintain a temperature slightly
higher than the dew point. The simplest
way to do this is to attach one of those
little chemical hand-warmer packets to
the underside of your lens with an elastic
band. A better (though more costly)
solution is an electrically powered dew
heater. Some run off 12-volt DC power,
while others use a USB connection.
Typical devices consist of two parts:
a controller plus one or more heating
bands that strap (usually with Velcro)
around the lens.
Keeping Chill
Cold weather can also affect the focus
of your lens. As the temperature drops,
WIRELESS WONDER The screen of an iPad dwarfs the rear display of this Canon 80D camera.
Using Live View on a larger display makes obtaining perfect focus far easier, and because the connection
is wireless, the iPad can be placed inside a warm car or house.
sk yand tele scope .o r g * DECEMBER 2023 55
FUZZY M42 Moist winter air and cold optics
don't get along. Moisture condenses on any
surface (including your lens) when temperatures
fall below the dew point. The first signs
can be quite subtle, with faint halos appearing
around bright stars. As dew continues to form,
dim stars won't be recorded, and the overall
image becomes fuzzy and low-contrast. The
effects of dewing are present in this two-minute
exposure of the Orion Nebula, M42.
the metal and plastic components in
the lens barrel will contract slightly,
throwing stars out of focus. My advice
is to check your photos regularly during
a winter imaging session. Just be careful
not to breathe on your camera or
lens when tweaking focus - the warm
moist air you exhale will instantly condense
on the glass.
Camera batteries are impacted by
cold conditions as well. It's worth keeping
a spare one (or two) warm in your
pocket or indoors while your camera is
clicking away. Better yet, you can get a
" dummy battery " adapter that allows
you to power your camera directly from
either an AC outlet or a portable power
bank. These are essential accessories if
you're planning to shoot a long, timelapse
sequence, a star-trail photo, or
meteors. The adapters sold by camera
manufacturers can be quite expensive,
but you can usually find a low-cost
alternative from a third-party source.
Some of the newest mirrorless cameras
even allow you to power them via a
USB-C cable. Check your manual to
see if this option is supported on your
particular model.
Cold weather isn't all bad, however
- digital cameras actually produce
better images at lower temperatures. As
imaging sensors heat up, thermal noise
increases, creating a colorful, confettilike
pattern of speckles in long-exposure
images. High-end, dedicated astro
cameras have electronic coolers built
in to minimize this effect, but you can

https://skyandtelescope.org/

Sky & Telescope - December 2023

Table of Contents for the Digital Edition of Sky & Telescope - December 2023