50 ASTRONOMY • FEBRUARY 2022
advertise rated temperatures
that are much lower than the
real temperature at which you
might feel comfortable. If you
take a bag out at its actual
rated temperature, you might
find its insulation inadequate.
This is why veteran campers
will tell you to add 15 degrees
Fahrenheit (8 degrees Celsius)
to a bag’s rated temperature.
That will give you the real
temperature at which you’ll
start to feel cold in the bag.
Some sensitive souls may want
to add 20 F (11 C).
The worst way to use a
sleeping bag is to place it
directly on the ground. Even a
slight bit of insulation will cut
down the amount of heat loss
between your body and
Mother Earth. I start with a
lightweight, plastic, queen-
size, fold-out platform that
lifts the bag about 18 inches
(46 centimeters) above the
ground. On the platform, I
put a battery-powered, self-
inf lating air mattress. That
raises the bag another foot or
so. These two steps make a
big difference in comfort, but
even a few layers of folded
blankets between the mattress
and the ground can help.
One item you should
always have is a Mylar blanket.
Often billed as survival blan-
kets, that’s exactly what it may
become. Wrapped around you,
Mylar will trap body heat,
allowing you to survive all but
the most severe conditions. As
a cover for your sleeping bag,
it will keep you warmer, pro-
vide a shield against wind
penetration, and keep frost off
your bag. These blankets are
also waterproof and weigh
next to nothing. You can find
Mylar blankets at any quality
camping or outdoor store, or
many places online.
What to expect
from the sky
OK, you’re all bundled up,
but have you chosen a good
night? The smart amateur
astronomer is usually also an
amateur meteorologist. As
you check the weather the
day before and day of your
observing trip, one of the
most important details to
look or listen for is informa-
tion about air masses.
A cold air mass (air colder
than the ground) is likely to
produce convection, which
will probably result in
cumulus clouds. The
atmosphere in such
a region is generallyclean, so transparency will be
good. But because the air is
turbulent, the seeing (atmo-
spheric steadiness) will suffer.
In this case, you can expect
to see celestial objects to the
limit of your vision, but
they’ll be twinkling.
On the other hand, a
warm air mass (warmer than
the ground) is likely to create
stratus clouds, haze, or mist.
The atmosphere in this region
probably will hold a lot of
dust. This makes the
air steadier but less
transparent. So,the transparency will not be
great, but the seeing might be.
You also can usually count
on bad seeing for at least
24 hours after a front (the
boundary between warm
and cool air) or a trough
(a stretched-out area of low
pressure) passes through your
area. And believe it or not,
seeing is often best when ultra-
thin cirrus clouds are over-
head. The exception occurs
when those clouds are moving
in one direction and there are
lower-level cross winds.Acclimating
your telescope
One of the biggest threats to
high-quality winter observing
is tube currents in your tele-
scope. While the seeing might
be good, the image through
your scope may not be. This is
often because the temperature
of air in the tube is different
than the ambient temperature
outside. But how can you tell
whether it’s the sky or your
scope causing you trouble?
Aim at a fairly bright star
and defocus the image. This
will allow you to detect airWinter nights offer some of the year’s clearest skies — and coldest
temperatures. Gloves, hats, and boots are a must, with several layers
underneath for added warmth. ALAN DYERThe Witch Head Nebula
(IC 2118), located near the
bright star Rigel, is one of
winter’s most stunning
deep-sky targets for
astrophotographers. But
you’ll need to make sure
your equipment is
temperature acclimated
and dew free to get the
best shot. GIUSEPPE DONATIELLO