WWW.ASTRONOMY.COM 51currents in your telescope by
looking for circular motions
inside the image. If your out-
of-focus star has “hair” or a
“ring of fire” around the outer
edges, and if these patterns
change subtly over a period
of 30 seconds to five minutes,
you have image-damaging
tube currents. But note that
you may not always be able to
see these effects. To deal with
the problem, your goal is to get
your mirror to the same tem-
perature as the outside air. The
easiest solution is to wait an
hour or so; but if you’re impa-
tient or on a timetable, you can
use a small, battery-powered
fan to blow the warmer air out
of the tube.
How to prevent dew
The second problem you
might face while out observing
is dew, those small droplets of
water that condense on cool
items at night. I happen to be
writing this from the desert
environment of Tucson, where
dew is rare. But I’ve lived and
observed in many other loca-
tions, so I know that dew on
a telescope objective can be
a session ender.
Dew forms on optics when
the temperature of your tele-
scope falls below the dew
point (the temperature at
which atmospheric water
vapor will condense on an
object). This happens because
when the weather is clear,
your telescope essentially
radiates its heat to the sky,
which is at a colder tempera-
ture than the atmosphere
around it. If the air tempera-
ture is near the dew point, the
heat loss will cool your scope
and dew will form on it. And
if it’s really cold, frost can
form on your optics.
Of all telescope designs,
the one most affected by dew
is the Schmidt-Cassegrain.
This type of scope has a large
glass corrector plate at the
front end of the telescope
tube, and that glass plate can
easily promote dew formation
once it gets cold enough. But
refractors and ref lectors are
certainly not immune, nor are
eyepieces and finder scopes.
To prevent dew from forming,
you have to bring theABOVE: In addition to the well-known Orion Nebula (M42), M78 should also be on
your winter observing bucket list. This bright reflection nebula belongs to the same
star-forming complex of gas and dust as its more famous neighbor. STEPHEN RAHN
LEFT: The Orion Dew Zapper system is just one of many options you can use to
prevent dew from forming on your telescope — or clear it off once those pesky
droplets have already condensed. ORION TELESCOPES & BINOCULARStelescope back to a higher
temperature than the dew
point. This is somewhat of a
balancing act, though. You
don’t want to overheat the
optics because then thermal
currents (see earlier) will
degrade your observations.
If you notice dew forming
on your scope, you can
deal with it in a
number of ways.
But there’s one
way you absolutely
should not deal
with it: Never wipe
the dew off with any
type of cloth. The danger
of scratching your lens or
mirror is too great.
In the field, you can turn
on your car’s heater and move
the tube inside to warm it up
just a little. Leave the optics
uncovered so the dew can
evaporate. You also can use
a cordless hair dryer or one
that plugs into your car’s
power outlet. Set it at the low-
est air volume and run it for
only a few seconds. Make sure
to keep it several inches away
from your equipment.
Those who often observe
in winter usually buy a dew-
removal system, which installs
on the telescope. Several
are available, such as the
Kendrick Dew Removal System(www.kendrickastro.com)
and the Orion Dew Zapper
(www.telescope.com).
Finally, when you’re ready
to pack up, beware of conden-
sation forming on items after
you transfer them from the
cold outside to a warm inside
location, be it your car or
house. Observers have
told me that they put
small objects (cam-
eras, eyepieces, etc.)
in zip-close plastic
bags before bringing
them inside. If you do
this, any condensation
will form on the outside of the
bag and not on the delicate
equipment inside.The key to
winter observing
Comfort is everything —
especially in winter. If you’re
uncomfortable, you won’t
concentrate as well and, worse
yet, you may cut your session
short. Having the right gear
with you will assure high-
quality observations of the
Orion Nebula (M42), Sirius B,
and lots more.Michael E. Bakich is
a contributing editor of
Astronomy who lives in Tucson
and no longer suffers through
subzero observing sessions.