As well as making a big lens hood from blue flexible foam, you can
also follow these tips.
(from http://www.skypub.com/tips/basics/dew.html )
Dealing With Dew
By Alan MacRobert
The most common equipment hassle that observers face at night is
water on the telescope, which comes as a surprise to newcomers who
expect things to stay dry in clear weather. Unfortunately, the steadiest,
sharpest telescopic views are often had under precisely the
atmospheric conditions that cause dew to form. At the eyepiece you
first notice dim stars and galaxies becoming harder to see, then bright
stars develop fuzzy halos -- and a check with the flashlight reveals wet
haze coating the optics. In severe cases the whole telescope may be
soaked. Wiping never helps; more water condenses the moment you
stop. At this point many observers pack up, defeated.
However, you can keep your lenses and mirrors crystal clear in even
the heaviest dewing conditions. You just need to understand the
enemy and take effective countermeasures.
Dew does not "fall" from the sky. It condenses from the surrounding air
onto any object that's colder than the air's dew point. The dew point,
often mentioned in weather broadcasts, depends on both temperature
and humidity. When the humidity is 100 percent, the dew point is the
same as the air temperature. At lower humidity, the dew point is below
the air temperature. If it's below freezing, you get frost instead of liquid
water.
An example of dew physics occurs when you take a bottle out of the
refrigerator. If the bottle is colder than the air's dew point, it drips with
condensation. Your telescope is the bottle.
"But my telescope can't get colder than the air!" a new Schmidt-
Cassegrain owner once told me. "It was warmer than the air when I
brought it outdoors. The Second Law of Thermodynamics says that
can't happen!"
If only life were so simple. Objects do try to come to the same
temperature as their environment and then stay there, as the Second
Law says. But they don't exchange heat just with the air around them.
They also exchange heat with objects at a distance by radiation.
That's why the Sun can feel warm on your skin even though it's 93
million miles away. At night the heat flow goes in the opposite
direction. The effective temperature of the dark night sky is just a few
degrees above absolute zero, and a telescope in an open field is
exposed to a whole celestial hemisphere of this cosmic chill.
The first line of defense against dew, therefore, is to shield your optics
from as much exposure to the night sky as is feasible. The traditional
dewcap extending beyond a refractor's lens often serves this purpose
well enough to keep the lens dry. The longer the dewcap, the more
likely it is to work. One of the nice things about a Newtonian reflector
is that its entire tube acts as a dewcap to shield the mirror in the
bottom. An open-tube reflector, however, needs a cloth shroud around
its open framework to gain this benefit. The cloth itself, of course, will
get wet on its sky-facing side.
The worst dew problems appear on exposed parts that are thin (or
have low heat capacity) and rapidly radiate away their warmth.
Schmidt-Cassegrain corrector plates are notorious for dewing; so are
Telrad sights with their exposed glass. A dew shield is reportedly the
first accessory that Schmidt-Cassegrain owners most often come back
to buy.
You can easily make your own. A piece of tough 5/8-inch foam rubber,
the kind sold in sporting-goods stores to go under sleeping bags,
makes a dew shield that's cheap, durable, and very lightweight. The
foam is an excellent insulator, for maximum effectiveness. If you're
concerned that the cap might vignette the image (block some starlight
near the edges of the field of view), you can cut the foam so it flares
open at a very slight angle. A 3° opening angle should allow a 3°
unvignetted field of view.
As a rule of thumb, a dewcap should be at least 1½ times as long as
the aperture is wide. A side benefit is that the cap also cuts down on
stray light getting into the telescope.
Eyepieces too are prone to dewing. Warm radiation from your face
slows the process, but humidity from your eyeball and breath speeds it
up. A tall rubber eyecup, the kind that extends above the eye lens all
around, not only blocks stray light while you're observing but acts as a
miniature dewcap when you're looking away.
The same principle works on large scales. Early on a clear morning,
have you noticed grass in the middle of a field white with frost or dew
while grass near a tree has none? The tree is a giant dewcap, and it
can work for you too. If you'll be looking at only one part of the sky, it's
nice to have trees around and behind you. Not just your telescope but
your charts and accessories will stay dry longer.
Trees also reduce wind problems, but a slight breeze is a good thing.
Radiational cooling is slow and inefficient compared to heat transfer
with the surrounding air, so even the mildest breeze will keep your
telescope nearly up to air temperature.
Then there's the observing umbrella, not a widely known accessory
but one that works. A beach umbrella blocks the chill of absolute zero
the same way it blocks the heat of the Sun. It can help shield all your
gear and keep the chill off you too. On a still night a thermometer
under an umbrella can read more than 10° Fahrenheit higher than
when it is exposed to the open sky.
The Heat is On
There will be times and places where none of this is enough. You then
have no choice but to warm your optics, usually electrically.
A 120-volt hair dryer, used gently from a distance so it doesn't
overheat the glass and warp it, will blow off dew for perhaps five
minutes. Then you have to use it again. And again. A 12-volt auto
windshield defogger gun is somewhat less effective. A better way is to
apply a little heat continuously. Heated dewcaps that run off a 12-volt
battery are advertised in this magazine (see "The Kendrick Dew
Remover System" for a review of one such system). Or with just a little
electrical know-how you can make an antidew heater to any size,
shape, and specification you want. Here are the details.
Warmed optics can have unexpected benefits. Dew works its first
subtle evils before you notice anything. The late Walter Scott Houston
used electric warmers on both the objective and eyepiece holder of his
4-inch refractor. When he turned off the power, the telescope could
lose a whole magnitude of light grasp before the objective actually
looked dewy.
"Even on nights when dewing is not noticeable," Houston wrote, "the
star images seem better with the heaters on than without them!" This
may be because, contrary to what you might think, gentle heating
keeps a telescope close to the temperature of the surrounding air.
After all, the whole idea is to stop it from growing colder than the air.
Not-So-Cold Storage
The most destructive dewing happens when a telescope is in storage.
No telescope should be closed up and put away until it is thoroughly
dry. Water with nowhere to escape, or condensation that forms and
evaporates repeatedly in a sealed environment over months and
years, may attack optical coatings and ultimately etch the glass itself.
How, you may ask, does water get into an airtight space that was dry
when you sealed it? The answer is it was there all along. Air contains
water vapor, and if your telescope gets colder than what the dew point
was when the air was sealed in, water will condense. This is why so
many puzzled telescope owners discover water stains on the inside
surfaces of their corrector plates and refractor lenses.
Several approaches can prevent this. Don't move a sealed telescope
from warm to cold storage. In fact, sealing may be a bad idea
altogether. The best telescope covering is cloth, which will "breathe." It
keeps dust off but lets water vapor out. And you might want to leave
the eyepiece holder covered only with cloth, just enough to keep dust
and spiders out.
The worst problems occur when a warm front of humid air blows in
after cold weather, as often happens in early spring. Everything cold
gets drenched. A cloth wrap may be the best defense here too; it will
greatly reduce the amount of humid air that can flow over cold parts.
The usual advice is to store a telescope at the outdoor temperature to
minimize tube currents when you set it up. But this old rule may need
modification. Keeping the telescope a little warmer will tend to thwart
condensation. An enclosed porch or attached garage may provide the
extra few degrees you need. And really long-term storage should
probably be inside your living space. Never leave a telescope in a
damp basement or garage or, as a rule of thumb, any place where
tools grow rusty.
You can take active countermeasures too. A 4- or 7-watt light bulb
inserted into a blanketed telescope makes a nice low-power heater.
Position it just below or right next to the glass, or else you may merely
drive off water from other parts of the tube that will condense onto the
cold optics. Running the bulb continuously will cost about a dollar per
watt per year. You might turn it on only in the damp season, or attach
it to a humidistat switch.
Silica gel desiccant will dehumidify the air in a tightly sealed enclosure.
I keep a ¾-pound bag in plastic webbing attached to the inside of one
of the tube caps of my 12.5-inch reflector. Every month or two when
the bag's indicator slip turns from blue to pink, I heat the bag in a
toaster oven in my observatory to drive off the collected moisture. The
more tightly you seal your tube or storage case, the less often you'll
have to do this. Silica gel is available from many sources. I got mine
from Hydrosorbent Products (25 School St., Ashley Falls, MA 01222;
call 413-229-2967 for a price sheet).
Water can be an insidious enemy for astronomers, but a little
knowledge will keep it permanently at bay.
Alan MacRobert is an Associate Editor of Sky & Telescope magazine
and an avid backyard astronomer.
------- Quidquid latine dictum sit, altum viditur -----------------
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