documents construction of an
heater and C8
foam dew shields.
The dew heater and dew
shield were constructed so they can be used individually or together as
a single unit. Total project cost was about $15 and both projects were
completed in a single afternoon.
Electrical modifications and construction of electrical accessories can
be dangerous. Do not attempt such activities unless you are qualified
to do them
safely, without injury to yourself or your property. Anyone
not qualified for these activities should secure the help of a
licensed electrician. It is always recommended to consult with the
appropriate manufacturer prior to modification of any equipment, as
this may void warranties and/or alter the
clear nights, objects
will loose heat by radiating
electromagnetic waves (radiative
heat loss) to the open sky
and become colder than the
surrounding air. When a telescope lens cools to the dew
point, small drops of water will condense out
of the air and form dew
on the lens. As the lens becomes covered with dew
or frost, observing
becomes difficult or impossible. There are several solutions to
eliminate dew: shielding, heating, or a combination
A passive solution is to block the radiative heat transfer by wrapping
shield around the telescope objective lens.
The dew shield is just a
cylinder that fits around the lens and shields it from the open sky.
A dew shield blocks
from radiating to the open sky in the same way that parking
to a house on a cold night prevents frost on the car window
the house; the house blocks heat radiation (losses) to the sky and
slows the windshield cooling. Dew shields are simple
solutions that can reduce dewing, but
not completely eliminate dew problems-they only slow the heat loss. Dew
heaters work by gently
heating the telescope lens to replace the heat that is lost to the open
sky. A dew heater will keep the telescope lens temperature above the
dew point, but is not intended to drastically warm the telescope; too
much heating would cause optical problems affecting image
quality. Dew shields and dew heaters can be used together, the
heater supplies heat to keep the lens above the dew point and the dew
shield blocks radiative heat losses. The dew shield also insulates the
directing the supplied heat inward to the telescope lens.
is a nickel chromium resistance wire used
in heating elements such as: electric toasters, hair dryers,
electric ovens, heating blankets, etc. Nichrome wire becomes hot when
an electric current is passed through the wire. The relation between
voltage, current, and resistance is given by Ohm's Law: E = i x R (E =
voltage, i = current, R = resistance). A long length of Nichrome wire
will have more resistance than a short length. If the Nichrome wire is
connected to a constant voltage, then Ohm's Law gives that the current
will vary inversely proportional to the resistance (wire length): short
high current and long length gives low current. So as we cut shorter
sections of Nichrome wire and apply the same voltage, we get more
current and the wire becomes hotter: Shorter Wire = Greater Current =
For this project I used 30 gauge Nicrome wire
(several meters cost about $5.00).
Several very important things to remember:
directly to house current (the AC current from your wall
outlets). This is dangerous and could result in electrocution!
Nichrome wire to a low voltage, low current DC source
lengths of Nichrome wire can become very hot and there is the
possibility of burning yourself
powered by house current and used outdoors must be connected via a
ground-fault interrupt (GFI) circuit to avoid injury
Loss and Resistance
An 8" SCT dew
supply 3-5 W of power. The power loss through a resistor (in this case
nichrome wire) is given by P = E2/R (P = power
loss, E =
and R = resistance). Since I want to supply 5 W of power and will be
using a 12 V power source, the only unknown quantity is the resistance,
but this can be calculated. Rearranging the power loss equation gives R
= E2/P and substituting the known quantities (E
and P) gives R = (12
volts)2/ 5W = 28.8 ohms resistance. This means
that a 12 V power source
connected to 28.8 ohms resistance will give 5 W power losses.
Using a multimeter, I determined that 2 meters of 30 gauge nichrome
wire gave 29 ohms resistance (below photo).
a 2 meter
length of nichrome wire by inserting it in clear heat shrink
tubing (below left photo) and heat shrinking the tubing around the
nichrome wire. I cut a length of aluminum tape
to my SCT circumference and placed it on my workbench with the adhesive
side up. I pressed the 2 meter insulated nichrome wire into the
aluminum tape adhesive side, and tried to distribute it as equally as
possible along the tape (below center and right photos). The below
center photo also shows
the green foam floor underlay used for the dew shield.
A second piece of aluminum
tape sealed the assembly
together and a strip of Velcro was added to form the strip
ring. I attached a low current variable voltage DC power source and
insulated the soldered connections with black heat shrink tubing. The
completed dew heater is shown in the below left photo. The aluminum
tape eventually tore near the Velcro strip. After several repairs, I
decided to replace the aluminum tape with ordinary duct tape (below
right photo). The duct tape works just as well as the aluminum tape and
is considerably stronger.
The dew shield is a section of foam used
underlay for laminate flooring. Another option would be the foam
camping pads that are placed under sleeping bags, but these are much
heavier than the foam laminate underlay. I wrapped the foam around my
C8 and secured it with heavy tape. I left the bottom several inches
untaped and added a piece of Velcro; this allows the dew shield to be
opened and placed over the dew heater, then tightened with the Velcro.
The below photos shows the completed C8 dew shield installed over the
heater. I fabricated a similar C90 dew shield.
The dew shield and dew heater have functioned extremely well.
The dew heater alone works so well that a dew shield is
not necessary. This simple homemade dew heater has kept my
C8 free of frost during entire
observing sessions at -10 deg. C.