SCT Dew Heater

Homemade C8 Dew Heater and Dew Shield

Nichrome Wire Dew Heater and Dew Shield Projects

This webpage documents construction of an 8" SCT nichrome wire dew heater and C8 and C90 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.

Safety

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 equipments performance.

Dew Heater Principle

On 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 of both. A passive solution is to block the radiative heat transfer by wrapping a dew 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 heat from radiating to the open sky in the same way that parking next to a house on a cold night prevents frost on the car window facing 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 dew 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 dew heater, directing the supplied heat inward to the telescope lens.

Nichrome Wire

Nichrome wire 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 length gives 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 = Hotter!

For this project I used 30 gauge Nicrome wire (several meters cost about $5.00).

Several very important things to remember:

Never connect Nichrome wire directly to house current (the AC current from your wall outlets). This is dangerous and could result in electrocution!
Only connect Nichrome wire to a low voltage, low current DC source
Short lengths of Nichrome wire can become very hot and there is the possibility of burning yourself
Anything powered by house current and used outdoors must be connected via a ground-fault interrupt (GFI) circuit to avoid injury

Calculating Power Loss and Resistance

An 8" SCT dew heater should 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 = voltage 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).

Dew Heater Construction

I insulated 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 equal 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 into a 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 three photos.

Dew Shield Construction

The dew shield is a section of foam used as an 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 dew heater and a similar C90 dew shield.

Project Status

The dew shield and dew heater functioned extremely well. They kept my C8 and C90 free of frost during an entire observing session at -10 deg. C.