Passive anti-dew system (i.e., requiring no power like a lens/corrector hood) may be able to help but only up to a certain extent (it delays the formation of dew, perhaps for a few hours, but will not completely stop it). As soon as the temperature of the scope drops below that of the dew point, dew will start to form and you will have no choice but to end your observation early. Without an active anti-dew system (heating by using electricity), it is simply impossible to completely eliminate dew. In this article, I will describe how an inexpensive DIY dew heater could be constructed using nichrome wire as the heating element.
This problem may not be very evident on a reflector since its mirror is located at the bottom of the telescope’s optical tube, thus, the tube itself acts as a large anti-dew ‘hood’, unlike in telescopes with an optical element located at the front: a lens in the case of a refractor and a corrector plate for Schmidt-Cassegrain. Without a dew heater, it will only take a few hours before you notice the formation of dew (wiping the lens with a cloth is highly discouraged for the fear of damaging the delicate coatings).
WARNING: I will not be responsible for any damage caused to your equipment. Follow instructions at your own risk!
In this particular DIY, I have decided to use nichrome wire (instead of resistors) as the heating element. After building a resistor-based heater, I believe nichrome wire proved to be a simpler and more practical heating element. Nichrome wire is available in most hardware stores. The one I have used has a diameter of 0.27 mm.
We will also need a flexible plastic strip (you can just cut one from a plastic folder), a shrinkable tube (optional, but it will make the heater professional-looking), some masking/electrical tape, scissors, some Velcro (optional), and a soldering iron.
It is also assumed that you already have a field battery (heaters are power-hungry!), and as an optional accessory should you wish to have a means of controlling the heater’s temperature, you need a power converter with variable output voltage (e.g., 1.5 to 12 V). (Note: I will write a separate DIY post for constructing a field battery.)
When electricity runs through a nichrome wire, it heats up. The shorter the wire, the hotter it gets (careful, it may even burn!). For the purpose of setting a safe temperature level, we need to determine the length of the nichrome wire that will produce a high temperature but still comfortable enough for you to hold (as warm as a typical coffee in a paper cup). This will ensure that even at maximum power, the temperature of the system never gets too high (otherwise, you’re risking burning your telescope!).
WARNING: The next part needs to be done with extreme care. You may burn your hands if you are not careful.
Tape a length of nichrome wire about 4 feet on to a table then connect its two ends to the power supply. As soon as the connection is made, the nichrome wire starts to generate heat. At a length of 4 feet, a 0.27 mm nichrome wire powered by a 12V 2A converter (or even directly via batteries) would not really generate much heat (it would be safe enough to hold). Let it warm up for 2 minutes and then sever the connection. You should be able to feel that it has increased its temperature a bit. Now try to shorten the wire by sliding the connector, perhaps by a few centimeters, then repeat the procedure described above. With each trial, you will notice that it becomes warmer and warmer. Since active heaters only need to raise the temperature of the lens/corrector plate by a couple of degrees, a ‘lukewarm’ or warm-water-hot-coffee-temperature should be more than enough to eliminate dew. Note that we don’t want it too cold either, since without enough heat, it would not be powerful against nights with exceptionally high amounts of dew! Stop when a comfortably warm temperature is achieved. Measure the length of wire. It should determine the minimum length of nichrome wire that we should use. In my setup, I used a 2-feet long nichrome wire.
If you are comfortable dealing with calculations, you could just calculate the resistance in the same manner resistor-based heaters are calculated, and then use the ohmmeter to determine the proper wire length. For instance if you have an 8-inch objective, Sky & Telescope suggests that we deliver 3 watts of power for an 8-inch lens/corrector plate and 1½ watts for a finderscope objective or eyepiece. The required resistance that will generate the desired heat is determined by the following formula:
Resistance needed = The square of the voltage of the power source divided by the amount of power or watts needed.
In symbols, R = V²/P (where R = resistance, V = volts, P = power)
For the 8-inch objective, we need 3 watts (as recommended by S&T). If you have a 12 V power supply, the required resistance would be:
R = 12²/3 (That’s 12 volts squared divided by 3 watts.)
R = 48 ohms (Meaning we need a length of nichrome wire with resistance of 48 ohms if your objective/corrector is 8 inch and your power supply is 12V)
For the eyepiece and finderscope, we need 1½ watts (as recommended by S&T), and you have a 12 V power supply, the required resistance would be:
R = 12²/1½ (That’s 12 volts squared divided by 1½ watts)
R = 96 ohms (Meaning we need a length of nichrome wire with resistance of 96 ohms for your finderscope/eyepiece if your power supply is 12V)
You can then use an ohmmeter to determine the length of wire that would yield the required resistance.
Wrap the nichrome wire around the flexible plastic strip. Make sure that it is evenly-spaced and that no connection is bridged or shorted out. Use a strip of insulated wire (see red wire below) to bring the connection to the other end, then secure everything with an electrical tape. Slide the whole assembly onto the shrinkable rubber tubing (blue) as shown below:
Devise some means of securing/attaching the heater on to the telescope. You may just use tape, but for easy attachment and removal of the heater, I opted to use Velcro in my setup.
I have prepared a few more units which I intend to use to build a more powerful heater in case I’d be needing more heat, since exceptionally clear nights are the ones with the most dew! Each coil must have a length of nichrome wire that is no shorter than the minimum prescribed length.
By sewing a piece of cloth holder, it is possible to create a professional-looking heater. Connect each coil in parallel with each other.
Shown below is my DIY dew heater. It has 4 coils of wire which should be powerful enough to zap away any dew!
I have prepared 2 more coils for my finderscope and my guidescope. These coils are taped directly on to the lens hood.
A total of 7 coils (heating elements) were used in my setup. Four coils on the main imaging scope, 1 coil on the guidescope, and 2 on the finderscope. All these costed me less than 2 US dollars (approximately 100 Php), and only took me a couple of hours to build.
I have been using this setup since November 2011 and have not had problems with dew. Should you have any questions, feel free to leave a comment. Clear skies!
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© Anthony Urbano (Manila, Philippines)