Home Made Adjustable Dew Heater

by Nick Zivanovic

INTRODUCTION

After putting up with ending several great observing sessions early because of the inevitable dew problems, I decided some active dew suppression was in order. I researched several different home-brew and commercial solutions and decided to build my own.

The Kendrick and Orion systems looked attractive, but expensive. Most of the home-brew systems involved gluing the resistive elements to the corrector of the scope, a route I did not want to take.! decided to design a system that would give me the best of both worlds: a neat, finished-looking end-product similar to the commercial ones, yet inexpensive to build and easy to use. My goal was to produce a unit that would not have a tacky homemade look but would complement the appearance of my scope (we all know how important that is!).

This project was completed by my wife and me in about 4 hours.! started by calculating the desired heat output in waifs at the maximum voltage setting on my battery pack and calculated the total resistance necessary at that level.! use the "GreatLand Power Source," a rechargeable lead- acid battery pack which is available at Target stores in the camping section. You can use whatever 12v source you want but I recommend this unit for it's selectable outputs.

The plans detailed below are for a 8" SCT. To adapt to other-sized scopes, you will need to vary the length of the cloth tube holding the resistors and possibly change the resistor configuration to get the desired wattage (details below).

The resistor configuration ended up being somewhat complex since I was limited to getting the materials from Radio Shack, and the exact desired resistance and wattage values were not available there. The local electronics supply company where I had gotten these type of items in the past decided to change their store hours and be closed on Saturdays so ! had to make due with supplies from Radio Shack instead. If you have a local electronic component supplier I suggest you go there. It will be easier to assemble the unit with the proper components and they will probably be much cheaper than RS.

This article is written assuming no electronics knowledge so there are some sections where theory is explained. You can skip these if you know how to compute resistances in parallel, power dissipation and the like. !f not, I've tried to make them as simple as possible.

Read through the whole article before starting assembly. There are some optional steps you may or may not want to do.

RESISTOR THEORY

(Skip if you already have a knowledge of electronics)

A resistor is a device whose purpose is to restrict the flow of electrical current in a circuit. The level of this restriction is called resistance and is measured in a unit called the ohm, designated by the capital Greek letter omega. when a resistor does its job, the "friction" of reducing the current produces heat. We measure heat in the unit of power, watts.

The other variables which determine the amount of heat produced are voltage and current, measured in volts and amperes (amps) respectively. Voltage is the unit of electromotive force. Current is the amount of electrons which pass though a given point in a circuit during a certain amount of time. You can use water as an analogy to understand these relationships. A larger pipe can carry more gallons per minute than a smaller pipe (this is current). A larger pump can push more water through a given pipe than a smaller pump (this is voltage). A corroded pipe will restrict the flow of water through it limiting the amount of current (this is resistance.)

The relationship of voltage, current, and resistance is defined by Ohm's Law: E = I x R, where E is voltage in volts, I is current in amps, and R is resistance in ohms. If a circuit is driven at 1 volt, and has 1 ohm of resistance, 1 amp of current will flow (E/R = I, 1/1=1).

The relationship of power to voltage and resistance is given by P= E²/R, where P is power in watts, E is voltage in volts, and R is resistance in ohms. In the above example circuit, P would equal l watt (1²/1). In a example case, say you wanted to generate 15 watts of heat and you had a 12v source. To calculate the total amount of resistance needed, a little algebra will show you that R=E²/P. In this example 12²/15 =9.6 ohms. How much current is flowing through this circuit? I=E/R=12/9.6 or 1.25 amps. The current is important because 1) there are safe current limits on power sources such as batteries, and 2) battery life is measured in amp-hours (AH). A battery rated at 4 AH will power this circuit for 3.2 hours.

Enough theory, now let's get practical. The Greatland Power Source (GPS) has a switch selectable output of 12, 9,6, or 3 volts nominal. The actual voltages are not regulated and so are dependant on battery condition. My unit measured about 14, 11, 7.5, and 4.5 at the respective settings when fully charged but dropped closer to the nominal after some use. I used the averages of the fully charged condition and the partially depleted condition in my calculations, about 13, 10, 7, 3.5 volts at the different switch settings. I wanted around 15 watts at maximum setting so I settled on 12 ohms of resistance (13x13/12=14.08 watts). The other settings gave me wattages of 8.3, 4.1, and 1.0 respectively. Current draw, affecting battery life, at these settings are approximately 1, 0.8, 0.6, and 0.3 amps, respectively. A 4AH battery like the GPS will power this setup for 4 hours at 14 watts, 5 hours at 8 watts, 8 hours at 4 watts, and 13.3 hours at 1 watt.

A little more theory now (sorry!). The idea behind the dew heater is to spread out the heat producing elements as evenly as possible so as to not create thermal "hot spots" in the optics. While a single resistor of 12 ohms would produce the desired amount of heat, it would all be in one spot. A single resistor capable of handling 14 watts would also be quite large. Resistors are rated by the amount of heat they can safely dissipate before the current flowing through them destroys them. The standard ratings on small resistors are 1/4, 1/2, 1 and 2 watts. Any higher and the size becomes unwieldy. My ideal design would have been twelve 1-ohm, 2-watt resistors spread evenly around the corrector, but the aforementioned hours change of my electronics supplier made finding that value impossible for me.

when resistors are connected end-to--end, in "series," the total resistance is additive. Twelve 1-ohm resistors connected this way yields a total of 12 ohms. When connected in parallel however, the resistance drops. This is due to the fact that the current has multiple paths to follow, more current gets through, and the overall resistance is lower. (This is where the water analogy breaks down. when you open more faucets, the water pressure is reduced. when you give electricity multiple paths the current increases.) when resistors of equal value are placed in parallel, the total resistance is the value of a resistor divided by the number of them.

A commonly available value is 10 ohm, ½ watt. RS sells these 5 in a package for 49 cents. Five 10 ohm resistors connected in parallel equals a 2 ohm resistor Since the current is spread evenly over all five, the total wattage rating of an "array" like this is 2.5 watts.

If you connect 6 of these arrays in series, you get the total of 12 ohms needed, with a power capacity of 15 watts. This is the basic design of my dew heater.

The thirty resistors needed costs about $3.

A final note on resistors: A third rating you will see is the tolerance, given in percent. Due to manufacturing processes, a resistor will not have the exact value it's rated for but will fall within a certain range called its tolerance. Most common resistors have a tolerance of 5% meaning a 10 ohm resistor will have a true value somewhere between 9.5 and 10.5 ohms. For our purposes this variance can be ignored.

DEW PROTECTION

The objective of effective dew removal is not to "heat up" the corrector, just to replace the heat lost due to radiant heating of the glass to the night air A large glass corrector will quickly radiate its heat away causing it to drop below the current air temperature. when the heat loss is great enough that the temperature of the glass reaches the dew point, dew will form on it, quickly ending your observing session. A dew cap offers limited protection by slowing the heat loss of the glass but will not prevent it entirely. A dew heaters job is to actively replace the heat that is being radiated away, keeping the glass at a constant temperature above the dew point.

A delicate balance must be struck so as not to introduce too much heat in the system which will cause optical distortion. A properly operating system will not even cause the scope to feel warm to the touch. It will simply not be as cold as it would have been without it.

A dew heater and dew cap in combination are most effective. The dew cap offers passive protection which reduces the amount of heat needed from the active dew heater system. Fewer watts equals longer battery life which equals longer observing sessions. The wrap-around style flexible dew caps can fit over the dew heater, acting as a thermal "blanket" reducing the amount of heat wasted and directing more of it inward toward the corrector, again reducing the necessary wattage. On an average Midwestern night, I find 4 watts in combination with the dew cap gives me plenty of protection and enough battery life to last all night. Nights of higher humidity, and thus higher dew point, may require more.

THE PLANS (Finally!)

THE ELECTRONICS

You need to make up 6 resistor arrays as described above out of the 30 resisters. This is where the "helping hands" tool is a great help. If you don't know what it is, ifs two alligator clips on articulated arms attached to a heavy cast~metal base. You can get them at RS or a hobby store. Bend the resisters leads into an S shape. Clamp one resistor lead into each of the helping~hands and position them so they overlap. Solder the overlapped leads making sure there is a good strong bond at least 1/4 long. The trick to effective soldering is to heat the joint (not the solder) and to touch the heated joint with the solder until it flows into place.

Continue the process until you have an array of 5 resistors. Test the job by checking the resistance on the remaining leads. It should measure 2 ohms (within the tolerance of the resistors). If it does not, you have a bad solder joint in tile array and you should re- heat the leads until you have a good connection. Make up the additional five arrays the same way.

Split about 30" of the speaker wire so you have a single wire. Cut five lengths of the wire about 6" long arid strip each end about 1". Wrap the ends of the wire around the end leads from 2 arrays and solder in place. Connect all six arrays in the same fashion. Test the resistance like you did on the individual arrays at the ends of the string. The whole string should measure 12 ohms.

Be careful when splitting the wire. The RS brand does not split easily and it's easy to break the insulation and expose the bare wire.

Now split a length of wire long enough to go around the scope plus about 10 inches. Leave the final 10 inches unsplit. Strip 1" of one strand and solder it to the far end of the string. Cut the other end arid strip and solder it as in fig. 3.

Carefully measure the circumference of the scope and lay out the resistor arrays in an equal spacing. You'll need to coil up the wire that connects each array in the string to get them close enough. Cover the entire length with electrical tape being careful to make sure tile whole assembly is flat like a long tape.

Strip and solder the exposed end leader to an RCA jack. Be very careful when assembling the connector so the two leads don't touch and short out. Make sure the solder joints are solid on tile connector. Remember- heat the connector and let the solder flow into the joint; don't drop molten solder onto the connector since it will break off very easily. It's a good idea to wrap a small strip of electrical tape between the tabs on the completed connector to prevent shorting.

For a nice, finished appearance use heat-shrink tubing around the wire where it leaves tile connector and around the connector itself If you desire the heat-shrink tubing, make sure you slip it on the wire before soldering on the connector. You'll need some larger-diameter tubing to fit around the connector itself but I recommend it if you can get it. It keeps the steel connector from banging up against the side of the scope and gives it a nice, soft rubber grip.

The final electronic assembly is the power cord itself You can make this up different ways depending upon the power source you use. The instructions for the GPS follow. Cut a piece of cable to the desired length (mine's about 6 feet long) and strip both ends. Attach an RCA plug to one end as described above for the RCA jack. Attach the cigarette lighter plug to the other end as described on the package (the package says crimp the wires; I crimped AND soldered).

Another home made dew heater