Making Dew Heaters
This page was inspired by all of the people who visited our web site and asked about the dew heaters that they saw there. I have had numerous requests for instructions on how to make eyepiece heaters, so one day I sat down with the camera and the scanner and came up with this.
The history of making dew heaters stretches back to the early eighties when we lived in downtown Toronto. The best inner city observing takes place in the hours between midnight and dawn. These are also the prime dewing times on most nights. Taking a page out of car rear window clearing, I figured that the same idea could be applied to eyepieces and objectives. At first, I used resistors. I would wire numerous resistors into chains and then sew them into bands to make heaters. These worked very well indeed. Then I discovered Nichrome wire at Active Surplus (345 Queen Street West, Toronto) and since this wire has quite high resistance per centimetre of wire, it makes the perfect material to wrap around an eyepiece, if properly housed.
In 1984, we moved to Kingston, where through the RASC Kingston centre, I met Larry Manuel. He is an extremely keen and accomplished observer and he immediately saw the usefulness of ocular heaters, so much so, he asked me if I wanted to manufacture and sell them. I had all ready learned my lesson when it comes to mixing hobbies and work, so I told him that he was welcome to do it himself, but I would not want to get involved. Not only would it ruin observing for me, I am a lousy business person, so I passed on the offer. Thus I gave my full blessing to "Ruby Optics". Unfortunately for Larry, he had a great product at the wrong time and his business did not take off like he hoped. At some point, Jim Kendrick cottoned on to the idea and started making eyepiece heaters as well. He bought up Larry's stock and trade and took it to Starfest at just the right time. An extremely dewy, clear night saw Jim's supply all disappear in a weekend and the success of Jim Kendrick's dew removal systems was assured. Thus I like to think of myself as the grandfather of optic heaters.
Making eyepiece heaters or objective heaters is a simple task. Outlined below is a ten picture display on how to make your own heaters. The small pictures can be clicked upon to bring up a larger version of each image. Be warned, you need to know how to do three things, basic electricity (E=IR), solder and sew. If you are going to sew, be sure to have the permission of the person who uses the machine. Better yet, ask for help making your first one. There is nothing I hate more than having people messing with my sewing machine. I hate blood on my bobbins.
I will put a complete materials list at the bottom of this page. The first picture shows all of the materials and equipment needed to make dew heaters. In the second shot, you can see the start of the process. You need to know how much heat you are going to require. It depends on your location, relative humidity, season, amount of eyepiece or objective to heat and your ability to supply power. The heater shown in this pictorial uses one heater band to create six watts of heat at 12 volts DC supplied power. In the second picture, you can see the start of the wiring process. You cannot directly solder to NiChrome wire, but you can use the solder to hold the wire in place. First cut the length of heater wire. In this image, about 35cm was used, about 26 ohms of wire. Cut a length of 1.5mm heat shrink tubing so that it is two centimetres shorter than the piece of NiChrome. The wire will be all distorted from being put onto a reel. Smooth it out in length as best as you can, avoiding kinks and hang a weight on one end. I use my hemostat. Then use a lighter to flame the wire. It will glow red hot, but it will instantly straighten out. Run the flame up the wire until you have a nice straight piece of wire from one end to the other. Then thread it through the heat shrink tubing. Cut three pieces of three millimetre tubing and one piece of six mm tubing and put them on the speaker wire as shown. Snip the insulation between the wire so that you can fan out the wire into two strands and clip one to be about 5mm shorter than the other. Strip off a centimetre of insulation from each wire end and splay the strands so that you have two halves. To attach the NiChrome to the speaker wire, match the end of one splayed end to one end of the NiChrome wire and twist the strands around to make the two separate wires mesh.
Now fold the Nichrome wire over and use the remaining strands to wrap around the whole wire to make a tidy bundle. Move the heat shrink tubing away from the joint and clip the wire with your hemostat and solder the copper wire with the NiChrome wire inside the twist. Once the solder joint cools, slide the heat shrink tubing on the Nichrome wire back as far as it will go to free up the other end of the wire. Put it into the splayed end of the other piece of speaker wire in the same fashion as the first. When it is, solder in place, then centre the 1.5mm heat shrink tubing on the NiChrome wire and shrink the tubing. You can use a candle or a lighter, but make sure that you do not get too close to the flame. Move up the three mm pieces of tubing to cover the solder joint and a small section of the tubing on the heater wire.
Then move up the second piece of tubing to cover the two shrunken pieces and finally the last piece to strain relief the whole set of joints as shown in the first picture. You now should have a completely covered piece of heater wire with a resistance of about 24 or so ohms.
Now comes the fun part, the sewing. Cut a piece of the light pack cloth to be eight centimetres wide by length of the heat loop stretched out plus about seven centimetres. Cut a strip of velcro. Place the velcro on one half of the material as shown, this will be the outside face of the material. Sew the felt velcro in place using a straight stitch, going all the way around the outside of the piece. The sew the heater loop in place on the other half of the outside face of the material using a double point zig-zag stitch that goes from one side of the wire to the other. I like to sew down one side to near the end, tie off the stitch, go back to the start and sew down the other and then go around the tight loop at the end, carefully moving the machine by hand to go around the corner. Zig-zag over the whole heat shrink tubing at the end bending the wire down so that it leads off the fabric as shown. I like to cut a small piece of fabric and sew it to the end of the hook side of the velcro to make a pull tab. Then I sew hook velcro onto the fabric with the hooks down at the wire end of the belt. Take the material and fold it in half down the length so that the outside faces are together on the inside. Pin around the edge of the material from the wire down the long part of the tube and across the end. Sew along the outside seam starting at the fold and moving to the corner of the end of the tube and then turn and come down the long side of the tube until you reach the wire. Be sure to use a small stitch at the fold and at the corner. Overcast the start at the fold and at the end near the wire. You now have what is called a tube, in sewing terms, one that is inside out. Pull the felt velcro out of the tube to bring the outside, now temporarily in the inside of the tube, to the outside. When this is down, you will have a smooth edged tube with an opening at the end with the hook velcro sticking out. Pin the material. Folding under the hem allowance created along the long side by making the tube, top stitch near to the edge of the opening from the wire to the short end. Overcast stitch across the band to the fold edge over top of the loop velcro using a multi-point zig-zag stitch on a fairly short stitch setting, thus sealing up the tube. Iron the finished tube.
I then use Goop E2000 type automotive glue to further strain relief the heat shrink tubing where it comes onto the band. Solder an RCA plug on the other end of the speaker wire and you are now done. The heater is ready to go.
You can do a variation on a theme by using two wires and a switch to give you multiple heats. This is especially handy in our climate where in the summer, a small amount of dew heat will be needed, but in the winter, a large amount will be required to keep back the frost. For objectives, two wires can each be separately tubed and then run through a larger tube to give one heat tube with two elements. Hook them in series for low heat, use one alone for medium heat and two in parallel for maximum heat. I use two ten watt bands to give five, ten or twenty watts of heat. I usually only need five watts, but the two extra settings are handy to clear frost and then maintain clear optics on really dew laden or frosty night.
If you go to the equipment page you can see examples of the very effective dew heaters used on our Tel Rad and Rigel Quickfinders.
Light pack cloth, thread, velcro, Nichrome wire 34 gauge or 70 ohms per metre, solder, 18*2 speaker wire, 1.5mm heat shrink tubing, 3mm heat shrink tubing, 6mm heat shrink tubing, RCA phono plug, E2000 type glue.
Soldering pencil, sewing machine, scissors, hemostat or long nose pliers, wire strippers, wire cutters, volt-ohm meter, pins.
Stuck? Send mail to: firstname.lastname@example.org
Back to our Astronomy Web Page.
Good luck and clear skies!