NOTE -- The following instructions will assist you in building dew heaters for your telescope. While this project is not difficult, use of these instructions is done entirely at your own risk. These heaters are Kendrick compatible, but a mistake in construction could damage non-DewBuster™ controllers and void the warranty. The DewBuster™ controller has built-in fault protection so the warranty will not be affected. These heaters are only for use on 12-Volt power and require a controller to regulate the heat, do NOT connect them directly to 12-Volt power since the heaters will generate too much heat and burn up. If you do not wish to use a controller then I can offer no advice, try searching the internet for alternate heater plans. NEVER POWER A DEW HEATER ON 120 VOLT DUE TO THE RISK OF ELECTROCUTION.

Homemade Heater Strips
by Ron Keating

Search the Internet and you will find many different arrangements for building dew heaters. While they do work, most of them wire the resistors in series and if you need a different size heater then all the values change. You must also be careful not to exceed the resistor wattage or it could overheat and possibly even cause a fire. To avoid such problems, the instructions below will use 330-Ohm 1/2 Watt resistors wired in parallel. No calculations are needed with this method, simply space the resistors 5/8 inch apart. These heaters are compatible with the DewBuster™ controller and other controllers that accept Kendrick heaters. There is no risk of exceeding the wattage rating and each resister will generate its full rated 1/2 Watt of heat when the controller is set to full power which means you will use the fewest possible resistors for the heater. Any size heater can be built by simply adding more 330-Ohm resistors. To determine the wattage of the heater simply multiply the number of resistors used by 0.5 Watts. To determine heater current just multiply the number of resistors used by 0.036 Amps. For example a 28 resistor heater will generate 14 Watts and draw 1 Amp. If you plan to build a very large heater be sure the current does not exceed the capabilities of your controller. Also, this type of heater can not be used without a controller as it would get too hot.

To estimate the number of resistors you will need, divide the circumference of your telescope tube by 0.625. For example an 8" SCT will use about 45 resistors, a 10" about 55, an 11" about 60. A 4" Refractor would use about 22 resistors depending on the tube diameter.

If you have a broken hair dryer, heating pad, or electric blanket from which to get it, Nichrome wire can also be used for building heaters. See building heaters with Nichrome wire for more information.

Materials needed:

330-Ohm, 1/2-Watt Resistors -- Make sure you have the correct resistors, a 330K or 330M resistor will not work.  There should be only 4 color bands and they should be orange-orange-brown-gold meaning that the resistor is 330 Ohms plus or minus 5%. If there are more than 4 bands or if the 4th is not gold, then consult the manufacturer for proper identification. A 1/2-Watt resistor should be about 1/8" diameter, do not use the smaller 1/4 Watt resistors because they will overheat and burn up. Before spending time building a heater, apply 12 volts to one of your  resistors, it should get hot to the touch but should not smoke or discolor. You can save money by ordering resistors in quantity from an electronics outlet such as Mouser Electronics (Mouser# 293-330-RC ) and the leftovers can be used whenever you need to build another heater. If you do not need enough to offset Mouser's shipping costs, you may purchase resistors through me.

Solid hookup wire --  20 AWG recommended for heaters up to 8", 18 AWG for larger heaters up to 14". Mouser Electronics carries many gauges of wire such as 20AWG 602-1563-100. You may use stranded wire, but try to keep the solder from bonding the strands together and making the wire stiff.

Speaker cable -- at least 18 AWG speaker cable with RCA-type phono plugs. Do not use VCR/Stereo shielded or patch cables because the small wires will get hot and short out, possibly damaging your controller. IF you solder your own RCA plugs make sure the RCA plugs can handle the current. Radio Shack has pre-made 18 AWG speaker cables 42-2478 that are ideal.

Duct tape -- Home Depot carries black duct tape that looks nice. An even better material is black nylon "camouflage tape" sold in hunting outlets (thinner so you may have to use two layers but it looks like commercial heaters). You can also sew a fabric covering on but make sure the material does not restrict heat flow toward the telescope. For ideas see Mark Kaye's instructions for making heaters with fabric covers.

Velcro® -- Available at Home Depot and many department stores. Black 1/2" width suggested.

Insulation -- used on the outside of the heater to reduce wasted heat loss to the air so more heat travels into the telescope. 1/2" wide foam weather strip (for sealing around doors) works well and can be found at Home Depot.  Since the controller regulates the heat, the resistors do not get very hot so high-temperature materials are not needed.

HEATER CONSTRUCTION:

1. Cut two lengths of solid conductor hook up wire at least as long as the heater will be (long enough to go around the object the heater is for) and strip off any insulation. These will form the top and bottom wires to which the 330-Ohm resistors will be soldered. To make it easier, drive two nails into a board 5/8 inches apart and wrap one end of each wire around a nail. Drive two more nails into the board at the opposite end and stretch the wires between them as shown below. Note: When building several small heaters, it is easier to make one long ladder of resistors and then cut it into smaller lengths afterwards.

2. Attach the resistors by wrapping the leads around the solid wire and then soldering (polarity does not matter). Wrap snugly to allow a good solder joint but not so tightly that it damages the wire. Space the resistors 5/8 inches apart. As shown above and below, the color bands on the resistors should be orange-orange-brown-gold, if not then you have the wrong size resistors and it will probably not work.

3. Clip off excess resistor leads leaving only the ladder of resistors for the heater.

4. Cut the speaker cable in half yielding 2 lead wires for heaters (set one aside for next time). Solder the speaker cable wires to the top and bottom wires of the string of resistors (polarity does not matter). You should now test the assembly by applying 12 volts across the resistors (be sure the power source is fused). The resistors should get hot to the touch but should not smoke or discolor. If they do not heat up, measure the voltage from the top to bottom wire, it should be 12 volts. If the voltage is low, you either have a bad connection, too small a wire gauge on the RCA cable, or your power source is not able to supply enough current.

5. Lay out a piece of black duct tape and stick the resistor string to it.

6. Wrap the assembly around a can or other round object of similar diameter to what the heater will fit. Wrap the insulating material (weather-strip) around the outside and stick it to the duct tape. Fold the edges of the duct tape over and stick it to the insulation.

7. Apply another layer of duct tape to the outside of the heater to cover the insulation.

8. Cut a length of Velcro® a few inches longer than the heater and stick it to the outside of the heater with one end even with the end of the heater (the end with the speaker cable attached). The excess Velcro® should overhang several inches past the end of the heater. Note: The illustrations show the "loop" side of the Velcro being used here, but you can use whichever you prefer.

9. Cut a 1/2 inch wide strip of duct tape or electrical tape and stick it to the very end of the overhanging Velcro® on the adhesive side. This forms a a pull tab for easy removal of the heater.

10. Cut a piece of the opposite type Velcro® ("hook" side in the diagrams) and stick its adhesive side to the overhanging Velcro®'s adhesive side. When the heater is placed on the telescope the mating Velcro® surfaces will stick to each other.

11. Before connecting your heater to your controller, test it with a multimeter to insure it has no shorts. The expected heater resistance can be calculated by dividing 330 by the number of resistors in the heater. For example if you used 33 resistors, then 330 / 33 = 10 so the heater should read about 10 Ohms resistance. Note: After measuring the heater resistance, touch the meter leads together to measure the resistance in the leads. Then subtract the lead resistance from what you measured on the heater, and this will be the actual resistance of your heater.

A Few Tips:

On a finderscope heater, connect its eyepiece and objective heaters by a short length of wire so that they both use the same RCA plug. This means one less wire to deal with.

If your DewBuster is mounted on the telescope base and you want to reduce the number of wires running up to the scope, run a single wire from the DewBuster's AUX output to a 4 RCA Jack Panel. You can then plug 4 heaters into it. Make sure the wire is heavy enough gauge to handle the combined current of the heaters. Usually #18AWG is sufficient.
 

When making a Telrad heater, try to place the resistors on the back side of the glass near the edges. This will do a much better job of heating than putting them in front of the glass. For this you need a slim heater so wire it as shown in the following diagram. It looks different but is actually the same parallel wiring as in the above instructions. The main difference is that you must use insulated wires to prevent shorts.

For a Rigel Quickfinder, the resistors are wired as shown and glued to the inside of the black plastic housing about 1/8" from the Rigel Window (to allow for window movement when aligning). The wiring to the RCA plug can be routed through the inside of the Rigel and a small hole drilled near base to allow wires to exit.

You can build a star diagonal heater as shown above. This will warm the diagonal which will in turn warm the eyepiece, so an eyepiece heater is not needed. This is much easier than having to refit a heater every time you change an eyepiece. Use about 20 resistors for a 2" diagonal, about 15 resistors for a 1.25" diagonal. The resistors can be arranged similar to the Newtonian Secondary Mirror heater shown below.

If you are building a heater for a Newtonian Secondary Mirror it is better to stagger the resistors evenly across the back side of the mirror. If the back of the mirror is aluminized do not allow any electrical connections to touch it as the aluminum coating will conduct electricity. Shown below is a matrix of resistors. Notice how jumpers connect every second connecting wire so that odd numbered wires are +12V and even numbered wires are ground. Thus each resistor has 12 volts applied to it. To determine the number of resistors to use, divide the circumference of the mirror by .625

Above is a variation that works well with small Secondary Mirrors. Notice that the center connection is +12V and all outer connections are ground. Only 4 resistors are shown for clarity, but the actual number is determined by dividing the circumference of the mirror by .625

www.dewbuster.com