This information is provided for those with the proper electrical knowledge to build 12 Volt telescope heaters using Nichrome wire. If you do not have such knowledge then you should build heaters with 330 Ohm resistors. This page only covers aspects of Nichrome wire, refer to building heaters with 330 Ohm resistors for information on how to assemble the heaters.

Building Nichrome wire heaters involves calculating the desired resistance and then using parallel lengths of Nichrome wire to obtain that resistance. The calculations are to obtain a wattage that is consistent with commercial heaters and this is very important for good  results. You can obtain Nichrome wire out of an old blow dryer, toaster, heating pad, etc. It can also be purchased from companies such as Scientific Instrument Services.

Do not cut any pieces of Nichrome wire until you get to step 7. These instructions deal with very low resistance values so it is important to subtract the resistance in the ohmmeter leads from the reading on the meter. Touch the meter leads together to measure the lead resistance. Subtract this lead resistance from every measurement taken in the following instructions. For example if touching the leads together reads 0.5 ohms, and when you measure a piece of Nichrome wire the meter reads 15.0 ohms, then the true resistance is 14.5 ohms.

1. Measure the circumference of what you want your heater to fit. For example a C11 measures 38".

2. Divide 190 by the circumference to find out the desired heater resistance:    190 / 38"  =  5.0 Ohms
(Why 190? Because this ratio produces 0.76 Watts per inch of circumference when operated at 12 Volts, matching the wattage of similar commercial heaters.)

The next step will determine the minimum length that we can use for any piece of Nichrome wire.

3. Without cutting the Nichrome wire, stretch it out and use alligator clips or some other means of temporarily applying 12 Volts to the full length of the wire (make sure it's over 30" long). Assuming the Nichrome barely gets warm, move one alligator clip  a few inches closer to shorten the portion of wire the 12 volts is being applied to. Keep moving it closer until you find a point that it gets warm enough to generate heat but not so hot as to melting anything in contact with the Nichrome wire (be careful not to burn yourself, the wire may get hot!). In the above example I found that a 30" length of wire was the shortest piece I could use without it getting too hot.

We do not want the wire to melt the heater strip or any part of the telescope. The shorter we cut a given piece of Nichrome wire the hotter it will get because the shorter the wire the less resistance it has and the more current flows through the wire. Now that we know the minimum length of wire, as long as the wires are this length or longer the wire will not get too hot.

4. Remove power from the Nichrome wire and touch the meter leads to the places on the Nichrome wire where the alligator clips were placed in step 3 to measure the resistance of the minimum length of Nichrome wire.  In this example the 30" length of Nichrome wire read 14.0 ohms.

5. Divide the minimum Nichrome wire ohms (step 4) by the desired heater resistance (step 2) which in this example was 2.8.  Increase the result to the next higher whole number (do not round downward) which in this example was 3. This is the number of  pieces of Nichrome wire you will have to cut, but we do not know the length yet.