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. 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 and subtract this "lead resistance" from what you measured on the heater to find 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.
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 if the resistors were placed on the front side of the glass. For this you need a slim heater so wire it as shown in the fabove diagram. Although it may look different, it is actually the same parallel wiring as in the above instructions. With this arrangement you must use insulated wires to prevent shorts.
For a Rigel Quickfinder, the resistors are wired as shown above and glued to the inside of the black plastic housing. You must allow about 1/8" separation from the Rigel Glass Window (the window moves when the alignment is adjusted). 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 makes changing eyepieces much easier. Use about 20 resistors for a 2" diagonal, or about 15 resistors for a 1.25" diagonal. The resistors can be arranged in an array as shown below to provide more surface area.
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 in an array similar to the above diagram. 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 resistors are laid out in a radial pattern with center connection +12V and the outer wire ground. Only 4 resistors are shown but the actual number is calculated by dividing the circumference of the mirror by 0.625
Copyright 2020 by Ronald & Teresa Keating. All Rights Reserved.