Adding A Heater To Your Star Diagonal
Tired of fiddling with the heater strip every time you change eyepieces? Wish your Star Diagonal (or Paracorr) had a built-in heater? Well here's a solution, just follow the instructions below to add a nichrome wire heater to your Star Diagonal that is compatible with the DewBuster™ Controller and other 12-Volt dew-heater controllers. Your Star Diagonal will not be modified and the heater takes up very little space. Dob owners can use these same instructions for their Paracorr since the principle is the same.
Tools and Materials:
  • Ohmmeter - Used to measure resistance of wire.
  • Nichrome Wire - Should measure about 17 Ohms resistance from end to end. If purchasing new nichrome wire, divide 17 by the wire's Ohms per foot to determine how many feet of wire to purchase. A common source of nichrome wire is from an old hair dryer. Note that the low heat setting uses a smaller gauge heater wire and this is the best choice for your heater as it will allow the shortest wire length to get 17 Ohms.
  • Copper wire - for heater cord, Use 2-conductor #24 AWG or larger wire long enough to reach from the Star Diagonal to the DewBuster™ Controller.
  • Insulation - For wrapping around  the heater to reduce heat loss. Neoprene soft-drink can insulators work well and is thin so it keeps the heater compact.
  • RCA Plug - for connecting to DewBuster™ Controller. Mouser 17PP058 works well but must be soldered onto your wires. For a solder-less connection you can use an RCA to screw-terminal adapter.
  • 2 Crimp Terminals - Ring Terminal or Butt Splice connectors (commonly used to make electrical connections) that fit #18 wire (example Mouser 644-BS18-M)   The ring size does not matter as this part will be cut off.
  • 1/16" Heat Shrink Tubing - used to keep nichrome wire from shorting against itself as it is wrapped around the eyepiece holder (example Mouser 5174-11161).
  1. Measure nichrome wire from end to end with Ohmmeter to ensure it measures at least 17 Ohms. If much less it will not work because the nichrome wire will get too hot.
  2. Twist end of nichrome wire to one of the copper wires of heater cord, then insert into terminal and crimp. For increased reliability you may solder the joint after crimping (solder does not stick to nichrome wire so you must crimp first).
  3. Connect Ohmmeter to the crimped end of the nichrome wire and touch the other lead to the opposite end of the nichrome wire.
  4. Move test lead along the nichrome wire until you find the spot where it measures 17 Ohms and cut nichrome wire about a 1/4" longer.
  5. Cut a piece of Heat Shrink Tubing about an inch longer than the nichrome wire, slip it over the nichrome wire, then use a heat gun to shrink the tubing.
  6. Wrap two turns of electrical tape around the eyepiece end of the Star Diagonal (as shown above) to insulate the wiring from the metal Star Diagonal.
  7. Wrap the heat shrink covered nichrome wire tightly around the star diagonal as shown above (the wraps can touch each other since the heat shrink keeps them from making electrical contact).
  8. Cut back enough heat shrink to expose 1/4" of nichrome wire and twist it to the other copper wire of the heater cord (see above). Insert into Butt Splice or Ring Terminal and crimp. For increased reliability you may solder the joint after crimping.
  9. Wrap remaining portion of the nichrome wire around the Star Diagonal so that the entire length of nichrome wire is in contact with it (see above). Wrap electrical tape around the nichrome wire to insure it does not slip out of position.
  10. Using an Ohmmeter, recheck the heater resistance to insure it is about 17 Ohms. If it reads much lower then you have a short.
  11. Wrap the heater with insulation to reduce heat loss to the atmosphere.
  12. Wrap the outside of the insulation with black electrical tape to keep the insulation in place.
  13. Install an RCA plug onto the heater cord (polarity does not matter).
  14. Recheck the heater resistance to insure it is still 17 Ohms.
  15. You are finished. You may connect the heater to your dew-heater controller to test it out. Remember that it will not make the star diagonal fee "hot", it will however get warmer than the air so the diagonal should not feel "cold".
NOTE: To allow the diagonal to be neatly stored in your eyepiece case, simply wrap the heater cord around the diagonal when storing.

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