LX50 Dew Heater

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The permanently installed 12 volt DC dew heater.

I would like to thank fellow amateur telescope enthusiast Chris Heapy of Macclesfield in the U.K. for the basis of this heater. 

The heater is unpainted in these views to improve clarity.  It was scuffed and then painted Krylon "Telescope" (satin) Black.  The corrector cover fits on normally and there is no need to remove the heater once it is unplugged.  Mine is held in with small strips of 3M double sticky foam tape.

Here are the parts ready to start building.  My scope is a 10" so anything larger or smaller needs a different set of calculations.

I used 1/4" OD soft copper plumbing.  Make sure the resistors will fit into the tubing.  The resistors I got from RadioShack.com fit quite well.  The better the fit, the better the heat transfer.

The 13 resistors are 2 watt 3.3 ohm 5% Metal Oxide resistors (RS# 900-0758), you could also use 9 4.3 ohm resistors.  You want to have 40 ohms total.  A little less total resistance will produce more heat, a little more, less.  Measure the outside of the corrector housing and very carefully cut a length of the tubing using a razor saw, don't use a tubing cutter, it rolls in the cut end and you will have to file the face of the tube flat, much more work!  Place the tubing inside the corrector housing (get help to prevent scratching the corrector) and make a mark on the other end of the tubing 1/4" back from the freshly cut end.  Remove and cut off excess.  Measure the length of the tubing.  Divide this figure by the number of resistors you are going to use.

This will even space the resistors around the corrector, providing even heating.

 Draw a line on a sheet of paper with marks this distance apart.  The resistors should be centered on these marks.  Strip the insulation from some 16 ga wire and cut it into 3/4" pieces  Slip a piece of this onto one of the resistors before soldering the next resistor into place.  The slide the tubing over the solder joint.  Continue this process until all resistors are used or you get bored and go to watch the Simpson's.  :o)

Cut a piece of bare copper single strand "bell wire " a few inches longer than the tubing.  Solder the end of the wire to the end resistor and push the wire through the tubing.  Carefully pulling the wire and having your assistant feed the resistors through the copper tube until the resistor network is evenly fitted inside.  Solder one end of the resistor string to the tube.  Slip a length of the insulation on the other end of the resistor string so it is slightly extended from the copper tubing. 

Now might be a good time to test the heater.  Mine (using 13 of the 3.3 ohm resistors) drew .25 amperes at 12 volts.

Prep the connector for the cord by cutting off the locking tabs.  I feel this is needed just in case the cable hangs up on something.  I used a RIA Electronics # 31249103-TA2 (RS# 910-4164) 3 position connector for the plug and a #31220103-TA2 (RS# 910-4150) for the heater.  The two outside terminals are soldered to the tubing and also act as the ground.

Solder one of the outside connector terminals to the tubing.  Install the heater into the corrector housing and after waking up your helper, mark where the other outside terminal should be soldered to the tubing to maintain a nice snug fit in the corrector housing.  Remove the heater and then solder this connection.  Make sure the positive end of the resistor string is not touching any part of the tubing and solder this lead to the center terminal of the connector.

Test the heater again!

If all is fine, scuff and paint the tubing and install in the corrector housing.

Here's the heater connected to the 12 volt power supply.