Dew Heater

 

Even with the LX-200 dew-shield in place at the beginning of an observing session after a couple of hours of damp British weather you often find the telescope images fading to a blur as the corrector plate dews up. I looked at several designs of dew-heater on the web but most seemed to be designed for portable (12v dc) operation. Since , at present, I only use this telescope in a fixed observatory I wanted to use a mains powered (240v ac) heater.

I had to hand a large quantity of second hand trace-heating cable which used to keep the gutters of a mainframe computer centre clear of snow and ice. This kind of cable can be connected directly to mains power, it consists of fine resistance wire wound around a flat rubber core and produces approx 30W/m length. The wire is joined at regular intervals to the main power conductors which run the length of the cable.

Construction of the heater was very simple BUT PLEASE NOTE THAT THIS INVOLVES THE USE OF MAINS VOLTAGES - YOU MUST TAKE SUITABLE PRECAUTIONS AGAINST ELECTROCUTION IN ALL SUCH WORK:- 

  1. Determine the interval along the cable at which the resistance wire is joined to the main conductors. You can do this either by stripping the outer sheathing back and inspecting connections 
    OR 
    cutting a length of cable about 3 times the circumference of the telescope, insulate one with electrical tape end and connect the to a temporary supply. Find the furthest point along the cable where it is getting warm. Disconnect the supply, cut the cable back to just below the warmest point. Re-insulate and re-connect the supply, you will now have a length of cold cable which gives you the interval between connections.
    The cable I used had a connection interval of about 0.76m
  2. Measure the circumference of your telescope. For my 10" LX-200 this is approx 38" or 0.96m
  3. Determine how many complete sections of cable to use. e.g. 2 sections of 0.76m = 1.52m, which means that there will be 1.5 turns around the telescope actually providing heat.
  4. Starting with the insulated end wrap the cable neatly around the telescope as close to the corrector plate as possible while still allowing the dew-shield to be fitted. Mark the cable where the free end crosses the insulated end.
  5. Remove the cable and , using a contact adhesive, fix a pair of velco pads to the positions marked (make sure you stick it to the correct side of the tape otherwise you will end up with a twist!).
  6. When the glue is set wrap the tape around the scope and fix in position with the velcro. This arrangements makes removal or repositioning very straightforward.
  7. Use a lightweight two-core cable to connect the permanent supply to the heater. It is preferable to include a light dimmer control in the circuit which then allows the power of the heater to be varied as conditions require.

To test the heater I put the telescope out on a cold night without the dew-shield. After an hour or so the corrector plate was heavily dewed up. I turned on the heater at full power and the dew was completely cleared in about 25 minutes, the alloy ring supporting the corrector plate was then just warm to the touch. Under normal conditions the heater is only needed to operate at about 1/4 power, with the dew-shield in place this can keep the plate clear all night.

 


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