A Do-It-Yourself

Dew Heater

for an SCT

Theory

Based on the commercially available products and comments from others on the Net, I decided that about 30 watts would be the maximum necessary power. (An 8" scope would probably need only about 15 watts total. You could use just the three lower power sets and eliminate half the work.) But there will be many times when that much power is way too much. So I wanted a minimum setting of about 2 watts. It then seemed reasonable to have intermediate settings of about 4, 8 and 16 watts. A rheostat is a really lousy way to control the power setting since all the extra power is wasted as heat in the rheostat. A fancified circuit to turn the whole thing on and off rapidly at various rates would accomplish the goal but is too complicated for my level of electronic "expertise". So I settled on a system with separate 2, 4, 8, and 16 watt heaters controlled with simple switches. It will generate those power levels with just one heater on and 30 watts when all four are on simultaneously (and as a extra added bonus for the arithmetically gifted various other combinations, too).

So there needs to be a little box to hold the switches. I originally planned to mount the box inside one of the fork arms, but the difficulty of neatly running all the wires across the Declination bearing led me to decide to simply stick the box on the side of the scope tube near the corrector end. This way only the power input wire needs to worry about the scope's motion. And so long as we have a box we might as well add some LEDs as pilot lights and a connector for the power input so the wire can be removed with the heater is not in use.

The basic circuit is very simple (don't worry it gets more complicated). The amount of power dissipated by a resistor is calculated by the formula:

P = E * I

where P is the power (in watts) and E is the voltage (in volts) and I is the current (in amps). We assume E=12 but what's the current? Ohm's Law to the rescue:

E = I * R

where R is the resistance (in ohms). A little high school algebra (you do remember that, don't you?) gives:

    E²
P = -
    R

So if we want 30 watts we need 144/30=4.8 ohms.

But we wanted four separate heaters with varying power levels. Doing the math gives about 80, 40, 16, and 8 ohms for the resistors.

Now we add the main pilot light. The extra resistor in series with the LED is to limit the current thru the LED to its rated level; it works out to about 1000 ohms (usually written 1k ohm). (Electronics cognoscenti will note that I've used the wrong symbol for a LED; sorry 'bout that but I couldn't find one to paste in so I used the symbol for a plain diode.)

But, of course, we want separate pilot lights for each heater, too.

And finally a switch to dim or turn off the lights during star parties. (Where wires cross without a dot there is no connection.)

There are two more interrelated considerations (and then we're ready to start soldering). First, we want to distribute the heat evenly across the whole circumference of the corrector (heck, we would like to distribute across the whole surface, but putting resistors in the middle of the aperture wouldn't do). And secondly, a resistor can't dissipate an infinite amount of power; they are rated for a particular maximum amount of power dissipation. So we use a number of resistors in combination for each of the heater resistors in the diagram above.

The resistance of two resistors in series is simply the sum of their individual resistances. So we implement the 80-ohm resistor with 16 5-ohm resistors in series.

The resistance of two resistors in parallel is given by the formula:

    1
R_t = -------
     1     1
     -  +  -
       R₁    R₂

So we implement the 40-ohm resistor with 8 5-ohm resistors in series each of which is a pair of 10-ohm resistors in parallel.

Similarly, the 16-ohm resistor is constructed of 8 sets of 5 10-ohm resistors and the 8-ohm resistor is 8 sets of 10 10-ohm resistors. For better heat distribution, the 16-ohm resistor is physically divided into two: 8 pairs plus 8 triplets. Similarly, the 8-ohm is 16 pairs plus 16 triplets.

In all cases, each individual resistor dissipates less then 1/4 watt so small 1/4 watt resistors can be used. And there are a sufficient number of resistors to distribute the heat evenly.

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Bill Arnett; last updated: 1996 September 17 ; suggestions or corrections appreciated