Reprinted from CONSTELLATION (VOL XIV, NO 2), February 1999
the official newsletter of Bucks-Mont Astronomical Association, Inc
©1999 BMAA, Inc
Tips for February
Making your own dew zappers
- by Bernie Kosher
With the poor observing conditions for the past several weeks, there is no recent observing to pass on. Perhaps someone will offer a suggestion for future columns.
Keeping in line with our hobby, here are some formulas which may be of use to you. I have simplified them for ease of use for the observer who is not familiar with electronics.
I recommend using a separate connector for each set of leads to your battery. Phono plugs, also called RCA, are excellent and cheap at Radio Shack. For that matter all the items mentioned are available at Radio Shack. Use about 6" of lead to the plug. Put the male plug on the wires to the heater, and the female to the battery. This will prevent the leads shorting to metal surfaces. Make each set of leads just long enough to reach a terminal strip, and then you'll have only one set going to the battery. Use red wire for the positive side and black or blue for the negative. This is the commonly accepted method. At least, keep all positive leads of one color and negative another. Be consistent in which goes to the outer and inner rings of the plugs, making troubleshooting easier.
Use about 28 gauge stranded, not solid, wire as the leads will be flexed and solid wire will break from fatigue.
With our high humidity these are pretty much essential. Using a good long dewlap limits the amount of heat needed. Usually about a watt to the inch for refractors is all you need, and can be less if the objective has a long cap. I use 2.5 watts for my 4.5" and have no trouble. It is unlikely the main mirror of a reflector will dew, especially with a closed tube. The diagonal may though. Use about 1.5 or 2 watts on your Telrad or 50mm finder.
Users of the popular 8" SCT's can safely use 5 to 8 watts for the corrector plate, as long as the heaters are in touch with the metal rim and not the glass itself.
Focusers need about 2 watts. Wrap the assembly around the upper part of the focuser tube.
In keeping with ease of use, we can round off the simple power formula. Car batteries run over 12 volts, and cheap resistors are usually 10% or 20% tolerance, so we are justified in this.
Resistors are rated in ohms, which is a measure of how much they oppose current flow. The higher the resistance, the lower will be the current. They have a current rating measured in watts. The value can vary as much as 20% in cheap resistors, but for our use this is not critical. They are not polarity sensitive, so it doesn't matter if they are connected to the positive or negative leads.
When resistors are in SERIES, that is connected end to end in line with the battery, add the value of each resistor in the series to come up with the total resistance.
When resistors are in PARALLEL, that is all have one lead connected to the same battery terminal and all of the other leads to the other, the resistance lessened by the formula . . .
Value of one resistor / Number of resistors ( if all are the same)
If you are using resistors of different values the formula becomes more complex.
Use the easy formula as follows to figure wattage . . .
150 / total resistance = wattage
That's pretty simple to use. Be sure to get resistors with adequate wattage ratings. It's better to use several resistors in series (wired end to end) than one big high wattage job. For example, I used four 16 ohm 1 watt resistors in series on the refractor. In series you add the individual resistors and comes up with 64. Since 150 divided by 64 is just under 2.5, I have about that total in watts.
Since the resistors are in series, each consumes 1/4 of the power and the rating of 1W each is more than adequate to prevent burnout.
Be sure to insulate the leads from contact with metal, using a plastic tape or tubing. Wrap the assembly around the outside of the objective cell and you're all set. I use springs and elastic to keep the units in contact with the cell. For more help, contact me.
For your Telrad, fix the resistors in place using silicone adhesive (RTV, aquarium cement etc...small tubes about $3.00) under the clear window but out of the light path.
To figure the total current drain on your battery, add the wattage's for each component, then divide by the voltage.
In my case, I use 2.5 watts for the main objective, 2 watts for the reflex finder and 2 watts for the eyepiece heater. This adds to 7.5 watts. Divide 7.5 by 12 and you arrive somewhere around 6/10 amp. Sure enough, this measures about that on a meters current range. About 650 milliamps.
Since most car batteries are at least 30 amp hours, this tells you not to worry about draining the battery in one evening. At that rate it would require at least 30 hours to use over half the starting power. I have only a small motorcycle battery, and it runs the thing for two nights with no problem.
Be very careful of short circuits. They can drain a battery very quickly, and overheat whatever is draining all that current. Most likely it will melt something.
It's probably a good idea to add a 1 amp or at most 2 amp fuse to the line.
Dew heaters can be controlled with manual switches, or pulsed with a 555 chip gating a transistor, or with a rheostat type of circuit. If you are not familiar with these setups, the simple method described will suffice
Perhaps we can take time at a meeting to do demo.
Good observing . . . BK
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- BMAA member Bernie Kosher provides the monthly 'Tips' column for the CONSTELLATION. [ -ed]