The two things, which seem to happen simultaneously at star-parties, 
are the emergence of a beautiful, clear, dark sky and the formation 
of dew on the optics. Much has been written about how and why dew forms, 
there is no point to repeating it all. I built dew zappers for the 
finder scope on my 6" Newtonian prior to my Florida several years ago. 
They worked so well in Florida that I decided to build similar ones for 
the Schmidt Cassegrain. At the several star parties they have kept the 
dew from forming until sunrise. Total cost for the four anti-dew heaters 
was under $20.00 and about two hours of work on one cloudy night. There 
are four heaters installed on the finder eyepiece tube and objective lens 
cell and on the SCT eyepiece tube and corrector plate. Dew caps made from 
automobile windshield sunshade material completed the anti-dew effort.
I am going to describe how the heating elements were designed, built and 
installed. There are several very simple formulae used to design the 
heating elements. All can be solved easily in your head or with a calculator. 
If you have never soldered two wires together, this is an excellent project 
to learn on. The results are dramatic and will keep you observing long after 
others have given up. For binoculars, simply make two heaters instead of one.

The tools needed are:

	1) 25 watt soldering iron 	(Radio Shack)
	2) Tweezers
	3) Cheap nail clippers

Materials needed are:

	1) Resistors: quantity and value to be determined
	2) Wire: 16ga, solderable, any color
	3) Plugs: power, not stereo 		(Radio Shack)
	4) Vinyl electrical tape 		(Radio Shack)
	5) Thin electronics solder		(Radio Shack)
	6) Masking or duct tape
	7) Protective eyewear

Support gear:

	1) Radio
	2) Crunchies
	3) Drink 


1) Design the heating elements:

	To keep things simple, we will go through the process to design, 
	build and install dew heaters for a 50mm finder scope of all-metal 
	construction with a right angle star diagonal. The process can be 
	applied to any size optical device normally found on an amateur telescope. 
	Definitions used used in the rest of this article are listed below.
	
	Heating element: the heat generating device we will design and build, 
	not unlike a toaster, 
	but cooler and with no visible light.
	
	Volt (V) /voltage: unit of electrical energy applied to the heater. 
	Think of it as water.
	
	Amp / Ampere: also referred to as current. 
	Think of it as water pressure.
	
	Resistor: the individual components used to build the heating element. 
	Usually carbon encased in a ceramic shell with a stiff wire on each end
	and colored bands to identify its resistance value. 
	Think of it as a faucet or water valve controlling the pressure.
	
	Ohm (W): Term and symbol used for resistance.
	
	Watt (W): Term and symbol for electrical power. 
	The higher the wattage of a light bulb, the brighter and hotter it is.

	A) 50mm Objective heater: this will be installed on the outside of 
	the metal lens cell. 

		1: Measure the outside diameter of the lens cell. 

		For this exercise, use a diameter of 2-1/2".

		2: Do the arithmetic.
		
		The design constant used is 1-1/4 (1.25) 
		Watts of heat per 1" of diameter to be heated.

		Wt = Watts (total)			D = diameter to be heated, in inches
		Rt = total resistance (Wt)		# = number of resistors needed
		Wr = resistor value			V = volts, (12 volts)

			1) Formula 1: 	Wt = (1.25) x D	Total heat needed
			     		Wt = (1.25) x (2.5) = 3.13 watts of heat needed, 
			     					   round to 3W
	
			2) Formula 2: 	Rt = (V xV) / Wt	Total resistance needed
					Rt = (144) / 3 = 48W
	
			3) Formula 3: 	# = Wt / 0.4 	Number of resistors needed
					# = 3 / 0.4 = 7.5 resistors needed, round to 8
	
			4) Formula 4: 	Wr = Rt / # 	Value of each resistor
					Wr = 48 / 8 = 6W per resistor

	B) 1-1/4"  Eyepiece heater: This will be installed on the eyepiece holder-tube. 

		1: Measure the outside diameter of the tube.

			For this exercise use a diameter of 1-1/4" (1.25)
		
		2: Do the arithmetic
				
			The design constant used is 1-1/4 (1.25) 
			Watts of heat per inch of diameter to be heated.

			1) Formula 1: 	Wt = (1.25) x D 	Total heat needed
					Wt = (1.25) x (1.25) = 1.56Watts needed, 
					                       round to 2w 

			2) Formula 2: 	Rt = (V x V) / Wt 	Total resistance needed
					Rt = (144) / 2 = 72.3W ~ 72

			3) Formula 3: 	# = Wt / 0.4 		Number of resistors needed
					# = 2 / 0.4 = 5 resistors needed 

			4) Formula 4: 	Wr = Rt / # 		Value of each resistor
					Wr = 72 / 5 = 14.4W per resistor ~ 14 W


Note: 	If the eyepiece holder-tube threads into a plastic diagonal mirror box or focuser, 
use a factor of 1-Watt per inch of diameter to be heated.  The plastic will not conduct 
heat away to the telescope tube, therefore less heat is needed to warm the eyepiece.




2) Build the heating elements: 

	A) Construction
		1. Measure the circumference of the eyepiece tube / finder scope lens cell by 
		    wrapping a piece of string around it, mark the string where the string  
		    overlaps itself with a felt tip marker. Stretch the string out on your work 
		    surface. The distance between the marks is the circumference of the lens 
		    cell.
		2. Using small pieces of tape, place the eight resistors evenly spaced between 
		    the marks on the string. Trim the wire leads with about 1/8" overlap and 
		    solder the wire leads together.
		
		   		 Leave two leads on the ends long.
		
		3. If you have multi-test meter, check the resistance of the heating element
		    against your arithmetic. It should be close.

	B) Installation
		1. Apply a single layer of tape around the surface where you will place the 
   		 heating element. This will serve as an insulator to prevent short circuits.

		2. Place your newly made resistor string around the objective lens cell and 
   		 tape it in place with electrical tape. Bend the two long leads so that they 
   		 stick outward but do not touch one another.

		3. Solder about 24" of wire to each of the wire leads and tape each with 
  		  electrical tape.

		4. Solder a cigarette lighter plug on the other end of the wires.

		5. Plug the assembly into an auto cigarette lighter socket, wait a minute or two 
    		and the  heating element should warm up.
	
		Note: 	All heaters are built the same way, it is up to you to decide how to 
		attach them to your equipment. 
		
		**The length of wire will vary depending on your equipment.




3) Battery Size

	A) Total Battery Power needed to run both heaters at once:

		50mm Objective Heater 	  3W
		Eyepiece heater		+2W
					Wt =5W

			Formula 5: 	Amps = Wt / V
					Amps = 5w / 12v = 0.42Amps,
					 (power used by the two heaters)

	B) Assume a two-night Star party with continuous running = 12 hours 'on' time.

		Batteries are usually rated in AmpHours.

			Formula 6:        AmpHrs = (Amps) x (Hours)	Battery capacity needed
				           AmpHrs = (0.42) x (12) = 5.04AmpHrs ~ 5AmpHrs 

I have used 7-AmpHr sealed-lead-acid batteries with success for a couple of years to run 
both the heaters and the RA drive motor. The sealed lead acid battery does not need water 
and won't leak sulfuric acid as will an auto battery if you carry it around. If you set up 
near your car most of the time, just buy or make an extension cord with lighter plugs 
on it and use your car battery.





Go Forth and enjoy dew-free observing