Page last updated: 2001 Apr 17
Start of the Project: 2000 August 24
Every year I and many other Astro-Dudes partake of the Star Party known as StarFest, held at Mount Forest Ontario. This year it Rained on Tuesday Aug 22. On Wednesday the 23rd the sun went down, it was clear and the dew fell out of the sky. Fell .. and fell.. and the water was running off the scopes like an indoor fountain. Needless to say we needed dew zappers.
With all due respect to the Kendrick system (which is very good), $45Can for a small eyepiece heater is ridicously expensive. So I built some of my own for under $5
Dew zappers are basically devices that generate enough heat to keep dew from forming or to heat away the dew, from telescope parts, namely eyepieces and mirrors/lenses. They can turn a night with heavy due from a complete failure to a very reasonable observing session.
Projects: Eyepiece heater, secondary mirror heater, camera lens heater, primary mirror heater
Let's assume you use 12vdc as that's what I use. (see the power box project). Some formulas:
V=voltage (volts) I=current (amps) R=resistance (ohms)
So for example, if you want a 1 watt heater:
1w=IV, I=1/12 = 0.083 amps = 83 milliamps. V=IR, R=V/I=12/0.083= 145 ohms
This is a linear relationship, so for:
power resistance (ohms) current (milliamps) 0.5 watts 290 42 1.0 watts 145 83 2.0 watts 72 166 3.0 watts 48 249 5 watts 29 415 10 watts 14 830
OK, now the problem is to find a resistor that (for 1 watt heating) is about 145 ohms (Radio Shack, 150 ohms, no problem), and can handle a load of 1 watt. Most "regular" resistors can handle only 1/4 or 1/2 watts. Let's assume you can only find 1/2 watt resistors. So you need 2 of them to handle 1 watt.
If you want to wire them in series you would need two (145/2=) 72 ohm resistors.
If you want to wire them in parallel you would need two (145*2)= 300 ohm resistors.
Another point to consider is that (on an eyepiece) a single resistor may not heat uniformly and in fact may cause optical distortion. So from a mechanical heat transfer perspective, 2 or more resistors would be a good idea.
(Not only can you do series or parallel connections, you can also do series-parallel connections.)
1. Design - Eyepiece Heater
Not having a clue, I picked a 1 watt design for the eyepiece heater (this was a complete guess as to how much heating power I would need), two resistors (which would be 1/2 watt, 290 ohms, wired in parallel). Remember that absolute resistance values are not critical by any means. 1 watt, 1.05 watts? who cares? (especially in the next project we build a pulse modulated duty cycle dew zapper controller!).
You could also use nicrome wire (toaster wire) but the longer the wire, the more resistance and I have no idea how much power it can dissipate without melting the mechanical assembly.
The first one I made was with a large strip of "female" velcro (1" wide by 4" long). I then glued a short piece of "male" velcro to the end to attach back after looping around the eyepiece. The second was with a velcro loop that had both male and female parts.
---------------------------- | ---------- ---------| |------------- | | | | | velcro male | | R1 R2 | |------------- 1 or 2 metres wire | | | |--|--------------------- | ------------------------|---------------------RCA Male plug -----------------------------Velcro female strip <==inside loop
-velcro strip 1"x5" female
-velcro strip 1-4" male narrower than the female
(could also replace the above with a single loop with both velcro sexes)
-two resistors (1/2 watt, 300 ohm)
-hookup wire (2 metres of 20-24 gauge)
-one rca male plug
-shrink wrap tubing (size dependent on your wire size)
$5-$10 depending on quantity built and if you have any of this stuff laying about.
After the Fact
It's a good idea to use a dew zapper before it is needed (ie easier to keep the dew off than to try and get rid of it later). I tested this under heavy dew conditions at Starfest Thursday Aug 24th and it kept my 1.25" eyepiece completely dew free. I may have to replace the duct tape with a better adhesive but it conducted the heat from the resistors well enough.
2. Design- Secondary Mirror Heater
I figure that a secondary mirror would need less heat than an eyepiece (after all it is sheltered and has less thermal mass) so I went for a 1/2 watt design for the secondary mirror (which did indeed fog up on the Wednesday night of Starfest 2000, I ended up building two of these the next day, on site). The 1 watt for the eyepiece heater actually worked so I knew the power requirements were in the ballpark.
In this case I used two 600 ohm resistors in parallel (300 ohm total), carefully positioned on the back of the secondary mirror and duct taped in place (later to be siliconed in place). The wires came up past the secondary holder and onto the back of the spider, thence across the spider to the outside of the tube. The wire was taped onto the spider to minimize its cross section. Depending on your spider, you may be able to use one or two spider arms as conductors and eliminate any additional cross section.
From there, add a strain relief using shrink wrap tubing or even just more duct tape, then add on the RCA male plug to the end of the cable. Measure the reistance using a meter to check that it is what you think it should be. Presto you now have a secondary mirror heater. Add a label to the RCA plug indicating what it is heating, its resistance and wattage.
You'll note that we are only using up to 1/2 watt of power, the two resistors can dissipate up to 1 full watt.
Parts List - Secondary Mirror Heater
two resistors (1/2 watt, 600 ohm)
some silicon glue
hookup wire (2 metres of 20-24 gauge)
one rca male plug
shrink wrap tubing (size dependent on your wire size)
Total costs: $2-$5 depending on quantity built and if you have any of this stuff laying about.