Astronomy.CJDawson.com

Dew Heater


Note: Since building this Dew heater, I've done several modifications and enhancements. In the end, I did not alter the original Dew Heater as laid out below, it does work. However, I recommend that you also take a good look at the PowerBox project as this contains the details about the enhancements that I've made since the original. The new main heating element that I made, about the same power requirements of the element from this project. Being mounted closer closer to the correcter plate allows it to work alot more efficiently.

So you've been out out, it's a perfect clear night. You start off targeting your favorite Messier objects, then having adapted your night vision, you decide to get hunting for that something special. You've been out for a couple of hours already, then suddenly you find that you can't see a thing! You look at your optics find that they're covered in mist, dripping with dew. What a nightmare, you're evening is spoiled, you pack up and go inside and sulk. SOund familier? Well, this nifty project will solve this problem. No doubt you've alreay done a search on the net, and found out about the kendrick system? Well this project works in a similair way, only it's a whole lot cheaper. Works out at about 1/2 the price, and you have the satisfaction of knowing that you built it yourself.

Parts List
One Cigertte lighter plug, with built in fuse and LED.
6 Meters dual core multistrand cable rated to take 4 amps at 12V or higher.
One DC 2.5mm jack plug.
3 Amp Fast Blow fuse.
Electrical Solder
Velleman kit K8004 (Pulse Width Modulator)
60 * 270 Ohm Resisters
12.4mm HeatShrink (1m length is enough) Large Project box. PCB Posts. 1K Linier Potentiomiter with built in switch 6 * Phono Sockets 6 * Phono Plugs Battery connectors (The type used for models, you need both the male and female parts)
1 * Phono Plug. 6 * Phono Sockets (To be mounted into the project box) Project Box, large enough to 1K Variable Resister, with Single Throw switch.

Tools used
Wire Cutters.
Wire Strippers.
Soldering Iron.
Phillip head screwdriver
Multimeter
Drill, with Various bits
Hair dryer

Assembly Instructions

I'm not going to go into detail about assembling the Vellman kit, their instructions do a pretty good job of that. You'll add the 1K Variable resister to the places indicated to give you the control that you need.

Vellman K8004
Drill hols in the project box to take the Variable Resister, DC socket, power LED (If you decide to add one, LED and a resister is all that you need) and the 6 Phono sockets. You may also need to drill holes to attach the K8004 circuit too.

Fix the circuit into the box, attach the power terminals to the switch, then the switch to the DC socket, it's best to attach the Positive to the center of the socket.

Next attach the Load terminals to the six Phono sockets. you want to attach these in parrellel. i.e. so that the wire goes from the +load terminal, to the middle terminal of every phono socket. Do the same with the negative terminal.

Next you need to make the power cables. You'll need two. One with a cigeratte plug at one end, and a DC Jack at the other. See the LX90 12V power cable project for full instructions. Make this about 2 meters long. The other will need a Phono plug at one end and the Male battery terminal at the other.

Now for the heater itself. Take the 60 resisters, bend both legs of each resister to a right angle. Next take two resisters, place them 12mm apart, so that the legs are touching. Solder the legs together. Repeat this process for all sixty resisters. You'll end up with a ladder type construction. now solder the female batter connector to one end. One wire attaches to each side of the resister chain. This is still a very flimsy construction, so we'll add some strength, push the ladder into the heat shrink, once the entire ladder has been slipped inside, cut off any excess. Then use the hair dryer to make the tubing shrink, this will grip against the ladder, adding strength and insulation.

Now place a piece of foam insulation along one side of the construction, attach at one end. Next form a ring, it's best to form the ring using your scope as a template. You should find that the resister ladder will stretch all the way around, and just fail to meet, if should miss by 12mm. Form the insulation around next, and cut it so that it forms a neat ring. I wrapped my foam in electrical tape to form to attach it to the resister ladder.

Finally, attach some velco to the ends of the foam ring to fasten it to the scope.

Plug the whole lot together and turn on. You'll find that at full blast the circuit pulls about two amps, the resisters should get very warm after about 10 seconds. In real use you should never need that much power turning the variable resister down to about 10% should be enough. That's about 200 milli amps.

The other 5 phono sockets are so that you can make additional heaters, for your eyepieces, finderscope, telrad, Autostar, or even your seat. Just remember that the more heater you add the more current that you'll use.

Phase 2 upgrade
Having use the heater several times now, I was really disappointed when it failed on me. The worst part was that I couldn't tell where it had gone wrong. So I've just made the following modification which will tell me both just how much the heater is on, and also that the controller is giving an output.

Open the heater box, Then solder an LED and resister combination in parrallel to the end of the phono socket supply loop. Also drill a hole in the box and mount the LED so that it can be seen from outside.

Phase 3 Extra Heaters.

Having now used the heater several times succesfully, I've found that my eyepieces are dewing up. Also I'm really limited on the AstroPhotography side of things as my camera also has dew problems. I realised this when I first put the control box together, but didn't realise that I'd be needing to solve the problem quite so soon. Anyway, I've done the work now and how three more heaters, one for the camera, one for my small eyepieces and one for my large eyepieces. There's still room on the controller to add two more heaters, so all in all, it's pretty well designed.

The Math. In phase one I showed you how to build a heater for an 8 Inch scope. Building secondary heaters for the scope mean that you need to scale everything in ratio, so that the controll will provide enough heat to all the elements in one go. The alternative is several control boxes, but that's just plain silly. I worked out the relative powers that the heaters need to be, then worked out the relevant number of resisters and their respective values. Here's a table that shows all the math.

Heater name Diameter to heat Percentage of OTA Size Wattage Required Resister Number Used Combined Resistance Max Current Power Power per Resister
  (Inches)     (ohm)     (Amps) (watts)  
Main 8 100 32 270 60 4.5 2.67 32 0.53
Eye Piece 1.25 15.625 5 430 15 28.66666667 0.42 5.023255814 0.33
Camera 2.25 28.125 9 250 15 16.66666667 0.72 8.64 0.58

So that's enough with the math, the principal for making the heaters are the same, the only difference being the actual length of that the element should end up.

To make a heating element

Materials that you'll need.

  • String
  • Heat Shrink
  • Electrical Insulating tape
  • Eyepiece (or the item that you're taloring the element for)
  • Power Connector (Plug and socket)
  • Resisters (See table above for number and type
  • Phono Plug
  • Power cable
  • Part of a camping mat.
First we'll need to create the measure the distance around the eyepiece, for this use a piece of string.
Next measure the length of the string and divide this buy one more than the number of resisters that you're using.
This is how far apart each resister needs to be placed.
Take your time to get the soldering as accurate as you can. This is what provides the element with not only it's power but also it's strength. Notice that the element doesn't quite match the length of the string. This is deliberate, so that there's room to add the connecting cables.
Next attach the connector plug to the element and use a little electrical tape to cover the ends. Cut some heat shrink to length so that it will completely cover the resister ladder. This will add water resistance to the project, as well as a little safety protection.
Next cut out a portion of the camping matt. This will become thermal insulation to direct all the heat into the area to be heated. Make sure that you make the piece longer than the element to allow for the extra distance that it will need to travel in order to wrap around the outside of the element.
Next task is to form cover the element with the heat shring. Then use a blow heater to shrink it tight around the element. After that form the heating element by carfully bending it around the object to be heated. It'll be impossible to do this once the camping matt is in place.
Finally add the Camping matt and wrap it tight in electrical tap. You can also take the opportunity to shape the connecter plug and cables so that they lay neatly within the finished element.
It doesn't matter how big or small the heating element is the principal is the same for all. The only diffence is that the small ones are alot more fiddly to make.
Addition: Since building my original main dew heater element, I've found that it does work effectivly. For best restults combine it with a dew shield, and you'll never get a dew problem even in the worst weather conditions. Also after getting a couple of cabling problems I decided to retro fit a 3mm Red Led to the main 8" heating element, this is in series with a 500 ohm resister. It's purpose is to provide a visual indication that the element is recieving power.
WARNING: Heat build up from this system could cause a fire, you should never leave your scope unsupervised when this project is attached. I will accept no responsibility for damages occuring from the use of information from this page.