I have always had problems with dew forming on my 14 Dob secondary mirror and decided to do something about it.
Was very successful so I thought I'd share my project for those interested.
I had read quite a bit about dew shields but they were either costly or just too fiddly.
My idea had to be real cheap, easy to make and most importantly, had to work.
This is what I came up with.
I'll try to explain in steps.
Step 1: When your loved one is away for the day, take the electric blanket off the bed.
You can see where the wires are so snip through one.
Step 2: Make a slit in the blanket and pull the wire out. Try and get as long a length as you can.
The wire in mine had the resistance value printed on the insulation, some do, some don't.
Through trial and error I needed about 4 watts to heat the mirror up and about 2 watts to keep it warm.
Applying a bit of Ohms law I worked out I needed 2 pieces at 80 ohms each. That would give me 3.6 watts and 1.8 watts
Step 3: Strip the ends off and you will see the nichrome wire is coiled around some cotton.
Step 4: place a ferrule over each end and crimp the metal with some pliers.
Step 5: Solder some flexible wire to the metal ferrule and slide some shrink wrap over the join.
The ideal wire to use is multi-strand. I cut some off an old computer power cable.
Step 6: With a hot glue gun, I carefully glued the cables to the back of the secondary mirror, and ran the other end to my battery supply.
If you do not have a fancy controller, just hook them up to a battery. I have 2 switches for now so I can turn one off when the mirror has warmed.
When it came to testing it out, I turned the heaters on half an hour before going outside.
The mirror was warm by then so I turned one heater off. Dew formed just about everywhere except on the secondary mirror.
Well that's it. Nice and easy and only took a couple of hours from scratch.
I am going to experiment now to determine the minimum and maximum wattage I actually need.
EDIT: Sorry, just realised I should of posted this in the DIY Projects section
Last edited by Colin_Fraser; 14-08-2012 at 10:42 PM.
