How to make a DIY secondary heater

Dew forming on a secondary mirror can be a problem even when using a dew shield at the end of the tube to try to avert the problem… You can buy some heaters but they are very pricy and in my case, because of the shape of the spider on my scope, no commercial one would fit. So, I decided to make my own one. The principle is very simple. To avoid due forming on cold night on the secondary (I am fine with the primary) it has to be heated.

In my case the simplest way to was to wrap a resistive wire around the secondary. Power control is done by using a DC motor controller found on Amazon https://www.amazon.fr/ARCELI-Contr%C3%B4leur-Commutateur-Commande-r%C3%A9glable/dp/B07RHMMM9W . This allows power adjustment from 0% to 100%.

The controller is supplied with 12V DC and it’s maximum current is 2A which is enough by a long way for what we are trying to make! Now the question is: what maximum power do we want to be able to dissipate on the secondary mirror? For reminder, a rise from ambient by a couple of degrees is enough to keep dew away, and we do not want to heat so much as to form surface turbulence on the secondary. Power is calculated with:

We can aim for a power of around 20W so current I is about 1.6Amps and the overall resistance of the wire is 67 Ohms. Using a 2M length of 30 Ohms/M wire will do th trick. You just need to wrap th wire around the secondary and tape over it and voilà!