RC secondary mirror dew heaters - options?

[Jonk]

I've run some tests and here's what I've found

The PWM controllers arrived yesterday (ebay ~£11 for 4).

I previously bought some 5W 56ohm ceramic resistors to use on the telrad finder.

It works perfectly.

Anyway - I wanted to explore the idea of copper tape along the spider veins for the secondary mirror heater (nichrome wire awaiting delivery).

As the current is so low, Here's what I did.

Attached a strip of insulation tape and copper tape to measure the thickness.

0.2mm so negligable along a spider vein.

I ran the resistor from the PWM controller for an hour and set what I think is the optimum temperature. I had tried this on the guidescope the other day when it was clear and frosty, and there was no dew. The temperature of the guidescope lens was around 20 degrees (using the IR gun.

On the bench, here's what it looks like.

Based on this, I am more than happy to run a secondary dew heater using copper tape in place of wire, and thus keeping the spider vein slight increas in thickness to a minimum.

The tape / veins will be painted in stove paint once complete and hopefully it will not be visible at all.

The next step is use a usb temperature sensor to feedback to the laptop and display the mirror temperature to find the optimum setpoint. The usb power / signals can use copper tape too as the current is so low.

[malc-c]

A good means of taking power to the secondary with minimum obstruction is to use the wiring tape used for lighting dolls houses

Self adhesive backing and can be soldered directly through the insulation.

[malc-c]

You could use the same tape on an opposite support vein to run wires to a sensor probe.  Maplin do small LCD temp modules which could be mounted to the scope, or a PIC / Ardunio and LCD display if you wanted something more complicated.  I've used a DS 18B20 one wire digital sensor on a lot of my projects including my reptile thermostat project, in which the reading is used in a closed PID loop to maintain a very stable and precise temperature, which might be a bit of an overkill on a dew heater project, but it's doable

[Jonk]

Yes, I thought that too - I'm looking at this http://pcsensor.com/index.php?_a=product&product_id=160 as an option.

Not automatic but the sensor can be fitted to the mirror and displayed on the laptop.

After a few runs, I'll know the optimum setting on the PWM controller.

The next step is arduino based automatic PWM...for the future. There are plenty of articles on that around.