My dew prevention modifications.

[Doc]

Sorted out my dew prevention modifications.

One bought a astrozap controller as this was perfect size to install alongside my wixey platform.

Bought a telrad dew heater from Bern at Modernastronomy.

And finally made myself a wraparound secondry mirror heater. This was easy to make.

Calculations were.

1 Ohm = 30mm of resistance wire. I had 500mm so had a resistance of 16.6 ohms

12v supply gave 0.75 amps and 9 watts of power. Thats on full power, I only used half of this.

Below is how I made it.

1. Make a template out of paper that matches your stalk on the secondry.

2. Cut 3 pieces of foam and one piece of foil to match template.

3. Add double sided sticky tape to one piece of foam and lay your resistence wire in a coil patten making sure the wire does not toch each other push down so the wire is stuck on tape.

4. Secure ends into a terminal block doesn't matter which end goes into which terminal.

5. Place other foam template onto and push down firmly to secure.

6. Tape outside of the dew heater you have made and stick foil onto this and then stick the last of the foam template onto this. This then gives an envelope of foil between the foam so stops alot of the heat from escaping. More heat is then transmitted into the stalk. ( don't let foil touch wire or it will short out)

7. Run your RCA wire along the spider vanes securing with tape as you go and secure wires into the terminal block. Once again it does not matter which wires go where.

8. Plug RCA into astrozap controller and test.

If I keep the controller to about half of it's compacity this seems to input enough heat so the stalk and telrad are just warm enough.

While I was at it I made up a light to illumunate my wixey. This is plugged into the astrozap so is dimmable.

[arad85]

Hi Mick,

Nice mod. Where did you get the resistance wire?

Andy

[themos]

Great stuff! I'd love to know if you've overspecified the wattage as I expected you (wrongly, perhaps) to need about 1 W. It would be great to get some numbers.

[Doc]

I thought I needed about 1W as well but if you look at the Kendrick secondry stalk heater they specify 8 watts as well. I just followed their figure. They say as it's heating the stalk and not the mirror you need more wattage.

Anyway I'll try it and see but it is adjustable and is warm enough with just 1/4 turn of the dial so in theory it's adjustable from 1 watt to 8 watt.

I got the nichrome wire of e-bay.

[brianb]

They say as it's heating the stalk and not the mirror you need more wattage.

But you don't want to heat the stalk - it will heat the air & ruin the seeing!

The successful secondary heaters I've seen have all involved milling a recess into the secondary mount & installing a resistor into it so that the secondary is heated gently from behind; also insulating the resistor & secondary mirror with a thin layer of plastic or cork to prevent the heat leaking into the support. Done this way, 1W is more than adequate for a Newtonian secondary up to approx. 3" minor axis.

[Doc]

But you don't want to heat the stalk - it will heat the air & ruin the seeing!

The successful secondary heaters I've seen have all involved milling a recess into the secondary mount & installing a resistor into it so that the secondary is heated gently from behind; also insulating the resistor & secondary mirror with a thin layer of plastic or cork to prevent the heat leaking into the support. Done this way, 1W is more than adequate for a Newtonian secondary up to approx. 3" minor axis.

Sounds a fantastic way of doing things but kendrick who are a pretty big in the dew department sell these stalk heaters, that are outputting this much wattage.

I do think you are right but will give this a go. If the viewing is impared I'll reconsider.

[albedo0.39]

Hi Mick - good idea with the Dew heater

Coming to the question about construction, when (if ?) I make mine it will be with a surface mount resistor of a few watts coupled to a simple variable DC power supply. I would then bond the resistor onto the back of the mirror. You don't need too much power in this way because all the heat is straight into the mirror.

Anyhow good for you for taking the plunge!

PS - (don't forget - if you need any counterweights for your scope, I have a couple spare).

[Sunwind]

may I ask what the black thing is which the wixey and astrozap controller are attached to, is it something you made or what? you seem to have a dob and i'm getting one soon along with a wixey and the dew controller so i'd like to make a similar setup so any info would be great.

thanks

[Doc]

They sit ontop of a telrad base. The telrad base has two slots in it and I made a plastic plate with two bars superglued to it. These slot into the telrad base and then the wixey and controller sit on top.

[dark knight]

Nice job Mick, very neat and tidy install, let us know how it performs..I was think of doing an eye piece one, where did you get all your calculation info ?? I'm going to also have to try and make the controller too, unless I can get a good cheap alternative...

[Doc]

Well Carl I've used it twice now and I turn the dew controller to half way and the secondary heater starts to feel warm to touch and also you can just feel the telrad heater getting warm as well.

Thursday night was pretty cold and I stayed out pretty late and the heaters kept the optics dew free and ice free. I know it gets alot colder so it's not ideally a test but the important thing is I didn't see no deterioation in the view.