Dew heater advice needed please

[KG8]

I have constructed this dew shield
http://img.villagephotos.com/p/2006-...copedewS03.jpg
http://img.villagephotos.com/p/2006-...copedewS01.jpg
for a C11 out of polycarbonate sheet (bunnings) and some flocking paper I lovingly cannibalized out of another shield I had. Now I am at the stage of fitting a dew heater. I don't want to incorporate it into the shield itself, unless I have to. I was thinking I could make it out of a 30mm wide strip of the poly sheet and size it so that it just slips neatly into the recess in front of my corrector plate here. http://img.villagephotos.com/p/2006-...copedewS02.jpg

I have 30 1ohm 1watt resistors that I plan to link in series and power off the 12 battery. which will give me (12^2/30) = 4.8 watts of heating power. I read that 3 watts is recommended for a 8" sct so I figured I would bump it up a tad. Since an 8" has area= 201 square inches and my 11" has an area of 380 square inches.

Of course I want to do it NOW!!! But I thought it wise to run it by some others here on the forum, in case there were any issues about heating this part of the scope. Thank you.

[montewilson (Monte)]

I don't like resistors. Use Nichromium wire instead. You will get much better heat tx to the metal of the scope. Resistors are great except it is hard to get them in metal to metal contact with the scope. So in effect you may have 4.8 W of heating power but only a fraction is entering the scope. The rest is going off to the air. You can ge that wire from Jaycar or DSE. Get the highest ohms per metre you can but generally you will need to put a few winds around the tube.

[KG8]

That's a much better solution montewilson, I'll grab some this morning. Thank's for the info.

[Omaroo (Chris Malikoff)]

Hmm.. I'm sorry but I'm thinking differently. I'm doing exactly what you are KG - on my 12" LS200GPS. I want to heat the air near the corrector plate - but I don't want to heat the plate! You end up with all sorts of optical problems if you heat them directly - you are not looking to heat the scope parts, but to heat the air near the plate by only a couple of degrees to stop it from reaching dewpoint. The only reason that other systems (such as those external strap-on jobs) require good transfer qualities is that they rely on transferring heat THROUGH the OTA walls to get to where it's needed. With what you propose - you're already there. Tell me if I'm wrong, but I don't think that you want your scope to radiate heat.

I'd like to hear what others have to say on the nichrome wire - I've heard that it has disadvantages over resistor arrays but I need clarification.

Cheers
Chris

[mill (Martin)]

Chris with nichrome wire it just depends on where you fit it.
If you fit it in front of the mirror in the dewshield it will heat the air and not the mirror.
Resistors are in principel the same as nichrome wire only bulkier.
When you fit resistors on the tube it will heat the tube the same as nichrome wire.
Nichrome wire will look better than resistors and you can regulate the heat the same.
When you have say 10 windings of wire you could tap it in the middle and have more heat with the same voltage or just use pulse width modulated power and have any heat setting as you like.
On dobsonians you can run the wire on top or under the spider (insulated) and gently heat the secondary by glueing a coil on the back of the mirror.
The same can be done with the primary mirror.

[Omaroo (Chris Malikoff)]

I went the resistor route. Worked a treat. Thread here: http://www.iceinspace.com.au/forum/s...ad.php?t=20234

[KG8]

I bought the wire, did a few preliminary fits and checks and decided to toss it. It was easily fatigued to the break point and I found it impossible to solder. Also to fit so many turns of it around my 30mm dew heater ring, while insulating it from itself yet not too much from the inside of the shield, would be an unnecessary complication. I used the 30 1ohm 2watt resistors instead, connecting them in series into a long string and bending it into a sort of zig zag pattern around the inside of the ring. I actually did this while the poly was a flat strip, before I formed it into its final ring shape, bending it more and more circular as I went and putting thin strips of duct tape at intervals as "tacks" to hold the resistor string in place. When it was fully formed I glued together a slight overlap I had allowed for in the design with epoxy 2 pack, then covered the resistors with a single run of the black duct tape.

The ring is separate from my dew shield and scope. I have slipped it just up inside the bottom of the shield so that it is near to the corrector plate but not touching the body of the scope. I don't see the need to heat the scope or corrector plate itself, simply having the air above it a few degrees higher does the trick.

It's pulling 580 mA which should not be too much of a burden on my 70 Amp hour battery but I was advised to pick up one of those Dick Smith speed controller kits for $15 so I can lower the voltage while it's not particularly dewy. You have to get a larger pot for the controller apparently, and mount this on your control box. I'll grab that tomorrow and fit it to my power distribution box. If anyone is interested I can take some shots of the finished products and post them up.

[Omaroo (Chris Malikoff)]

Yep - nichrome ain't the way to go. I think you summed it up as per the comments I've already heard from others. Can I ask why you went with resistors in series (vs parallel) where, if one connection breaks, you're done?

Cheers
Chris

[Omaroo (Chris Malikoff)]

Originally Posted by mill

On dobsonians you can run the wire on top or under the spider (insulated) and gently heat the secondary by glueing a coil on the back of the mirror.
The same can be done with the primary mirror.

heat the primary mirror? I don't know about anyone else, but I struggle to keep it cool - not warm.

[KG8]

Originally Posted by Omaroo

Yep - nichrome ain't the way to go. I think you summed it up as per the comments I've already heard from others. Can I ask why you went with resistors in series (vs parallel) where, if one connection breaks, you're done?

Cheers
Chris

I chose series Omaroo because I had the resistors on hand and because I see the parallell method as an unnessesary complication for a simple system. If the job is done right, series should last a lifetime and if I get a dewy plate then I didn't take enough care with my solder joints and I deserve it. I'll put a tiny led in the curcuit as a positive check though.