Secondary Dew Heater

[gbeal]

OK, I need some help here, especially from your electronic tech types.
I am a klux, big time, when it come to electronics.
The requirement is to produce a dew heater for the secondary on my 16" truss dob, and after reading the recent posting by Astronut I reckon it is time to get amongst this.
I need help to decide which resistors to buy and use. In the past I have used short lengths of nichrome wire, encased it in a sheath and wrapped it around the objective or eyepiece, and powered it with 12 volts.
Assuming someone can help, what I need is the exact JayCar part number or whatever identifier, so I c an go buy it/them and solder it all together. It doesn't need a fancy PCB as far as I can tell and while the temp on/off system would be nice I can live without this as well.
Any takers?
Thanks
Gary

[Starkler]

Simplest solution would just be to buy one of these (http://www.astrosystems.biz/dewgrd.htm)

For around $50 you get a pre made unit thats temperature regulated.

[gbeal]

Thanks Geoff. It is looking that way. I just wanted to see if I could by-pass that and do it myself, albeit without temp compensation.
Gary

[Starkler]

Well its possible but IMO not worth it..

Lets see, the commercial unit says 2-4 watts of power in their design which should be enough to overheat the secondary if continuously on.

Ohms law says R=V^2/P

So for 12v , 144/2watts = 72 ohms worth of resistors.
Resistors come in standard values 10,12,15,18,20,22,27,33,47,56,68... .

4x18ohm resistors would theoretically do it, but in 1/2 watt format they will cook if their generated heat cant be adequately transferred to the mirror.

Personally I think its a bad idea to go down this path and have heat plumes streaming off your secondary mirror when such a cheap commercial solution will do it right and regulate the mirror temp to be a degree above ambient.

[CoombellKid]

Gary,

I'm with Geoff on the AstroSystems dew guard. I'm getting one for my
16" dob too. You can also source them from SDM (http://www.sdmtelescopes.com.au/) in Oz. I might work
out better on freight.

regards,CS

[gbeal]

Guys,
OK, tightwad as I am you are both right. My limited knowledge, plus the relative cost difference is insignificant. Why not.
Thanks for the link Rob, and post is free in Oz too.
Regards,
Gary

[CoombellKid]

Gary,

Well you could buy one (a DG2:thumbsup:) and post it to me :D. That way you
could take advantage of the free postage :D.

Let me know if I need to cancel my order :lol:

but seriously, I've heard good reports about them.

regards,CS

[erick]

Anyone have experience fitting one of these DG heaters to the secondary of a 12" GSO newtonian? I looked at the secondary in place last night while I was hitting it with the hair dryer and couldn't quite figure how to fit a heater?

Yes, OK, I'll make a dew shield and try that first :)

[CoombellKid]

Erick,

I think you would probably have to change your style of secondary holder.

regards,CS

[JethroB76]

Didn't Astronut fit one to his lightbridge?

[astronut]

Erick,
Have a look at the thread I started in this section.
If the secondary holder is the same in the G.S.O scopes as in the LB's, I can highly recommend that you carry it out.
Its probably a job left for a full moon period, so that you don't miss out on any observing.
If you need further info, just let me know.

John:)

[erick]

Thanks John. That looks very promising. Sorry, I should have searched better. Eric:)

[Garyh]

Hi Gary,
Some resistors in series to make up 1-2w output should work well to keep the dew off most nights I say.
Say like 4 - 1/2 watt 27 ohm resistors should get you around 1.3w or 4 at 22 ohm which shall give you around 1.7 watts.
Just solder them in series...ie: end to end like sausages ..lol
Should work well I say.
My 6" has only 1 watt of heating and it has never fogged up and you cannot feel any heat by touching the stalk or side of the mirror.
cheers Gary