Keeping dew off secondary - How much power?

[Garyh]

Hi Folks,
Just after a little advice as to how much wattage at 12v would I need to keep a 2.5" secondary dew free and just above ambient?
I would just like to make a resistor based warmer to go behind the secondary and would like to know what works for you?
Would 1/2 watt be too much and cause thermal problems etc?
cheers Gary

[gbeal]

I'll watch this with interest mate, as I am thinking to "slip" something behind my secondary as well. What will you use etc? I have in the past would/siliconed a piece of sheathed nichrome, but want to try something else. Stashing/routing the wires as well, ideas?
Gary

[Garyh]

Originally Posted by gbeal

I'll watch this with interest mate, as I am thinking to "slip" something behind my secondary as well. What will you use etc? I have in the past would/siliconed a piece of sheathed nichrome, but want to try something else. Stashing/routing the wires as well, ideas?
Gary

Hi Gary,
Was thinking of having a piece of 3mm aluminium plate the same size as the secondary with 2 resistors adhered with heatsink compound between the stalk and the mirror. Will have to hollow a bit of the stalk out to accommodate the resistors. The aluminium plate should distribute the heat nice and evenly over the whole mirror.
I might use some enameled copper wire along one of the vanes for power?
cheers

[gbeal]

Sounds like it could work. I don't have the luxury of alloy plate on the rear of my secondary(ies), but even some little heat can be helpful. Not sure what size of style of resistor to use either, but sparks wasn't my strong point.
If you are running power, just run one, and use the metal from the other as the wire, as long as one doesn't short to the other, LOL.

[erick (Eric)]

I've seen this:-

http://www.astrosystems.biz/dewgrd.htm

He says can run off a 9V battery and runs at 2-4W when "on full".

Might give you some ideas?

[Starkler (Geoff)]

The commercial Astrosystems dewguard system runs between 2-4W max depending on size. This is more power than needed as it has temp sensors and automatically regulates itself.

http://www.astrosystems.biz/dginst.htm

[erick (Eric)]

Thanks Geoff - I didn't dig deeply enough.

[astronut (John)]

Hi Gary,
Before I mounted my dew/light shield on my Lightbridge, the secondary would dew up even with the shroud in place.
Once the shield was mounted, I haven't lost one night due to a fogged secondary.
I've experienced very wet nights, where the scope looks like it has been immersed in water, the secondary has stayed dry.

[Starkler (Geoff)]

Originally Posted by astronut

Hi Gary,
Before I mounted my dew/light shield on my Lightbridge, the secondary would dew up even with the shroud in place.
Once the shield was mounted, I haven't lost one night due to a fogged secondary.

Another thing that works for many with tube dobs is a variable speed cooling fan at the back end pushing air up the tube. The air movement seems to serve a bonus function in keeping dew away.

[Garyh]

thanks Gary, Erick Geoff and John
what I have found on the net for a larger secondary is somewhere around 2-3 watts but being only 2-1/2" I think maybe 1-2 watts.
I thought about the light shield but being a imaging newt it will result in vignetting on the DSLR. So I might stick with a heater..
But having a fan to cool things down would serve to keep the dew away as well...good idea I have been thinking about a cool down fan for sometime but just havn`t got around to doing it..
cheers