Heater question

I really want to eliminate 'fuss' from the scope I'm building (I have computers to fuss with all day so I don't need more at night and besides, I have a goto Intes on a Nextar mount if I didn't get enough computer grief at work and want more at home!

So I don't really want to make a control panel where I'm fine tuning everything from fan speeds to heater output either. Actually I am thinking about putting all my heaters on a single pole single throw switch to really simplify.

But if I do that, I should pick power values that are suited for a dew-fight when on, otherwise I switch it off if dew is a non-issue that night. I live in Portland, OR which is wet 4 months a year but I don't observe while it's raining anyway The other 8 months I don't find humidity or sudden temp drops to be a huge problem, and I bet one could 'keep up' with the temp changes without much heat to the optics needed to fight the good fight.

So does 1.5W for the 2.6" secondary sound good? What about .75W for the telrad?
Also, do you find there is any need to switch one on and the other off. I would think that 99% of the time, they are either both heating or both off.

Thanks.
TomL

p.s. I think I will skip making an EP heater unless people here really pipe up in their favor. Those just seem to dangle unused on most people's scopes I've seen and who wants to fuss with that every time they change EPs (I don't even bother with my focuser set screws cause I change EPs frequently).

Tom, I've found that 1 watt on my Telrad is just right. It will stay clear on the dewiest of nights. If I neglect to turn it on and it starts to dew, it will clear off in about 5-10 minutes. I also have 1.5W behind my 2.6" secondary, but I've not needed to use it yet (I just added it this winter). My secondary is buried about 16 inches inside my 16" tube (13" mirror), so it's protected from dewing pretty well. If your's is closer to the end of the tube then it may dew more readily. But the Telrad is out in the open, so I would guess that it will dew up nearly every time. It does for me, while I'll have the secondary dew up only on the worst nights, maybe once a year.

Remember that heating the secondary may introduce convection currents that could harm the image, so I would put it on a separate switch so that you can heat it only when you need to.

I'm with you re: dew heaters for the eyepiece, I don't use one. I switch eyepieces often, and it's just a hassle to replace the heater all the time. I just keep the EP's covered when not in use.

Best regards,
Doug Scobel

Astrosystems sells a heater that has 2 temperature sensors - 1 in the air, and 1 on the mirror. You set it to keep the mirror ~1 degree above ambient (adjustable to even less if you want). That way it turns itself on and off as needed to keep the mirror just warm enough not to dew. I have used this system on 2 scopes now - it works very well.

Jarad