Dew Heater for Refractor

Post by **weixing** » Tue Jan 10, 2006 11:51 pm

Hi,
Just wonder anyone had made a dew heater for refractor before?? I'm making one for my refractor using 8 x 5.6ohms resistor in series and will be using my "standard" battery pack (8 x 1.2V NiMH 1.8AH) to power it, so it'll give off total of around 2W of heat and will last around 8.5 hours.

Anyway, the problem I face is where to mount the resistors?? The dewshield is in the way and the lens cell is a huge chunk of metal. There are three places that I think it is suitable to mount the resistors:
1. On the lens cell behind the dewshield (my dewshield is removable) - the problem is that the lens cell is a huge chunk of metal and may conduct the heat away before it reach the lens cell,
2. On the lens cell inside the dewshield before the lens - I think the best place to mount it for maximum effectiveness, but will be very difficult to mount it firmly and do maintenance.
3. On the dewshield - easiest place to mount, but the heat may have difficulty to conduct to the lens, unless use a larger power.

Currently, I mount the resistors on the dewshield itself (easiest method) and cover the dewshield with a heat insulator to prevent the heat from escaping via the dewshield outer surface. I do a test indoor and it seem that the heat had difficulty to conduct to the lens...

can't actually feel the heat when I put my hand in the dewshield, unless I put on the lens cap for a while before I put my hand in. Will try on the field on my next observation session (wondering when will that be??

).

Anyway, I hope that I don't need to increase the power of the heat, so that I don't need to carry any big battery and can last more than 5 - 6 hours. Anyone had better ideas???

Have a nice day.

Post by **VinSnr** » Wed Jan 11, 2006 12:01 am

why not just use a heatpack?

Post by **weixing** » Wed Jan 11, 2006 12:11 am

Hi,

why not just use a heatpack?

I hope to have a long term and cheaper solution...

Have a nice day.

Canopus Lim · Post by **Canopus Lim** » Wed Jan 11, 2006 12:19 am

How about buying copper tape and using certain length of nichrome wire to be sandwiched between the copper tape? Perhaps it will be easier to place and less bulky and diffcult to fold as resistors.

Post by **Airconvent** » Wed Jan 11, 2006 1:49 am

weixing,
your design only draw 0.2 A (1.76W) which is too low to generate enough heat. you should consider using a 12V Gel battery pack or use 10 NiMH batteries instead and reduce the total resistance from 45 to 30 ohms.
that should give you 0.4A (4.8W). A 4AH Gel pack should give you about 9-10 hrs while the Sanyo 2500mAH NiMH battery should yield about 6 hrs.

my heater (with inputs from Matthew and Canopus) is actually quite warm...in fact I was not sure if its too hot!

you solder the resisters in series, then wrap them in a heat shrink.
after that canopus recommends taping it although I left mine bare...no doubt mine will become sticky in time!

Tachyon · Post by **Tachyon** » Wed Jan 11, 2006 9:10 am

Hmm.. have you measured the temperature of the lens? A temperature increase of 1 to 2 deg C over the ambient temperature is usually sufficient to prevent dew formation. You also know that too much heat will generate tube currents. If you need to measure your tube temperature I have the sensors. Let me know when you are free!

Cheers!

Post by **acc** » Wed Jan 11, 2006 9:15 am

Ya agree with Andrew. A correctly-rated dew heater should not raise the temperature of the OTA perceptibly (the OTA shouldn't feel warm to the touch).

clear skies
cc

Canopus Lim · Post by **Canopus Lim** » Wed Jan 11, 2006 9:15 am

Theoretically the temperature of the heater should be slightly above ambient, but the amount of heat that actually gets to the optics will be far less as the metal or material underneath it will conduct the heat away as what Weixing says. Also, if the heater is not efficient, and the heat is not focused on one side, there will be more losses. Too high temperature will also have tube currents therefore experimenting with a controller will get the best result. Too low temperature on the other hand will not enable the optics to be free from dew.

cataclysm · Post by **cataclysm** » Wed Jan 11, 2006 9:41 am

you can try using tyre warmer for rc kit/cars. Its available in some hobbyist shop and they runs on any 6v to 12v source. Relatively affordable as compare off the shelf heater. At least it works on my scope.

Tachyon · Post by **Tachyon** » Wed Jan 11, 2006 1:27 pm

Interesting... never heard of tyre warmers. Care to elaborate more?