Re: [ATM] dew heater circuit questions

[Michael Lindner <mikell@optonline.net>]

On Sun January 16 2005 2:13 pm, Don Clement wrote:
> ...My circuit uses the only the minimum heater power even if
> the on time power is set to many watts....

The power should still be reasonably close to the "always on" required power. 
Just as a furnace has to be sized for a house, the power of a dew heater has 
to be sized for the optic it heats.

It's even more of a problem for heating glass, as it is a poor conductor of 
heat. Referring to your circuit, the further "D2" (the diode that measures 
the optics temperature) is from the heat source, the more the circuit will 
locally overshoot the desired temperature before D2 turns the circuit off. 
furthermore, the more powerfull the heat source, the more the circuit will 
locally overshoot the desired temperature before D2 turns the circuit turns 
off. It is that local overshoot that can deform the optic.

I'll also note that the goal is not to keep the optics above air temperature, 
but to keep them above the dew point. If the local air temperature is above 
the dew point, any amount of heating is counterproductive. If the temperature 
falls more than 2 degree below the dew point during the course of the 
observing session, then the 2 degree difference won't be enough (although 
perhaps it's better to quit then instead of heating the optics to the point 
where tube warm air currents off the glass get too bad). Of course, until we 
have solid state "dew point" sensors, it's moot.

I'm not trying to denigrate your excellent work on dew heater control, but to 
point out that there is no dew "silver bullet" and there's still some work 
needed in the area of sizing heating elements for optics. For my own part, I 
have a homemade secondary heater of 1.44W dew heater (four 400 ohm resistors 
in parallel mounted on a copper plate touching the back of the glass for a 
3.5" secondary) and I've yet to find a night where I had to run it at 100% 
duty cycle.

Clear skies.
-- 
Michael Lindner
http://www.starastronomy.org *** http://home.att.net/~mikel
http://www.atmsite.org *** http://www.atmlist.net
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