Last night I was set up with my C11 to image Saturn. The dew was pretty intense in that the eyepieces were getting dewed. A perfect night for the Dewbuster.
But I got into an argument with my wife when she saw the corrector plate had some fog on it at the end of the session. “Why did you turn it off!!!!!!!????”
Perhaps you can educate us with your opinion on this.
When photographing Saturn it is done at a focal length of 7000 mm.
At 10 PM I put the dewbuster on automatic setting “5” on the dial with both thermistors connected of course. All systems fine.
At 11:30 PM I looked at Saturn on the computer screen where it was obtaining the image from a DMK21AF04 camera.
The thin rings would go in and out of focus in a dance from the seeing of course.
I began to suspect tube currents from the Dewbuster keeping in mind that the scope had been outside since 7 PM, so it was equilibrated.
At 11:45 I turned off the dewbuster which was powering the heating band in an Astrozap dew cap.
At 12:30 the seeing seemed considerably better and photographing continued.
By 1:10 AM the corrector plate was noticed with a light coat of fog.
My wife said I was stupid in that I should have left the Dewbuster on. I disagreed thinking that the Dewbuster was creating tube currents that took a while to dissipate. This is not a fault of the dewbuster but my thinking goes like this.
The purpose of the dewbuster is to adjust the duty cycle of the supplied cycles of current so that it is just enough to prevent dewing by keeping the temperature in the dew cap region just above the dew point. . This also means battery consumption will be at a minimum for the job at hand.
The dewbuster will cycle off and on at full current. That is, I think the dewbuster output voltage is not lowered to produce less heating, but the duty cycle is decreased so that current flows in brief spurts.
When the dewbuster is heating, the air temperature is raised, thus creating tube currents as the warmed air forming around the heating element mixes with the cooler air surrounding it, raising the temperature to just above the dew point.
When the Dewbuster is used at much lower focal lengths for deep sky photography then these miniscule eddy currents are hardly noticeable and the dew buster can be simply left on automatic mode throughout the night. I must say that a friend who has a 14 inch scope refuses to use a heating strip with his dewcap as he feels that it is more appropriate to just drag out a 12V hot air blower. My experiences with hot air blowers is the seeing goes to hell immediately after the fog is cleared and you have to wait until equilibrium with the ambient temperature is reestablished.
Any takers to settle this argument?
The most important question is where have you got the heater installed? Is it attached to the metal of the scope or is it in the dew shield somewhere? You shouldn't see tube currents if it is around the corrector plate end of the scope.
I have seen some people try to keep the dew shield warm and they have the problem you are referring to.
