All-sky cameras are specially built digital imaging devices the purpose of which is continuous visual monitoring of and recording sky conditions at a certain location. While home brew units sometimes rely on standard inexpensive cameras coupled with spherical mirrors and other ingenious optics and camera mounts, professional ones utilise very wide angle fish-eye lenses aimed towards the zenith in order to “directly” cover the entire upper hemisphere.
All-sky all-weather all-seasons...
A common problem one might face when using all-sky cameras is fogging of the optics caused by the necessity that the camera operates outdoors under extreme weather conditions, and usually for weeks or months without any human intervention. We experienced such nuisance with one of the premium models "All-Sky 340" made by SBIG, a very respectable producer of professional astronomical equipment settled in USA. This camera is built rock solid (we are talking of several kilograms of machined aluminium) and its picture quality is superb thanks to a very high quality CCD sensor and crystal clear optics. Although the initial image quality of the camera was excellent, it started to degrade gradually over the first winter of its exploitation at the new astronomical observatory located at the peak of beautiful Vidojevica mountain in southern Serbia. The cause of the problem appeared to be a small amount of water trapped underneath the camera main housing, which served as a reservoir of moisture which eventually managed to find its way into the acrylic dome... Searching over the camera user manual we noticed that SBIG found a feeble 4W built-in heater appropriate for keeping the camera dry at their factory test site, which becomes a bit questionable statement if one remembers that S.B.I.G. actually means Santa Barbara Imaging Group - and that the vast majority of worldwide all-sky camera users actually don’t live in the “Sunny State” of California :)
So unfortunately it appears that the internal heater is unable to prevent neither condensation formation at low temperatures nor snowfall buildup common at European mountain tops in winter months. After an intense brainstorming session, we decided that the first solution to the problem that we should try next spring was boosting significantly the heater power, for two main reasons:
Air inside the small camera dome should never be let to become so cold that the moisture it contains condenses over the lens and the internal surface of the protective camera dome.
Snow that falls onto the dome should be melted and then gradually evaporated away from the outher surface of the dome, otherwise a personnel would have to periodically climb the roof of the main building and free the camera from contamination by hand.
In addition, we decided to insert a thin plastic tube into the camera dome and lead the other end of it into the nearby room of the main building. The idea was that if for whatever reason additional heating proved not to operate as expected so water condensed onto surfaces inside the dome, one would be able to easily, without climbing the snowy roof, insert the tube into a sealed box containing desiccant and “remotely” extract the excess moisture out of the dome. As of December 2013, after several months of operation, there was no need to activate the tube so we assume that the additional heater alone does the job well. In the rest of this article we describe the simple circuit that drives it and the control algorithm behind it.
Because secondary heater will have to be able to not merely warm the air inside the dome a bit but as well to melt the appreciable amount of snow that falls onto the dome, it has to be significantly more powerful than the 4W original one. But as the dome is made out of clear acrylic plastic that melts at not so high temperatures, one has to be cautious when firing the heater. Had the manufacturer provided a means of automatic control of the built-in heater, it would have been a trivial task to piggyback a power transistor or a relay onto it and let the original electronics turn both heaters on and off simultaneously. But unfortunately the primary heater can only be controlled manually (using the dedicated PC application) or via a predefined timed sequence, without any data being available about the temperature of either the camera or its dome.
The solution to the problem is presented in the diagram above. The additional heater operates in a semi-automatic mode i.e. it “listens” to the primary heater and lets it govern the operation but only if the temperature is inside a safe range set in advance. If dome temperature falls below the lower threshold or rises above the upper one, the secondary heater operates autonomously.
If temperature inside the dome is below +5°C, secondary heater is turned on, even if the primary heater is currently off. This prevents snow buildup and keeps surfaces inside the dome ice free. If temperature is above +5°C but still below +35°C, secondary heater follows the operation of the primary i.e. only if the primary is on, so is the secondary. If temperature reaches +35°C threshold, secondary heater turns off in order to prevent possible damages to the dome or camera electronics.