Introduction

Astronomy is my primary hobby and taking photographs of night-sky objects is my particular interest.  A downside to this hobby is that it is very weather dependent.  If it’s cloudy nothing can be seen.  Weather reports are important to monitor but they just serve the general area.  The sky conditions at my specific location are better monitored with an AllSky Camera.

An AllSky Camera is simply a camera with a fisheye lens that’s pointed up into the sky.  A program takes pictures of the sky all night long so checking the sky conditions can be done by looking at the latest sky image.  Is it too cloudy to take images?  Are clouds starting to move in?  Just check the AllSky Camera!

The Issue with Dew

Unfortunately, the dome of the AllSky Camera is prone to have dew forming on it when the humidity gets high.  Once that happens, the Allsky images are totally unusable.  To combat dew, many AllSky Cameras have a dew heater built into them.  The heater in my camera is very simple:  Apply 12 VDC and the heater is on.  Remove the voltage and the heater is off.  This applies about 10 W of power to the heater, and it does get hot enough to keep dew from forming on the dome.  Sometimes it gets too hot.

If the humidity is moderate the 10 W of power is way more than needed to keep the dome clear.  An unwanted side effect of too much heat is that cameras don’t like it.  The hotter a camera gets the noisier, or grainer, its picture becomes.  This is especially apparent with long exposures and AllSky Cameras can take up to 60-second exposures under a dark sky!  The better solution is to vary the amount of power applied to the heater so that only enough is applied to keep dew from forming, and no more.