I use an SBC2 with a Phidget IR thermometer and a phidget humidity detector installed in an external housing. The IR thermometer is pointing towards the sky. I compare the IR temperature to the ambient temperature. If the delta temperature is larger than 25 degrees then there are virtually no clouds. If it is between 25 and 21 degrees then there are broken clouds. If the delta temperature is less than 21 degrees then there are clouds. If clouds or more than a 5-minute interval of partial clouds are detected then a signal is sent to the observatory dome shutter control to close the shutter. If no clouds are detected (for more than 10 minutes) then a signal is sent to the observatory dome shutter control to allow the shutter to be opened. (This approach is based on the fact that clouds are reflective. If it is cloudy then the clouds reflect the ambient temperature so the IR temperatue is very close to the ambient temperature. If there are no clouds then you're looking out into space and IR temperature measures down to -51 C (the coldest I've measured).)
I use the humidity to calculate Dew point. If the ambient temperature is within 2 degrees of the dewpoint then I turn on a small heater at the IR Thermometer window to keep the dew off of it.
I have a 4x40 LCD screen that shows the realtime stats. I mimic this display on a static webpage (so I can read it remotely). An example display is (I'll use code format to maintain the spacing)
Code: Select all
22:35:29 It's Clear Dome Open
Sky -11 Amb 15 d-26 since 06-06 16:58
-12/2 15/24 Dewpoint 5 for 05:37
51%RH Cloud=10 Tran=4 See=3 IRt 15
This SBC2 is a great reliable controller for remote operation.
I am working on upgrading to SBC3 and adding an all-sky camera.
It is much more exciting to see this live.