A simple thermoelectric infrared sky temperature sensor
is mounted on the south west wall of the 1.3 meter telescope.
The voltage from the sensor (irvolts) and the case temperature
(ovolts) are measured and then a relationship is derived
to compute the aproximate cloud cover.
The field of view is about 60 degrees.
And therefore does not give an all sky view.
The sensor only responds to low level warm clouds and
Here is the drawing .
The command taux
server command will read the voltages and do the conversion.
NOT READY YET
The infrared sensor puts out a voltage depending on
the difference between the case temperature and the
The sensor voltage is denoted irvolts and the
case temperature is related to ovolts.
I determine the "cloud" calibration by observing the
range of irvolts and ovolts over as wide a range of
conditions as possible.
Here is a graph taken over December 2006.
The upper envelope is the clearest sky.
The bottom represents cloud.
The amplifier can only produce positive
signals and there unfortunately seems to be
a negative bias, which I will remove by installing
a resistor in the future.
However we can still continue.
By assuming the "clear" value is the
red in the graph and "cloudy" is 0 irvolts.
We come up with the crazy formula that:
cloud=1 - (irvolts - (ovolts-2.820)*0.467)/0.3
cloud ranges from 0 --> 1.0.
The formula does a good job for clear skies and lousy
for really cloudy ones.
In other words if cloud > 0.2 don't bother.
Picture of the cloud sensor on my roof.