Review Boltwood Cloud Sensor
The Boltwood Cloud Sensor is a device that will monitor the weather conditions continuously. In my opinion this is a must have tool for any serious astrophotographer. When I observe, I usually will go inside to sleep or do something else while the telescope is running. Only on exceptional clear nights I drag out the Dobson telescope and observe visually. Most of the other time I stay awake untill midnight and then get some sleep. Depending on the season, I close up the observatory somewhere between 4 am and 6:30 am.
The Cloud Sensor will keep you posted on the weather conditions during the time you are not watching the skies and will generate an alarm when it starts to cloud over or even worse, starts raining or snowing.
Features
The sensor can detect rain, snow, clouds and clear skies. There is also an ambiant temperature measurement. The principle of determining the cloud coverage is fairly easy. By measuring the skies ambient temperature in the 8-14 micron infrared band it can be determined whether it is clear or clouded. At -20°C to -25°C it usually is perfectly clear while a -10°C, a thick layer of clouds is covering everything. The sensor includes a relay that can close a dome when a certain event occurs.
Software
I ordered the sensor with a short cable (20ft). The sensor is mounted next to my radio meteor setup on a wooden pole. The sensor is connected to an adapter box and by means of a serial cable to the observatory desktop PC. Most unfortunatly the sensor cable sometimes suffers from a malfunctioning connection. The plug on the cable has pins that are to thin to connect to the sensor box. I have a desktop running 24/24 (the same one for the meteor setup. The default software delivered with the sensor is called 'Clarity'.
Clarity is a small application that can:
* read the data from the sensor
* show the data on the screen
* generate an alarm on the desktop
* log the data to a file
Clarity will not show graphs or trends of cloud % and temperature. I will also only generate alarms on the PC the sensor is physically connected. I wasn't too happy with this. First of all the Cloud Sensor is connected to the desktop PC at my desk. I am usually sleeping in the living room, with a laptop that has a remote desktop connection to the the observatory. So hard wiring the Cloud sensor to the laptop wasn't really is an option. So what good is a cloud sensor if I am not able to hear an alarm when clouds are rolling?
Too avoid this behaviour and to show some more meaningful graphs I decided to develop custom build software. The idea was to have a client-server application. The server monitors the Cloud Sensor and the client will connect to the server to find out what the weather conditions are. If clouded or clear, the client can wake up the sleeping astronomer.
The software is very flexible. It can be run from everywhere as long as there is a network connection to the server. Options can be set to generate an alarm following 10 minutes of being clear of 1 minute of clouds. The server will upload current weather charts to this website regularly.
Does it work?
The sensor is very reliable and stable. (apart from the malfunctioning contact). It performs exactly as advertised. The only types of clouds that are undetected are thin, high cirrus clouds. I did notice a seasonal change in the clear/clouded thresholds. It appears that in summer the threshold for clear can drop to as low as -15°C to increase to -25°C in winter. The sensor is always generating alarms before any drop of rain falls in normal circumstances. However, sometimes there is practically zero warning time. This usually occurs in spring due to a particalur kind of storm.
Alternatives
Before I used the Cloud Sensor, I used a webcam to do cloud monitoring. I basically counted the number of stars present on every frame to determine when it was clear or not. However this camera could only be pointed North as otherwise the Moon would mess up the counts. By means of counting the background counts, dawn could be detected. This system worked fairly well, but had to be set up every observing session as webcams aren't weather proof. Recently a European base firm developed a low cost alternative to the quite expensive Boltwood. (http://www.auroraeurotech.com/CloudSensor.php) This sensor does exactly the same as the first version of the Boltwood Cloud sensor, including the relay to be able to close a dome. Meanwhile the Boltwood Cloudsensor has evolved to version 2. The added features include wind speed, daylight and humidity.
Conclusion
Although expensive, the Boltwood Cloudsensor is a big help in daily observing. The sensor has allowed me to protect my equipment even better. Not only will the sensor detect when everything becomes clouded, but in clouded nights of fe meteor showers, the sensor can wake me up when the clouds have vanished! When I bought the sensor, no alternative was available. Due to the high difference in price, I would certainly consider to purchase the Aurora cloud sensor in stead of the Boltwood.
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