Weather Monitor

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William Bristow

Hi Nick

The principle weather monitor used is an AAG Cloudwatcher from Lunatico Astronomia. This has two (semi) independent methods of rain monitoring for observatory shutter control.

Normally, AAG software running on the observatory computer monitors the AAG hardware mounted on a pole adjacent the observatory where an I.R. sensor measures sky temperature to determine cloud coverage, a heated capacitive plate senses rain droplets directly and a light sensor detects sky darkness. I also installed the optional anemometer.

If user configured thresholds in the software for the I.R. sensor monitoring sky temperature are exceeded, water droplets fall on the capacitive plate, the sky brightens or high wind speeds are detected then the AAG software will issue a weather unsafe warning and the observatory management software (ACP) will initiate an observatory shutdown and issue a close shutter command to the dome controller. 

The above actions depend on the correct functioning of the observatory computer which can never be 100% reliable, even with UPS backup, so the AAG Cloudwatcher also has an independent pair of potential-free relay contacts on a dedicated socket built into the hardware mounted on the pole. When the capacitive plate senses water droplets the relay contacts close and via a separate direct wire feed back the the dome controller, the dome controller will close the shutter independently of any computer command.

The dome controller itself, which is a Rigel System from Pulsar Observatories (UK), uses a ‘heartbeat‘ monitor. If for any reason the observatory computer ceases to respond to regular ‘Are You Still Alive?’ queries then the dome controller will close the shutter. Also, as the dome controller communicates with the battery powered shutter via wireless it also has a ‘heartbeat‘ monitor and should the wireless link go down between shutter and controller then the shutter will independently close after a short delay.

The capacitive water sensor plate in the AAG Cloudwatcher hardware is heated to prevent condensation, melt ice or snow and dry the plate when rain stops. Because of this, power consumption of the AAG Cloudwatcher is quite high so the AAG hardware is powered from the mains supply via an AC/DC convertor.

Due to local topography and coastal location my observatory is subject to gusting wind speeds exceeding 90 mph several times a year. Should the open dome shutter be facing towards the wind it is possible the dome roof would be lifted off as the Pulsar Observatories dome design does not use a captive suspension system for dome roof to dome wall coupling, the dome roof rests on support rollers with a short skirt providing lateral location. As the observatory is under full automatic control of ACP I installed the optional AAG wind speed anemometer with the rest of the AAG pole mounted hardware. ACP monitors the AAG Cloudwatcher wind speed sensor and either will not startup the observatory and open the shutter or will close the shutter and suspend operations should windspeed exceeed my user defined threshold, currently this is set at a rather low windspeed of approx 30mph.

In practice, the AAG Cloudwatcher is not infallible as far as the I.R. cloud sensor works. The user defined thresholds need constant tweaking as the seasons progress and upper atmosphere temperature conditions vary, otherwise clear sky conditions are reported cloudy and vice versa. I had been working on writing a program in C++ code that could read my local public and ATC airport weather stations to automatically update the AAG thresholds for the I.R. cloud sensor but have found that my nearest airports ATC weather reports usually stop when the airport closes for the night so this project has been shelved for now.

My AAG unit is from an early batch and I read that Lunatico Astronomia have changed the specification of the I.R. sensor on the current shipped units to improve the stability of the the I.R. sensors response to upper atmosphere temperature changes.

The AAG Cloudwatcher also now includes a humidity sensor though apart from recording and reporting purposes I’m not sure it serves much purpose in active observatory management unless you incorporate this into dew heater management for example.

The AAG capacitive plate water sensor, light sensor and I.R. sensor are reliable but do need regular cleaning to keep them free of bird droppings etc so location for access is important.

At the time I purchased the AAG Cloudwatcher I also bought the AAG Solo weather server with the intention of mounting the AAG hardware high on one of the house chimney stacks, communication between the observatory computer and the AAG hardware would have been via the Solo unit and the LAN network. In the end I abandoned this approach for several reasons, partly planning since the house is a listed building but mainly accessibility to keep the AAG hardware clean (I did start down the path of building a heated washer system based around a salvaged automobile high pressure headlamp washer), possible issues with heat plumes from the chimmneys causing misreading of the I.R. sensor and the inabilty to use the built-in direct connection rain sensor relay contacts. In the end I decided to keep things simple and mothballed the Solo unit for now and placed the AAG Cloudwatcher hardware on a pole, 2mtr above ground and 1mtr from the observatory wall where it can easily be cleaned and wired directly to the observatory computer.

The anemometer is not heated and has frozen twice this winter where snow has melted and refrozen around the base. In practice as my location is just a mile inland of the South coast this has not caused a problem since we rarely experience snow, freezing conditions and high winds simultaneously but I will add an external heater cup to the base of the anemometer at some time. As an engineer I feel I can do this easily and at a much lower cost than Lunatico Astronomia’s own optional anemometer with built-in heater.

The ambient light sensor will respond to moonlight or domestic light spill so threshold settings need carefull adjustment to avoid false readings and unnecessary observatory unsafe conditions shutdown or delayed opening. Orientation of the AAG hardware so that the I.R. sensor faces the prevailing weather direction is important though there is no reason why you can not mount the AAG hardware on a pole that can be rotated to face into the prevailing weather direction and adapt to seasonal weather patterns.

Looking back at the observatory weather records for the last year I can see that with an observing session under way the AAG Cloudwatcher I.R. sensor detected cloud and initated observatory shutdown and shutter closure before it began raining on 37 nights while the capacitive rain sensor detected water droplets even though no cloud was detected and initated shutdown on 8 nights.

I did preferentially consider the Boltwood cloud monitor from Diffraction Ltd over the AAG Cloudwatcher The stability of the sky temperature detection of the Boltwoods active thermopile compared to the passive I.R. sensor used in the AAG Cloudwatcher is an advantage plus the no-moving-parts anemometer of the Boltwood is a simpler and more reliable device but I felt the higher initial purchase price and long term running costs of the Boltwood are too excessive for a purely amateur observatory. With a finite life of the thermopile and degradation of the thermopile window by airborne salt and organic contaminants, given my coastal  location and seagull – pigeon populations I would expect to have to return the Boltwood to Canada every three or four years for thermopile / window replacement.