A Precipitation Sensor

Getting rain or snow on the primary mirrors is bad.

Open Dome
The Keck 2 primary mirror
The normal method of detecting conditions that might lead to rain or snow is by monitoring the humidity or dew point. If the dew point approaches the current temperature, to within a few degrees the operators must close the domes. This is much the same thing as the humidity approaching 100%. Thus I have installed several new humidity/dew point/temperature sensors over the last few years. These replaced some old and troublesome equipment that had been in place for over a decade.

The problem… You can have both rain and snow occur with low humidity. Moist air above the telescope can produce rain or snow which falls into drier air at the summit level. This can catch the operators by surprise, a situation we have observed on a number of occasions.

To help detect this you can deploy a precipitation sensor, something some of the neighboring telescopes have done. The engineers over at CFHT were kind enough to show me the units they had installed during their remote operation project.

Keck Weather Mast
Various instruments atop the Keck Observatory weather mast
Yes, I just went out and bought one. Not just any old unit either… The best precipitation sensor I could locate on the market, a Vaisala DRD11A. The sensor uses a capacitive detection method. Moisture on the top of a plate will change the capacitance and trigger the sensor. The plate is tilted to allow rain to run off and heated to melt snowflakes and sleet. testing on my work bench showed this to be pretty effective and quite sensitive. Yes, you can visualize me sitting at the bench dripping water on a sensor… It worked.

Not that the project was finished there… The new sensor does not come with any sort of network interface. Rather surprising given that just about everything else Vaisala sells has either a serial interface or an ethernet interface. This sensor has just a couple simple outputs… A logic level indication of precipitation, an analog output representing roughly how much, and a frequency output representing the same thing. I needed to interface this unit to the network. As I have installed a terminal server in the rack below the weather mast, at the minimum I needed a serial port.

While I was at it there are a couple other little devices I want to install on the weather mast. These need a network interface as well. May as well put together another little PIC controller and assemble it all together in one neat little package. a few evenings of coding and I had my solution, an interface that allowed remote computer control and status read-back. How many microcontrollers do I have performing little tasks at the summit now? Quite a few.

How bad is it?

One of the miscellaneous systems in the observatory that I have inherited is the weather station. A critical set of gear that has been neglected far too long. Neglected to the point our telescope operators had been complaining, loudly, about a system that frequently gives erroneous data or provides wildly oscillating readings.

Weather mast covered with several inches of ice
The Keck weather mast covered with several inches of ice
The weather station is critical in protecting the all important optical surfaces of the telescope. The mirrors that gather light from distant galaxies depend on a thin coating of aluminum that is easily damaged. Snow, ice, fog or even simple dew can damage the coating and require the mirror segments to go through a laborious re-coating process. Thus the operators monitor the weather closely, when fog and humidity roll in, alarms go off, and the great shutters are closed to protect the telescope.

The first part I have replaced is the humidity and dew point sensor. In many ways the most important part of the system. The new unit is a modern sensor with a direct ethernet interface, simple to link into the observatory network. This is the same sensor used by the National Weather Service in their remote weather stations. All I had to do was spend a little money, and spend a day hanging off the weather tower on the observatory roof installing it. It was a beautiful, warm, sunny day up there, I got the job done, and got a slight sunburn in the process.

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