RS232 communication protocol for AAG CloudWatcher — v1.4

Relevant hardware changes since the last (1.3) document

New CloudWatcher units have replaced traditional light sensors (which measured sky brightness) with a new sensor able to measure the quality of the sky in terms of Magnitudes Per Square Arc/Second (MPSAS).

New firmware and underlying communication protocol have been updated to support these new sensors.

Commands added or modified since document Rs232_Comms_v130

From firmware versions 5.89 onwards.

Command "C!"

"C!" previously returned 4 values (see table below). If, and only if, the new light sensor is installed, an extra fifth value is included specifying the raw period (in 1/250000 sec) read by the new light sensor. For convenience, the table below includes all values, previous and new:

Notes on this command

• As always, developers should not rely on a fixed size of the response to a command. Use the handshaking block as an end-of-response marker.
• The order of each response block must not be assumed. Order may change in future versions of the firmware.
• Raw frequency of the new sensor is converted into MPSAS (Magnitudes Per Square Arc/Second) by the Windows Software and SOLO using the following formula:

  mpsas = SQReference - 2.5 * Log10(250000/period)

  Where SQReference is set to 19.6 by default, but is configurable in both Windows Software and SOLO to better tune the sensor.

  The final "mpsas" value also includes a small correction because of ambient temperature, using this formula:

  mpsas_corrected = (mpsas - 0.042) + (0.00212 * temperature)

  Where "temperature" is the ambient temperature in degrees Celsius. This correction is also applied in both the Windows Software and the SOLO.

• If the new light sensor is installed, the value of block "!4" (LDR voltage) is estimated by the firmware and maintained for backward compatibility. Developers are encouraged to use "!8" instead if available, since it is not estimated and represents a more accurate sky quality measurement.