Skybadger Observatory
Astronomer and tinkerer, This site is about projects in Astronomy..
Bibliography.
A Sky quality Meter (SQM) is the sensor that tells the observatory the state of the sky. It provides two values - the sky temperature and the sky brightness. The Sky brightness tells us how dark the sky is - typically for me at my location it varies from 17 mags/square arc-sec at full moon to 20.6 on the darkest nights. The device also tells me the ambient temperature of the sensor, which is useful at night but less so during the day when insolation - heating of the box fro the sun, raises it well above what you might consider the real temperature. The sky brightness reading helps to corroborate the all sky camera in terms of idenitfying whether its cloudy or night since hte brightness increases when there is more cloud due to light pollution. The sky temperature reading gives a more direct reading of when its clear, since the sky temperature drops below zero, as far as -35, when there are no clouds in the sky due to that being the ambient temperature of the bulk of the atmosphere above the cloud layer. The sky termperature reading is also subject to interpretation however. When my prevailing conditions apply, it will be clear at any termperature reading below 5 degrees C. somewhere between 5-8 indicates thin haze and anything above that full cloud. However when the wind turns round to from the East, It can still be thick cloud down to -12 degrees. What I think this means is that the weather from the East is inherently colder than from the SW and so I need to check the wind directions to draw clear sky conclusions. I also need to check the all sky camera but until I have coded some way of measuring the sky 'clearness' from the visual approach , the SQM remains the best way. Building The SQM consists of the common TLP4122 sensitive light sensor paired with the Melexis 9142 Infrared thermometer. The latter provides the ambient and sky termperature reasdings, while the first is a multi-range device that you can set the gain and collection time on. This is the same family of devices used in the Unihedron. Both of these devices use the I2C bus to present their readings. I implemented my standard pattern ESP8266/MQTT/web firmware with the i2c interface talking to the above devices. The readings are reported to the MQTT messaging service and visualised by the MQTT listener at the Node-Red dashboard for all sensors. Its a fairly straightforward assembly of voltage regulator, ESP8266-01, i2c bus pullups and the devices themseles. The code provides a routine for the light sensor to control the gain and collection time to keep the values in the centre of the range. The sensor is franed by a led torch lens focuser to collect asmuch light as pssible from the sky and covers an effective field of about 30 degrees.