Unihedron SQM-LU-DL, Operator'S Manual

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3 Theory of operation
Figure 3.1: SQM-LU-DL Block diagram
3.1 Light measurement
The SQM-LU-DL measures the darkness of the night sky to provide readings of magnitudes per square arc second through
the USB connection.
A light to frequency sensor (TSL237) provides the micro-controller with a signal whose frequency depends on the
amount of incoming light. And, readings from the temperature sensor are used to compensate the light sensor readings
through the range of operating temperatures.
In bright skies, the sensor produces a high frequency square wave (up to 500kHz). As the sky gets dark, the sensor
produces a low frequency. In absolute darkness, the sensor period is very long, and is defined on the calibration sheet as
“Dark period” with a 300s timeout.
Measurement of this wide range of frequencies is done by timers and counters inside the microprocessor. It is difficult for
a microprocessor to accurately measure the period of high frequency signals, and just as difficult to measure the frequency
of extremely low frequency signals. For this reason the microprocessor has two modes of signal measuring:
Frequency mode : The frequency mode is for bright readings where the frequency is high. The sensor signal is fed
into a counter that is gated at one second intervals to report a frequency measured in Hz.
Period mode : The period mode is for dark readings where the frequency is low. The sensor signal gates a high
speed internal counter which reports the period measured in µ s.
Both Frequency and Period results are available when connected directly to the meter and using the “rx” command.
The SQM-LU-DL computes mpsas by deciding if the period or frequency report should be used and then compensating
for temperature. Deciding between period or frequency mode reporting was originally done at a frequency of 354Hz, but
by selecting the “Ideal crossover firmware” setting on the “Report interval” page, the switchover will be 679Hz which
offers the best resolution at the switchover point.
The SQM-LU-DL holds the updated period and frequency recordings in memory while waiting for an asynchronous
user request for a reading. It is possible for the user to see stale readings if the light sensor cannot gather enough light
to produce a reading before the user asks for the next report (i.e. using the “rx” command too frequently). Starting at
firmware version 58, the SQM-LU-DL can provide a status of the reading that the user has requested by using the “r1x”
command which contains a status suffix character of F/P/S whose meaning is:
F : fresh FREQUENCY mode reading.
P : fresh PERIOD mode reading.
S : STALE reading indicates that this reading has not changed since the last reading because there was not enough
light gathered to produce a new reading.
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