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Sky Quality Meter - FAQ

What kind of sensor is used in the Sky Quality Meter?
A TAOS TSL237S sensor is used, you can view the specs here. The sensor is covered with a HOYA CM-500 filter, you can view the spectral response curves here.

Do you provide a calibration certificate?
There is no calibration certificate available. The NIST meter that we use to calibrate against is the EXTECH Instruments Model 401027. You can read more about Extech meters here

Does the Sky Quality Meter have an external port that can be connected to a PC?
No, the SQM does not have an external port.

What is the field of view for your device?

The "effective solid angle" is 1.532 steradians. The angular response if effectively that in the TSL237S datasheet.

It is worth pointing out that this is not a "spot" meter - it accepts light from a wide cone - roughly 80 degrees diameter on the sky (we measured the effective solid angle to be 1.532 steradians). To produce a spot meter, a fast lens and mounting hardware would have been required and this would have dramatically increased the price. In practice, we believe the reading is representative of the range of altitudes over which observers would typically observe.

Do you have any benchmarks for linking magnitudes per square arc sec with the Bortle scale?
We believe that if you check this Light pollution report by Richard Berry, the descriptions associated with each mag/sq arcsec are sufficiently detailed that you could draw up a pretty decent correspondence.

How was the solid angle of the detected light chosen?

We picked the large solid angle of the detector partly for greatest sensitivity and partly to be representative of the sky conditions over the part of the sky where people would normally be observing and imaging. It is straightforward to reduce this solid angle with a mask, but different observers have different preferences for beam size and so our design offers the maximum flexibility and customizability. The adoption of a different solid angle would require a fixed zeropoint correction to the meter reading.

For equivalent sensitivity at a smaller solid angle, a lens and mounting hardware would be necessary, significantly increasing the cost of the unit for little added functionality.

Have you measured the spectral response of the detector with the IR rejection filter? How closely does it match the response of the human eye?
We haven't measured the spectral response curve ourselves, but the sensor manufacturer has. It is very close to that of the human eye. The Hoya CM-500 filter cuts off the entire infrared part of the spectrum. The response is that of the "clear" line in Figure 2 of the TCS230 datasheet (which is for a different sensor in the TAOS line).

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