Sky Quality Meter FAQ

SQM Questions

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.

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.

SQM & SQM-L Questions

What is the difference between the SQM and SQM-L?

Here is a chart showing the differences at a glance.

The main difference is the field of view. The SQM-L (with lens) is an improvement over the SQM. The lens collects more light from a smaller cone so that the meter is not affected from lights or shading on the horizon.

The SQM spec for field of view is located in this technical report. A comparison of the angular response for both meters is here. Generally speaking, the SQM-L 'Half Width Half Maximum' is ~10 degrees as opposed to ~42 degrees for the SQM.

The SQM-L is better suited for astronomy and dark sky enthusiasts. It has a lens to narrow the field of view so that street lights and buildings or trees do not affect the reading very much.

If you expect to always take readings at dark sky sites in an open field then the regular SQM will do fine for that task.

Note: The SQM-LE has the same optics as the SQM-L.

Does the SQM or SQM-L have an external port that can be connected to a PC?

No, the SQM and SQM-L do not have an external port. These models have a minimum of components to reduce costs, and they cannot communicate with a PC.

The SQM-LE has the same optics as the SQM-L as well as a computer interface via an Ethernet connection. If you intend on just connecting the unit to a laptop rather than a network, an Ethernet crossover cable can be used.

Meter makes a quiet clicking noise instead of providing a reading.

Try replacing the battery with a fresh one.

How can I get the model and serial number from the SQM or SQM-L?

The temperature in degrees Celsius then degrees Fahrenheit are displayed when you press and hold the button a second time. Also, the model and serial number are displayed after the temperature.

For example, press and release the button once. While the display is still showing something, press and hold the button and watch the following results:

1. Temperature in degrees Celsius. This is the temperture inside the unit, not the outside temperature.

2. Temperature in degrees Fahrenheit. This is the temperture inside the unit, not the outside temperature.

3. Model number, like _2.17. Model 1.xx is the SQM, Model 2.xx is the SQM-L. The last two digits are the firmware revision.

4. Serial number, like 3647.

General SQM Questions

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.

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).

How does transparency affect the SQM readings?

The SQM's readings are assuming 'best transparency'.

You can get an updated definition of the transparency in your area from Attila Danko's Clear Sky Clock. Also, frequently local weather stations can provide "visibility" and "relative humidity" numbers which could potentially be used as surrogates for actual transparency measurements (which aren't possible with a handheld meter).

How does zodiacal light affect the SQM readings?

It is likely to be less than a 1 or 2 percent effect. The primary reason is that the brightest and widest part of the zodiacal light is nearest the horizon where the SQM has almost no sensitivity (due to it being a zenith-looking device). The portions at higher altitude are the narrowest and faintest and they would barely creep into the sensitivity cone of the SQM.

What are "Magnitudes per Square Arc Second"?

Magnitudes are a measurement of an objects brightness, for example a star that is 6th magnitude is brighter than a star that is 11th magnitude.

The term arcsecond comes from an arc being divided up into seconds. There are 360 degrees in an circle, and each degree is divided into 60 minutes, and each minute is divided into 60 seconds. A square arc second has an angular area of one second by one second.

The term magnitudes per square arc second means that the brightness in magnitudes is spread out over an square arcsecond of the sky. If the SQM provides a reading of 20.00, that would be like saying that a light of a 20th magnitude star brightness was spread over one square arcsecond of the sky.

Quite often astronomers will refer to a sky being a "6th magnitude sky", in that case you can see 6th magnitude stars and nothing dimmer like 11th magnitude stars. The term "6th magnitude skies" is very subjective to a persons ability to see in the night, for example I might say "6th magnitude skies" but a young child with better night vision might say "7th magnitude skies". You can use this nifty calculator created by SQM user K. Fisher to do that conversion, or this chart.

Each magnitude lower (numerically) means just over 2.5 times as much more light is coming from a given patch of sky. A change of 5 mags/sq arcsec means the sky is 100x brighter.

Also, a reading of greater than 22.0 is unlikely to be recorded and the darkest we've personally experienced is 21.80.

Reading accuracy

The value produced by the sensor in the SQM is affected by temperature. There is a temperature sensor in the SQM that compensates for this effect. However, when the SQM is first powered up, the light sensor is colder than when the power has been on for a few seconds. Depending on the ambient temperature this will result in the first reading being slightly higher than subsequent readings.

For the most accurate results, it is best to take many readings and disregard the very first reading.

This issue is due to the transient response of the TSL237 which briefly changes its light-to-frequency characteristic when it is warmed by applied power. Ironically, the normally very sensible practice of leaving it out at the environmental temperature probably makes it more significant.