Sky Quality Measurement Project - Sky Quality Meter FAQ
Copyright 2008 Unihedron Inc.
What is the difference between the SQM and SQM-L?
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. 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.
What kind of sensor is used in the Sky Quality Meter?
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).
Do you know the contribution from the Milky Way with the wide-angle of acceptance of your photometer? I would like to subtract the Milky Way if possible.
We are in the process of developing a web page tool for correcting the SQM reading for the Milky Way as seen from a given longitude, latitude, SQM reading, and date/time. We are basing this on Schlosser & Hovest (A&AS, 128, 417, 1998) "Collection of Major Surface Photometries of the Milky Way". We need to integrate over MW surface brightness (involving two filters), the responsivity of the SQM with angle, and extinction with zenith angle for each map cell. This is straightforward but time-consuming to verify and so it is not yet available.
See Surface Photometries of the Milky Way (Schlosser+ 1997) for more information
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 that 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 object's 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 a 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.
The "magnitudes per square arcsecond" numbers are commonly used in astronomy to measure sky brightness, below is a link to such a comparison. See the third table in section 10 for a good chart showing how these numbers in magnitudes per square arcsecond relate to natural situations: