SQM-LU Questions

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. You can read more about Extech meters here

Do you have any benchmarks for linking magnitudes per square arc sec with the Bortle scale?

We believe that if you check this Wikipedia Bortle Dark-Sky Scale link, the descriptions associated with each mag/sq arcsec are sufficiently detailed that you could draw up a pretty decent correspondence.

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 webpage 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 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.

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:

  www.stjarnhimlen.se/comp/radfaq.html

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.