The names Unihedron and Sky Quality Meter have become synonymous with measurement of sky brightness. Indeed, it’s hard to imagine a tool that has had greater impact on quantification of local light pollution.
Unihedron’s original SQM set the standard and has since evolved into a comprehensive range of specialized tools, including the SQM-L with lens for focusing on a narrow field-of-view, the SQM-LU with USB connectivity, the SQM-LE with Ethernet connectivity, the SQM-LR with RS232 connectivity and the SQM-LU-DL, a narrow field-of-view, USB-connected version with data-logging capability.
Unihedron’s SQM-LU-DL is specifically designed to deliver continual, connected, autonomous data-logging of sky brightness in magnitudes per square arcseconds. Why a data-logging version? Because relative sky brightness is variable, even beyond cyclical lunar phases.
How variable is the sky brightness of your favorite observing or imaging site? When are you most likely to encounter darkest local skies? Answering such questions requires accurate, continuous quantitative data, and Unihedron’s SQM-LU-DL is the most-affordable, most-accurate and most-convenient tool for gathering that data.
Potential uses for the SQM-LU-DL include: measuring the night-sky brightness during remote biological field experiments; comparing sky brightness at different sites quantitatively; documenting the evolution of light pollution in your area, setting planetarium dome illumination to mimic the skies people are likely to experience elsewhere in your city; monitoring sky brightness throughout a single night, from night-to-night, and from year-to-year; determining which nights show the greatest promise for finding those faintest fuzzies; calibrating the effect of sky brightness on qualitative measures such as the Bortle Scale; investigating how sky brightness correlates with the solar cycle and month-to-month sunspot activity; helping provide local ground truth for future sky brightness prediction with the Clear Sky Clock; and allowing CCD users to correlate between SQM readings and when the background reaches specific ADC levels.
The SQM-LU-DL includes an infrared blocking filter to restrict measurements to the visual bandpass and features field-upgradeable firmware. Its maximum light-sampling duration is 80 seconds and its minimum is one second. Its case measures a compact 5.5 by 2.6 by 1.1 inches, and it is powered by an included 5- to 6-volt DC adapter.
The basic SQM-LU-DL package is priced at $249.99US and includes the SQM-LU-DL unit, a printed user manual, as well as CD and PDF manuals and example software, a 15-foot USB cable, a battery-to-USB adapter with six AA battery cells and a battery holder, and a calibration sheet that indicates the unit USB serial number and calibration values.
A custom waterproof housing can be included for $39.95US. It features white surfaces to better reflect hot summer sunshine, a bottom ventilation port to allow moisture to escape, a scratch-resistant glass top of small aperture to minimize greenhouse heating, and sealed cable ports.