During a recent observing run near Ft. Davis, TX Dragan Nikin and I had the opportunity to compare frequent readings between our Unihedron SQM-L meters over multiple nights. We had both purchased these instruments with the intent to acquire quantifiable measurements of sky brightness and then able to use those measurements as a relative comparison of sky brightness between observing locations. We both understood the multitude of factors that affect the sky brightness measurements. However we were not prepared, nor are we able to explain, the wide variances we consistently encountered between our readings – acquired at the same times, same observing location, and literally with the instruments physically adjacent to each other and resting on a flat surface, i.e., pointing to the same angular segment of sky.

On every night except for one my meter's readings averaged 0.1 to 0.2 magnitudes darker than Dragan’s. On that one night Dragan's meter was consistently reading 0.1 to 0.2 magnitudes darker than mine. Throughout the week it was not uncommon to see the readings between these meters vary by 0.2 to 0.5 magnitudes. Even on a single meter acquiring a series of back-to-back measurements over a period of 60 to 120 seconds there were sometimes variances of up to 0.5 magnitudes. According to the Unihedron “Each SQM‑L is calibrated using a NIST-traceable light meter. The absolute precision of each meter is believed to be +/-10% (+/-0.10 magnitudes per arcsec squared).” My unscientific sampling does not seem to be in alignment with Unihedron's stated precision.

I’m interested in hearing from other SQM owners about their experiences. What degree of variance do you get in multiple back-to-back readings (after throwing out the first due to the internal circuitry changing temperature on initial power-up)? Have you gathered measurements using multiple SQM meters under the same conditions - what were your findings? Let us know.