About the comparison

Theo J. A. van Aerts, a fine customer of ours, ran this comparison to make an educated choice between the two devices—and was kind enough to share his results with Lunatico. Both devices ran simultaneously under the same sky. Before beginning, Theo adjusted the CloudWatcher's Darkness Reference Value from the factory default of 19.6 to 20.15, a small calibration step that aligned the device with his local conditions.

The comparison focused on SQM values of 12 mpsas or higher, which represents TESS-W's operational range. Two scenarios were analysed: general mixed conditions, and specifically dark skies above 18 mpsas.

Results

General conditions

  • Maximum difference between devices: 0.2 mpsas
  • Variance: 0.003

Dark sky conditions (>18 mpsas)

  • Maximum difference: 0.15 mpsas
  • Variance: 0.001

Theo also noted that the CloudWatcher reads more often than the TESS-W (we don't know if this is a limitation of TESS-W itself), so the TESS-W shows more peaks while the CloudWatcher stays much smoother.

"For the first time I got the real correct results for the comparison of both devices and for me the winner is your AAG Cloudwatcher! Extremely accurate and all errors and peaks in the plot are mainly caused by the extreme peaks of the TESS-W! The Cloudwatcher is simply doing a better job and maybe even more accurate than the TESS-W!!??? I don't know, I guess, but I certainly prefer your device." — Theo J. A. van Aerts

CloudWatcher vs TESS-W—SQM comparison chart 1

Reading frequency

A notable practical difference emerged in data consistency: the CloudWatcher produces more frequent readings than the TESS-W, resulting in smoother data curves when plotted over time. The TESS-W showed more pronounced peaks in the data, potentially indicating sampling limitations compared to the CloudWatcher's higher update rate.

CloudWatcher vs TESS-W—reading frequency chart CloudWatcher vs TESS-W—comparison chart

Conclusion

Theo's test confirms that the CloudWatcher's sky quality readings are consistent and accurate relative to the TESS-W across a wide range of sky conditions. The 0.2 mpsas maximum difference is within the expected variation for any two sky quality meters measuring at slightly different positions and times.

For observatories looking for a single device that combines sky quality monitoring with cloud detection, rain sensing, wind measurement and roof automation, the CloudWatcher covers all of these without needing a separate SQM unit.