Sky Quality Calculations¶
SQMeter converts TSL2591 light readings into three astronomical metrics. The conversion is only as good as the optical build and calibration: a bare TSL2591 is not a calibrated SQM instrument because it has a very wide angular response and can collect stray light from the horizon, ground, buildings, vehicles, and the enclosure.
SQM — Sky Quality Meter¶
Measures sky brightness in magnitudes per square arcsecond (mag/arcsec²). Higher is darker.
Firmware averages raw TSL2591 counts before this conversion. The night SQM path uses MAX gain and 600 ms integration, applies a rolling average, subtracts the saved dark visible offset, then applies the optional SQM calibration offset.
Dark calibration should be repeated whenever the lens, baffle, aperture, lens-to-sensor distance, or internal finish changes.
Typical values:
| Location | SQM |
|---|---|
| Excellent dark site | > 22 |
| Rural sky | ~21.5 |
| Suburban sky | ~19–20 |
| City centre | < 17 |
NELM — Naked Eye Limiting Magnitude¶
The faintest star visible to the naked eye under current conditions. Estimated from the SQM value using empirical formulae based on atmospheric conditions.
Higher = more stars visible. A dark rural sky gives NELM ≈ 6.5. Suburban skies typically give 4–5.
Bortle Dark Sky Scale¶
| Class | SQM Range | Description |
|---|---|---|
| 1 | > 21.99 | Excellent dark-sky site |
| 2 | 21.89 – 21.99 | Typical truly dark site |
| 3 | 21.69 – 21.89 | Rural sky |
| 4 | 20.49 – 21.69 | Rural/suburban transition |
| 5 | 19.50 – 20.49 | Suburban sky |
| 6 | 18.94 – 19.50 | Bright suburban sky |
| 7 | 18.38 – 18.94 | Suburban/urban transition |
| 8 | 17.00 – 18.38 | City sky |
| 9 | < 17.00 | Inner-city sky |
Cloud Detection (MLX90614)¶
When an MLX90614 IR thermometer is present, SQMeter estimates cloud cover by comparing sky IR temperature to ambient temperature. A large negative delta (sky much colder than ambient) indicates clear sky. A small delta indicates cloud cover blocking the sky's thermal emission.
This is a heuristic and works best in dry climates. Humidity and fog affect the reading.
Implementation¶
See src/calculations/SkyQuality.cpp and src/calculations/CloudDetection.cpp.