Atmospheric Sensing for ALPACA Observing Conditions

Instrument-based sky radiometry using AS7343 spectral sensing and MLX90614ESF LWIR radiometry. For continuous observatory operation and ALPACA ObservingConditions reporting.

Sky Spectral Radiance — AS7343

Multi-channel visible/NIR radiometry for quantitative sky brightness and spectral composition.

Sensor Characteristics

Discrete spectral channels spanning visible and near-infrared
Band-limited radiometric sampling with stable digital integration control
Low dark current and repeatable gain behavior for long-term trending
Reduced sensitivity to focus, star-field density, and tracking compared to imaging methods

Astronomical Relevance

The AS7343 is used to quantify sky spectral radiance (not imaging detail), supporting observatory-grade analysis of:

Moonlight contribution (phase/altitude) and twilight progression
Broadband brightening and color shifts from thin cloud / haze scattering
Airglow/auroral contamination and long-term site stability (light pollution drift)

ALPACA Mapping

AS7343 measurements contribute to SkyBrightness (relative or calibrated units) and can support derived SkyQuality when a site calibration model is applied. The system avoids star-count heuristics; it provides stable radiometric inputs suitable for downstream automation.

Radiative Sky Temperature — MLX90614ESF

Long-wave infrared (8–14 µm) radiometry in the atmospheric window for physics-based cloud indication.

Sensor Selection Rationale

The MLX90614ESF is a calibrated thermopile radiometer sensitive in the 8–14 µm atmospheric window, commonly used in professional atmospheric sensing for emissivity-based cloud detection and radiative sky temperature estimation.

Physical Measurement Principle

Clear skies exhibit strong radiative cooling and typically measure 20–40 °C colder than ambient (site/season dependent). Cloud, fog, and high humidity increase effective emissivity, driving the measured sky temperature toward ambient.

ALPACA Mapping

The MLX90614ESF provides the authoritative value for SkyTemperature. Combining this with ambient temperature yields the sky–air difference (ΔT), a robust discriminator for clear sky, thin cirrus, overcast, and fog/high-humidity conditions.

Complementary Sensor Fusion

Optical radiance and thermal emissivity respond differently to atmospheric conditions. Used together, AS7343 and MLX90614ESF provide cross-validation by separating optical scattering from infrared emissivity.

Condition	AS7343 (Optical)	MLX90614ESF (Thermal)
Clear, moonless	Low spectral radiance	Large negative sky ΔT
Moonlit	Elevated visible bands	Large negative sky ΔT
Thin cirrus	Increased scattered light	Moderately elevated sky temperature
Overcast	Broadband increase / flattened spectrum	Sky temperature near ambient
Fog / high humidity	Flattened spectrum, reduced contrast	Sky temperature near ambient

ALPACA Compliance Philosophy

The ALPACA ObservingConditions specification does not prescribe sensors—only outcomes. This implementation adheres to the intent of the standard by reporting measured physical quantities with transparent traceability from sensor to parameter.

Measured physical quantities (not camera-only heuristics)
Stable calibration behavior appropriate for long-term trending
No opaque “clear/cloudy” declaration without supporting context
Downstream clients (ACP, NINA, custom schedulers) apply site-specific thresholds

Long-Term Observatory Use

These sensors are designed for continuous outdoor operation with predictable long-term behavior. Unlike imaging sensors, there is no concern for pixel burn-in, no loss of sensitivity from daylight exposure, and no dependence on optical focus or image quality—making the subsystem suitable for robotic observatories, remote sites, and permanently installed monitoring stations.