SteadySky

Author: Alan Catovic

Current Seeing

Live

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Arcseconds FWHM

r₀ (cm)

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τ₀ (ms)

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Coherence time

θ₀ (arcsec)

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Isoplanatic angle

σ_scint (%)

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Scintillation rms

Data: Open-Meteo API | —

FWHM — Full Width at Half Maximum: angular diameter of the seeing disc in arcsec; lower = better (excellent <1″, poor >3.5″). r₀ — Fried parameter (cm): coherence length of the atmosphere; apertures larger than r₀ are turbulence-limited, not diffraction-limited. τ₀ — Coherence time (ms): timescale over which the wavefront changes significantly — sets the maximum frame rate for lucky imaging and AO loop speed. θ₀ — Isoplanatic angle (arcsec): angular radius over which a single AO correction remains valid; determines maximum guide-star separation. σ_scint — Scintillation rms (%): photon-arrival intensity noise caused by turbulence; sets the photometric noise floor.

Atmospheric Parameters

Temperature

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Wind Speed

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Humidity

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Pressure

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Dew Point

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ΔT dew

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✓

Dew Point — temperature at which air becomes saturated and condensation begins on surfaces. Always keep optical surfaces above this value. ΔT dew — safety margin between current temperature and dew point: ΔT >5°C is safe (no heater needed); 2–5°C use low-power heater (25–50%); <2°C run full-power heater to prevent condensation.

Wind Analysis

Beaufort

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Wind Chill

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°C felt temp

Moon & Sky

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🌑

Computing...

Illumination

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🌙 Rise

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🌙 Set

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Deep-Sky Impact:

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☀️

Sun & Twilight

☀ Sunrise

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☀ Sunset

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Civil Twilight

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Nautical Twilight

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Astronomical Twilight

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Solar Decl.

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Day Length

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🔭 Astro Night

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AO / Strehl Analysis

Aperture for Strehl (mm)

Strehl

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D/r₀

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Lucky %

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Good frames

Scintillation vs Airmass

AO / Lucky Imaging Recommendation:

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Strehl ratio (0–1) quantifies wavefront quality: S = 1.0 is perfect diffraction-limited performance; S < 0.8 represents noticeable seeing degradation. D/r₀ is the fundamental turbulence parameter: when D/r₀ < 1 the telescope is diffraction-limited; between 1–3 lucky imaging is effective; above 3 full AO is needed; above 6 severe turbulence dominates. Lucky % estimates the fraction of short-exposure frames where the instantaneous PSF is close to diffraction-limited — usable for frame-selection stacking. The Scintillation table shows photometric noise σ (%) vs. zenith angle for the entered aperture — values above 1% limit high-precision photometry.

24-Hour Forecast

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🟢 Excellent (<1.5") 🔵 Good (1.5-2") 🟡 Fair (2-3.5") 🔴 Poor (>3.5") ⭐ Best observation windows

Loading forecast data...

Jet Stream Wind Profile

Wind speed through atmospheric layers (5-20 km). High jet stream speeds indicate turbulence at tropopause level, degrading seeing.

Transparency vs Seeing

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Transparency

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Limiting Magnitude

Note: Seeing affects resolution (turbulence). Transparency affects depth (extinction, aerosols).

Equipment Optimizer

Dew Point Calculator

Dew Point

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Temp Delta

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✓

Dew Status

Heater Recommendations:

Cloud Cover Forecast

Low (0-2km)

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Mid (2-6km)

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High (6-12km)

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Total Cloud Cover

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Atmospheric Dispersion Calculator

Differential atmospheric refraction displaces short wavelengths more than long ones along the altitude axis, smearing stellar images into a spectrum. The effect grows rapidly with zenith angle and is independent of aperture. An ADC (Atmospheric Dispersion Corrector) is a pair of contra-rotating prisms that compensates this shift; it becomes essential when dispersion exceeds ~50 % of the seeing disc. The chart uses the Edlén/Ciddor formula for differential refraction: Δε = Δn · tan(z), where Δn is the refractive index difference between the two edge wavelengths of the selected bandpass. Colour zones: green <1″ (usually acceptable) · amber 1–2″ (ADC recommended) · red >2″ (ADC essential). Input fields are pre-filled from the current weather data.

Temperature (°C)

Pressure (hPa)

Bandpass

Telescope Thermal Equilibration

Mirror Diameter (mm)

Mirror Thickness (mm)

Material

ΔT Storage-Ambient (°C)

Thermal equilibration is critical for image quality. A mirror that is warmer than ambient creates a convection column (tube currents) that severely degrades seeing locally, often worse than the atmosphere itself. Rule: allow 30 min per 5°C ΔT as a baseline.

FITS Header Generator

Generate FITS header keywords for calibration and metadata.

Target Calculator

ADC Recommendation:

Session Logger

Sky Quality Meter

Check the light pollution map for your exact coordinates to get an accurate SQM reading. The estimated value below is computed from real-time cloud cover and humidity.

Manual SQM from map (mag/arcsec²):

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mag/arcsec²

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Naked Eye Limit:

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SQM (Sky Quality Meter) measures sky brightness in mag/arcsec²: higher = darker sky. The Bortle scale (1–9) classifies sites from pristine dark (1) to inner-city (9). Use lightpollutionmap.info with the SkyBrightness 2025 overlay to read your local SQM from the map and enter it manually above for a precise reading. The automatic estimate uses real-time cloud and humidity data as a proxy.

Multi-Site Comparison