Air Mass Calculator
Calculate atmospheric air mass to understand extinction effects and optimize observing times. Essential for photometry, spectroscopy, and high-quality imaging.
Observation Parameters
Altitude angle above horizon (0° = horizon, 90° = zenith)
Angle from zenith (0° = zenith, 90° = horizon)
Note: Enter either altitude OR zenith angle - they will be automatically synchronized
Height above sea level (optional, for precise calculations)
Atmospheric temperature (optional)
Barometric pressure (optional)
Quick Altitude Selection
Air Mass Results
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Atmospheric Path Visualization
Air Mass Calculation Methods
Simple Secant Formula
X = sec(z)
Where z is the zenith angle
Accurate for zenith angles < 60°
Kasten-Young Formula (1989)
X = [cos(z) + 0.50572 × (96.07995 - z)^(-1.6364)]^(-1)
More accurate for large zenith angles
Accounts for atmospheric refraction
Pickering Formula (2002)
Most accurate for all zenith angles
Includes temperature and pressure corrections
Used for precise photometric work
Atmospheric Extinction Effects
Extinction by Wavelength
| Band | Wavelength (nm) | Extinction (mag/airmass) | Notes |
|---|---|---|---|
| U (Ultraviolet) | 365 | 0.60 | Heavily affected by ozone |
| B (Blue) | 445 | 0.40 | Strong Rayleigh scattering |
| V (Visual) | 551 | 0.20 | Standard reference band |
| R (Red) | 658 | 0.10 | Less atmospheric absorption |
| I (Infrared) | 806 | 0.05 | Minimal extinction |
Extinction Sources
- Rayleigh Scattering: Blue light scattered more than red
- Aerosol Scattering: Dust, pollution, water droplets
- Molecular Absorption: Ozone, water vapor, oxygen
- Atmospheric Turbulence: Seeing degradation
Observing Guidelines by Air Mass
X < 1.2 (Altitude > 56°)
Quality: Excellent
- Minimal atmospheric effects
- Best for photometry
- Optimal seeing conditions
- All types of observations
X = 1.2-1.5 (Altitude 41-56°)
Quality: Very Good
- Slight extinction increase
- Good for most observations
- Acceptable for photometry
- Minor seeing degradation
X = 1.5-2.0 (Altitude 30-41°)
Quality: Good
- Noticeable extinction
- Adequate for imaging
- Photometry requires corrections
- Seeing starts to degrade
X = 2.0-3.0 (Altitude 20-30°)
Quality: Fair
- Significant extinction
- Imaging quality reduced
- Photometry challenging
- Poor seeing conditions
X > 3.0 (Altitude < 20°)
Quality: Poor
- Severe atmospheric effects
- Avoid precision observations
- Strong color effects
- Very poor seeing
Practical Applications
Photometric Observations
- Plan observations at minimum air mass (transit)
- Apply extinction corrections to measurements
- Use multiple air masses for extinction determination
- Consider spectral response of your equipment
Imaging Guidelines
- Image targets when X < 2.0 for best quality
- Longer exposures needed at high air mass
- Color balance shifts with air mass
- Guide star performance degrades
Spectroscopy Considerations
- Atmospheric lines become stronger
- Telluric correction becomes critical
- Wavelength-dependent extinction
- Consider differential atmospheric refraction
