Object Visibility Calculator (Airmass)

More atmosphere means more absorption, scattering, and turbulence. High airmass degrades image quality, reduces brightness, increases color effects (differential refraction), and makes photometric measurements less accurate. Observatories schedule critical observations near transit when airmass is minimized.

At sea level on a clear night: 0.20-0.25 mag/airmass in V band. At excellent mountain sites (Mauna Kea, La Palma): 0.10-0.15 mag/airmass. In hazy conditions or at low altitude: 0.30-0.50. Extinction is also wavelength-dependent: blue light is more heavily extincted than red.

The Hardie (1962) formula improves on the simple secant approximation for airmass. It adds polynomial correction terms that account for the curved atmosphere, giving results accurate to better than 0.1% down to about 5 degrees altitude where the simple secant overestimates airmass by several percent.

With standard optics, no. However, refraction can make objects appear slightly above the horizon when they are actually geometrically below it (by up to about 0.5 degrees). This calculator does not model extreme refraction near the horizon.

At low altitudes, the atmosphere bends blue light more than red light. This smears stellar images into a vertical spectrum and is a major problem for spectroscopy at low altitudes. The effect is negligible above 30 degrees altitude but significant below 20 degrees.

Higher airmass means fainter images, more noise, worse seeing, and more color aberration. For deep-sky imaging, most astrophotographers prefer targets above 30-40 degrees altitude. Narrowband filters (H-alpha, OIII) are more tolerant of high airmass because they are less affected by atmospheric extinction.

Seeing refers to atmospheric turbulence that causes stars to twinkle and telescope images to blur. High airmass means the light path passes through more turbulent air layers, generally worsening seeing. However, seeing is also highly dependent on local atmospheric conditions and is not simply a function of airmass.

Photometrists measure standard stars at multiple airmasses throughout the night to determine the extinction coefficient. They then correct all measurements back to zero airmass (above atmosphere). This allows comparison of brightness measurements made at different times and altitudes.

The path length (airmass) is the same, but extinction depends strongly on wavelength. The atmosphere absorbs and scatters blue light much more than red or infrared light. Extinction coefficients must be measured separately for each photometric band (U, B, V, R, I, etc.).