Build a scene
Atmosphere and sky
Extinction, refraction, airmass and the sky background at the station.
Extinction
Section titled “Extinction”The extinction coefficient k (magnitudes per airmass, V band) is the loss of brightness for a vertical crossing of the atmosphere. It depends on the site and the night: about 0.11–0.15 on dry high mountains, 0.2–0.3 in the lowlands. The extinction applied is k × X, where X is the airmass.
Two ways to set k in the Station tab:
Use the value of the site, or your own measurement.
Schaefer’s (1998) empirical model, computed offline from the site altitude and latitude, the season, relative humidity, temperature and an aerosol load factor (1 = typical, 0.5 = very clean air, 2 to 4 = haze or dust). The four terms (molecular scattering, aerosols, ozone, water vapour) are shown. Typical uncertainty ±0.05.
Airmass
Section titled “Airmass”The thickness of atmosphere crossed, relative to the zenith (X = 1), from the apparent elevation (Kasten & Young): about 2 at 30°, 5.6 at 10°.
Refraction
Section titled “Refraction”The atmosphere bends light and makes the object look higher than it is: about 0.5° at the horizon, 5′ at 10°, under 1′ above 45°. LightCurve uses Sæmundsson’s formula, corrected for the pressure at the station altitude and the temperature. The apparent elevation is used for the minimum elevation, the airmass and the display.
Sky background
Section titled “Sky background”| Value | Meaning |
|---|---|
| Night background of the site | Zenith sky on a dark night, without Moon or twilight (mag/arcsec²) |
| Estimated background toward the satellite | Night background + twilight (from the Sun’s elevation) + scattered moonlight (Krisciunas & Schaefer), in the direction of the satellite. It enters the SNR |
mag/arcsec² is the same unit as the mpsas of SQM meters: the larger, the darker.