Airmass: Why Stars Fade Near the Horizon

A low target means a longer path through air

Straight overhead is roughly one airmass. At lower altitude, light slants through more atmosphere before reaching you. At 30 degrees above the horizon, a simple flat-atmosphere estimate gives about two airmasses. Do not extend that approximation all the way to the horizon: Earth's curvature and the atmosphere's structure matter there. Airmass describes the path, not a direct measurement of how clear the sky is.

Reference: AAVSO: DSLR Observing Manual

Extinction reduces the light that reaches the telescope

Air molecules, aerosols, and other material scatter or absorb some of the light. The loss depends on wavelength and conditions. In visible light, blue is often attenuated more than red, so a star may look warmer near the horizon without changing intrinsically. This is different from seeing, which describes how air turbulence blurs and distorts an image. Both problems can affect the same observation.

Reference: ESO: La Silla Extinction Coefficients

Put a magnitude difference in context

Suppose an observing site has an extinction coefficient of 0.2 magnitude per airmass in the band you are using. Under the same conditions, a star at two airmasses would appear 0.2 magnitude fainter than at one airmass. That is an illustrative coefficient, not a value to apply everywhere. Site, wavelength, dust, and variable haze can change the actual loss.

Reference: ESO: La Silla Extinction Coefficients; AAVSO: DSLR Observing Manual

Compare one star as it climbs

Use the same instrument and watch a bright star at two elevations in one evening. Note color, apparent brightness, and haze separately. If doing a camera comparison, keep settings fixed and avoid saturating the star. For a faint object you have not found yet, try again nearer its highest point in the sky instead of assuming the chart was wrong.