Specifications

46 AUTOSTAR CCD PHOTOMETRY
Getting Started
A star's Air Mass, X, is the effective path length of air through
which starlight has passed to reach the observer. By definition,
X = 1.00 at the Zenith (straight overhead) and increases as one
observes stars closer to the horizon. Letting Z be the angular
distance of a star from the Zenith (0° ≤ Z ≤ 90°), then the simplest
relationship is:
X = sec Z
This would be correct if the Earth and its atmosphere were flat.
However, the Earth's curvature causes the relationship to
overestimate the air mass for large zenith distances. The correct
configuration is illustrated in Figure B-1.
a
b
c
a
b
c
Earth
Zenith
Horizon
Atmosphere
c > b > a
Atmospheric
Distances
Air Mass c
is much greater
than Air Mass a
+
+
+
Figure B-1. Illustration of a Star's Air Mass
(Accounting for Curvature of the Earth’s Atmosphere).