Specifications

AUTOSTAR CCD PHOTOMETRY 45
APPENDIX B
Calculating the Air Mass
Introduction
The listed corrected magnitudes of stars are given as they would be
seen outside the Earth's atmosphere. These values are the values
found in books and tables that list a star's extraterrestrial
magnitude.
When starlight passes through the Earth's atmosphere, it is
diminished by a variable factor. This attenuation, known as
atmospheric extinction, is caused by absorption of some of the
starlight by the atmosphere. The greater the thickness of the
atmosphere the starlight passes through, the greater the extinction.
Observations made at observatories located at high altitude have
less extinction than those at a lower altitude. This is one reason
why most major observatories are located on mountain tops.
Observations made directly overhead at the Zenith have the least
extinction for a given location. The extinction increases as a star is
viewed further from the Zenith, and is at a maximum at (or below)
the horizon. The section of atmosphere that the starlight travels
through is known as the Air Mass.
One of the first steps in determining a star's magnitude from
measurements taken at the Earth's surface is to know the
extinction. The key factor in knowing the extinction is in knowing
the star's air mass at the time of the observation.
The following is an explanation of how to determine air mass for a
given location, time, and star position. While seemingly simple,
there are many pitfalls in determining the air mass.