5.2 Altitude and Azimuth
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5.2 Altitude and Azimuth
The simplest coordinate system you will encounter in astronomy is the altitude and
azimuth system, also known as the horizontal system, but more commonly known as
AltAz. AltAz (or Alt-Az) is based on the two great circles formed by the observer’s
horizon and celestial meridian line. The celestial meridian line passes through the
zenith, the point directly above the observer, and the nadir, the point directly below
the observer, intersecting the north and south cardinal points. For an observer in the
northern hemisphere, the origin is the point on the horizon due south of the observer.
For observers in the southern hemisphere, it is the point on the horizon due north of
the observer.
The AltAz system is determined by factors local to the observer, and it is not fixed
to the celestial sphere. Hence, astronomical objects transit through the AltAz grid
during the course of the night as the Earth rotates. However, it is the AltAz grid that
is used to point a telescope, and the observer must translate between the coordinate
system of the target and AltAz. For an observer in the northern hemisphere, an object
is located using its azimuth (A), the object’s angular position along the observer’s
horizon, measured eastwards from the most northerly point on the horizon, and its
altitude (α), the angular elevation above the horizon. Hence, for an observer in the
northern hemisphere, an object lying due south on the horizon has an Alt = 0
◦ and
Az = 180
◦ , whilst the point lying due north on the horizon has Alt = 0
◦ and Az = 0
◦ . A star directly overhead, i.e., at the zenith, would have Alt = +90
◦ , while its Az
could be anything at all, as azimuth angles converge at this point. For observers in
the southern hemisphere, it would be an object to the north on the horizon that would
have Alt = 0
◦ and Az = 0
◦ , whilst the object at the zenith would have an Az of
−90
◦ .
As AltAz is a local system, it is used to determine not only when and where to
point a telescope but also whether the target is visible, whether it is in its optimal
position in the sky for observing, and whether the location of the target is going
to exceed the telescope’s limits of orientation. In many cases, the instrument you
will be using will have some limitation as to where it can point. This is typically
the minimum altitude that can be observed, which is rarely as low as 0
◦ , as often
there is some obstruction, for example the edge of the dome or trees, that prevents
this. Furthermore, with the increasing trend of ever larger astronomical cameras,
maximum altitude limits have also become more prevalent due to the inability of
these larger cameras to pass safely under the telescope forks. Although somewhat
rarer than altitude limits, some sites also have azimuth limits, due, for example,
to physical obstructions, problems with prevailing winds, or mount limitations. In
many cases, you may find that a telescope has declination limits. Typically, such
limits occur when the telescope is polar aligned (when the Az axis of the telescope
points at the pole), as most are. Declination limits are similar to altitude limits but
are adjusted for the tilt in the telescope’s vertical axis due to the polar alignment.
Before you observe, you should be aware of the telescope’s pointing limits whether
55
5.2 Altitude and Azimuth
The simplest coordinate system you will encounter in astronomy is the altitude and
azimuth system, also known as the horizontal system, but more commonly known as
AltAz. AltAz (or Alt-Az) is based on the two great circles formed by the observer’s
horizon and celestial meridian line. The celestial meridian line passes through the
zenith, the point directly above the observer, and the nadir, the point directly below
the observer, intersecting the north and south cardinal points. For an observer in the
northern hemisphere, the origin is the point on the horizon due south of the observer.
For observers in the southern hemisphere, it is the point on the horizon due north of
the observer.
The AltAz system is determined by factors local to the observer, and it is not fixed
to the celestial sphere. Hence, astronomical objects transit through the AltAz grid
during the course of the night as the Earth rotates. However, it is the AltAz grid that
is used to point a telescope, and the observer must translate between the coordinate
system of the target and AltAz. For an observer in the northern hemisphere, an object
is located using its azimuth (A), the object’s angular position along the observer’s
horizon, measured eastwards from the most northerly point on the horizon, and its
altitude (α), the angular elevation above the horizon. Hence, for an observer in the
northern hemisphere, an object lying due south on the horizon has an Alt = 0
◦ and
Az = 180
◦ , whilst the point lying due north on the horizon has Alt = 0
◦ and Az = 0
◦ . A star directly overhead, i.e., at the zenith, would have Alt = +90
◦ , while its Az
could be anything at all, as azimuth angles converge at this point. For observers in
the southern hemisphere, it would be an object to the north on the horizon that would
have Alt = 0
◦ and Az = 0
◦ , whilst the object at the zenith would have an Az of
−90
◦ .
As AltAz is a local system, it is used to determine not only when and where to
point a telescope but also whether the target is visible, whether it is in its optimal
position in the sky for observing, and whether the location of the target is going
to exceed the telescope’s limits of orientation. In many cases, the instrument you
will be using will have some limitation as to where it can point. This is typically
the minimum altitude that can be observed, which is rarely as low as 0
◦ , as often
there is some obstruction, for example the edge of the dome or trees, that prevents
this. Furthermore, with the increasing trend of ever larger astronomical cameras,
maximum altitude limits have also become more prevalent due to the inability of
these larger cameras to pass safely under the telescope forks. Although somewhat
rarer than altitude limits, some sites also have azimuth limits, due, for example,
to physical obstructions, problems with prevailing winds, or mount limitations. In
many cases, you may find that a telescope has declination limits. Typically, such
limits occur when the telescope is polar aligned (when the Az axis of the telescope
points at the pole), as most are. Declination limits are similar to altitude limits but
are adjusted for the tilt in the telescope’s vertical axis due to the polar alignment.
Before you observe, you should be aware of the telescope’s pointing limits whether