# Source code for astropy.coordinates.builtin_frames.altaz

```
# Licensed under a 3-clause BSD style license - see LICENSE.rst
import numpy as np
from astropy import units as u
from astropy.coordinates import representation as r
from astropy.coordinates.attributes import (
EarthLocationAttribute,
QuantityAttribute,
TimeAttribute,
)
from astropy.coordinates.baseframe import (
BaseCoordinateFrame,
RepresentationMapping,
base_doc,
)
from astropy.utils.decorators import format_doc
__all__ = ["AltAz"]
_90DEG = 90 * u.deg
doc_components = """
az : `~astropy.coordinates.Angle`, optional, keyword-only
The Azimuth for this object (``alt`` must also be given and
``representation`` must be None).
alt : `~astropy.coordinates.Angle`, optional, keyword-only
The Altitude for this object (``az`` must also be given and
``representation`` must be None).
distance : `~astropy.units.Quantity` ['length'], optional, keyword-only
The Distance for this object along the line-of-sight.
pm_az_cosalt : `~astropy.units.Quantity` ['angular speed'], optional, keyword-only
The proper motion in azimuth (including the ``cos(alt)`` factor) for
this object (``pm_alt`` must also be given).
pm_alt : `~astropy.units.Quantity` ['angular speed'], optional, keyword-only
The proper motion in altitude for this object (``pm_az_cosalt`` must
also be given).
radial_velocity : `~astropy.units.Quantity` ['speed'], optional, keyword-only
The radial velocity of this object."""
doc_footer = """
Other parameters
----------------
obstime : `~astropy.time.Time`
The time at which the observation is taken. Used for determining the
position and orientation of the Earth.
location : `~astropy.coordinates.EarthLocation`
The location on the Earth. This can be specified either as an
`~astropy.coordinates.EarthLocation` object or as anything that can be
transformed to an `~astropy.coordinates.ITRS` frame.
pressure : `~astropy.units.Quantity` ['pressure']
The atmospheric pressure as an `~astropy.units.Quantity` with pressure
units. This is necessary for performing refraction corrections.
Setting this to 0 (the default) will disable refraction calculations
when transforming to/from this frame.
temperature : `~astropy.units.Quantity` ['temperature']
The ground-level temperature as an `~astropy.units.Quantity` in
deg C. This is necessary for performing refraction corrections.
relative_humidity : `~astropy.units.Quantity` ['dimensionless'] or number
The relative humidity as a dimensionless quantity between 0 to 1.
This is necessary for performing refraction corrections.
obswl : `~astropy.units.Quantity` ['length']
The average wavelength of observations as an `~astropy.units.Quantity`
with length units. This is necessary for performing refraction
corrections.
Notes
-----
The refraction model is based on that implemented in ERFA, which is fast
but becomes inaccurate for altitudes below about 5 degrees. Near and below
altitudes of 0, it can even give meaningless answers, and in this case
transforming to AltAz and back to another frame can give highly discrepant
results. For much better numerical stability, leave the ``pressure`` at
``0`` (the default), thereby disabling the refraction correction and
yielding "topocentric" horizontal coordinates.
"""
[docs]
@format_doc(base_doc, components=doc_components, footer=doc_footer)
class AltAz(BaseCoordinateFrame):
"""
A coordinate or frame in the Altitude-Azimuth system (Horizontal
coordinates) with respect to the WGS84 ellipsoid. Azimuth is oriented
East of North (i.e., N=0, E=90 degrees). Altitude is also known as
elevation angle, so this frame is also in the Azimuth-Elevation system.
This frame is assumed to *include* refraction effects if the ``pressure``
frame attribute is non-zero.
The frame attributes are listed under **Other Parameters**, which are
necessary for transforming from AltAz to some other system.
"""
frame_specific_representation_info = {
r.SphericalRepresentation: [
RepresentationMapping("lon", "az"),
RepresentationMapping("lat", "alt"),
]
}
default_representation = r.SphericalRepresentation
default_differential = r.SphericalCosLatDifferential
obstime = TimeAttribute(
default=None, doc="The reference time (e.g., time of observation)"
)
location = EarthLocationAttribute(
default=None, doc="The location on Earth of the observer"
)
pressure = QuantityAttribute(default=0, unit=u.hPa, doc="The atmospheric pressure")
temperature = QuantityAttribute(
default=0, unit=u.deg_C, doc="The ground-level temperature"
)
relative_humidity = QuantityAttribute(
default=0, unit=u.dimensionless_unscaled, doc="The relative humidity"
)
obswl = QuantityAttribute(
default=1 * u.micron,
unit=u.micron,
doc="The average wavelength of observations",
)
def __init__(self, *args, **kwargs):
super().__init__(*args, **kwargs)
@property
def secz(self):
"""
Secant of the zenith angle for this coordinate, a common estimate of
the airmass.
"""
return 1 / np.sin(self.alt)
@property
def zen(self):
"""
The zenith angle (or zenith distance / co-altitude) for this coordinate.
"""
return _90DEG.to(self.alt.unit) - self.alt
# self-transform defined in icrs_observed_transforms.py
```