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# 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.
        """

    @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)
        location = EarthLocationAttribute(default=None)
        pressure = QuantityAttribute(default=0, unit=u.hPa)
        temperature = QuantityAttribute(default=0, unit=u.deg_C)
        relative_humidity = QuantityAttribute(default=0, unit=u.dimensionless_unscaled)
        obswl = QuantityAttribute(default=1 * u.micron, unit=u.micron)

        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