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# AltAz

class astropy.coordinates.AltAz(*args, **kwargs)

  Bases: `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.

  Parameters:
    data `BaseRepresentation` subclass instance

      A representation object or `None` to have no data (or use the coordinate component arguments, see below).

    az `Angle`, optional, keyword-only

      The Azimuth for this object (`alt` must also be given and `representation` must be None).

    alt `Angle`, optional, keyword-only

      The Altitude for this object (`az` must also be given and `representation` must be None).

    distance `Quantity` [:ref: ‘length’], optional, keyword-only

      The Distance for this object along the line-of-sight.

    pm_az_cosalt `Quantity` [:ref: ‘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 `Quantity` [:ref: ‘angular speed’], optional, keyword-only

      The proper motion in altitude for this object (`pm_az_cosalt` must also be given).

    radial_velocity `Quantity` [:ref: ‘speed’], optional, keyword-only

      The radial velocity of this object.

    representation_type `BaseRepresentation` subclass, `python:str`, optional

      A representation class or string name of a representation class. This sets the expected input representation class, thereby changing the expected keyword arguments for the data passed in. For example, passing `representation_type='cartesian'` will make the classes expect position data with cartesian names, i.e. `x, y, z` in most cases unless overridden via `frame_specific_representation_info`. To see this frame’s names, check out `<this frame>().representation_info`.

    differential_type `BaseDifferential` subclass, `python:str`, `python:dict`, optional

      A differential class or dictionary of differential classes (currently only a velocity differential with key ‘s’ is supported). This sets the expected input differential class, thereby changing the expected keyword arguments of the data passed in. For example, passing `differential_type='cartesian'` will make the classes expect velocity data with the argument names `v_x, v_y, v_z` unless overridden via `frame_specific_representation_info`. To see this frame’s names, check out `<this frame>().representation_info`.

    copy bool, optional

      If `True` (default), make copies of the input coordinate arrays. Can only be passed in as a keyword argument.

  Other Parameters:
    obstime `Time`

      The time at which the observation is taken. Used for determining the position and orientation of the Earth.

    location `EarthLocation`

      The location on the Earth. This can be specified either as an `EarthLocation` object or as anything that can be transformed to an `ITRS` frame.

    pressure `Quantity` [:ref: ‘pressure’]

      The atmospheric pressure as an `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 `Quantity` [:ref: ‘temperature’]

      The ground-level temperature as an `Quantity` in deg C. This is necessary for performing refraction corrections.

    relative_humidity `Quantity` [:ref: ‘dimensionless’] or number

      The relative humidity as a dimensionless quantity between 0 to 1. This is necessary for performing refraction corrections.

    obswl `Quantity` [:ref: ‘length’]
      The average wavelength of observations as an `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.

  Attributes Summary

  `default_differential` | Default representation for differential data (e.g., velocity)
  `default_representation` | Default representation for position data
  `frame_attributes` |
  `frame_specific_representation_info` | Mapping for frame-specific component names
  `location` | The location on Earth of the observer
  `name` |
  `obstime` | The reference time (e.g., time of observation)
  `obswl` | The average wavelength of observations
  `pressure` | The atmospheric pressure
  `relative_humidity` | The relative humidity
  `secz` | Secant of the zenith angle for this coordinate, a common estimate of the airmass.
  `temperature` | The ground-level temperature
  `zen` | The zenith angle (or zenith distance / co-altitude) for this coordinate.

  Attributes Documentation

  default_differential

    Default representation for differential data (e.g., velocity)

  default_representation

    Default representation for position data

  frame_attributes = {'location': <astropy.coordinates.attributes.EarthLocationAttribute object>, 'obstime': <astropy.coordinates.attributes.TimeAttribute object>, 'obswl': <astropy.coordinates.attributes.QuantityAttribute object>, 'pressure': <astropy.coordinates.attributes.QuantityAttribute object>, 'relative_humidity': <astropy.coordinates.attributes.QuantityAttribute object>, 'temperature': <astropy.coordinates.attributes.QuantityAttribute object>}
  frame_specific_representation_info

    Mapping for frame-specific component names

  location

    The location on Earth of the observer

    No default value

  name = 'altaz'
  obstime

    The reference time (e.g., time of observation)

    No default value

  obswl

    The average wavelength of observations

    Default: 1.0 micron

  pressure

    The atmospheric pressure

    Default: 0.0 hPa

  relative_humidity

    The relative humidity

    Default: 0.0

  secz

    Secant of the zenith angle for this coordinate, a common estimate of the airmass.

  temperature

    The ground-level temperature

    Default: 0.0 deg_C

  zen

    The zenith angle (or zenith distance / co-altitude) for this coordinate.