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- Astropy v1.1.1 »
- Astronomical Coordinate Systems (`astropy.coordinates`) »
- AltAz

# AltAz

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

  Bases: `astropy.coordinates.BaseCoordinateFrame`

  A coordinate or frame in the Altitude-Azimuth system (Horizontal coordinates). Azimuth is oriented East of North (i.e., N=0, E=90 degrees).

  This frame is assumed to include refraction effects if the `pressure` frame attribute is non-zero.

  This frame has the following frame attributes, which are necessary for transforming from AltAz to some other system:

  - `obstime`

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

  - `location`

      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`

      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`

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

  - `relative_humidity`

      The relative humidity as a number from 0 to 1. This is necessary for performing refraction corrections.

  - `obswl`
      The average wavelength of observations as an `Quantity`

        with length units. This is necessary for performing refraction corrections.

  Parameters:

  representation : `BaseRepresentation` or None

  > A representation object or None to have no data (or use the other keywords)

  az : `Angle`, optional, must be keyword

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

  alt : `Angle`, optional, must be keyword

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

  distance : `Quantity`, optional, must be keyword

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

  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 discrepent results. For much better numerical stability, leaving the `pressure` at `0` (the default), disabling the refraction correction (yielding “topocentric” horizontal coordinates).

  Attributes Summary

  `default_representation` |
  `frame_attributes` |
  `frame_specific_representation_info` |
  `location` |
  `name` |
  `obstime` |
  `obswl` |
  `pressure` |
  `relative_humidity` |
  `secz` | Secant if the zenith angle for this coordinate, a common estimate of the airmass.
  `temperature` |
  `zen` | The zenith angle for this coordinate

  Attributes Documentation

  `default_representation`
  `frame_attributes` = OrderedDict([('obstime', <astropy.coordinates.baseframe.TimeFrameAttribute object at 0x7f52386efbd0>), ('location', <astropy.coordinates.baseframe.EarthLocationAttribute object at 0x7f523a458050>), ('pressure', <astropy.coordinates.baseframe.QuantityFrameAttribute object at 0x7f523a458090>), ('temperature', <astropy.coordinates.baseframe.QuantityFrameAttribute object at 0x7f523a4580d0>), ('relative_humidity', <astropy.coordinates.baseframe.FrameAttribute object at 0x7f523a458110>), ('obswl', <astropy.coordinates.baseframe.QuantityFrameAttribute object at 0x7f523a458150>)])
  `frame_specific_representation_info`
  `location` = None
  `name` = 'altaz'
  `obstime` = None
  `obswl` = <Quantity 1.0 micron>
  `pressure` = <Quantity 0.0 hPa>
  `relative_humidity` = 0
  `secz`

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

  `temperature` = <Quantity 0.0 deg_C>
  `zen`

    The zenith angle for this coordinate