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_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_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