# AltAz¶

class astropy.coordinates.AltAz(*args, **kwargs)[source]

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.

The frame attributes are listed under Other Parameters, which are necessary for transforming from AltAz to some other system.

Parameters: Other 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, 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. pm_az_cosalt : Quantity, optional, must be keyword The proper motion in azimuth (including the cos(alt) factor) for this object (pm_alt must also be given). pm_alt : Quantity, optional, must be keyword The proper motion in altitude for this object (pm_az_cosalt must also be given). radial_velocity : Quantity, optional, must be keyword The radial velocity of this object. representation_type : BaseRepresentation subclass, 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. differential_type : BaseDifferential subclass, str, 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. copy : bool, optional If True (default), make copies of the input coordinate arrays. Can only be passed in as a keyword argument. 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 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 The ground-level temperature as an Quantity in deg C. This is necessary for performing refraction corrections. relative_humidity : Quantity or number. The relative humidity as a dimensionless quantity between 0 to 1. This is necessary for performing refraction corrections. obswl : Quantity 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, leaving the pressure at 0 (the default), disabling the refraction correction (yielding “topocentric” horizontal coordinates).

Attributes Summary

 default_differential 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_differential
default_representation
frame_attributes = {'location': <astropy.coordinates.attributes.EarthLocationAttribute object at 0x7f3878bb23c8>, 'obstime': <astropy.coordinates.attributes.TimeAttribute object at 0x7f3878bb2390>, 'obswl': <astropy.coordinates.attributes.QuantityAttribute object at 0x7f3878bb24e0>, 'pressure': <astropy.coordinates.attributes.QuantityAttribute object at 0x7f3878bb2400>, 'relative_humidity': <astropy.coordinates.attributes.QuantityAttribute object at 0x7f3878bb2470>, 'temperature': <astropy.coordinates.attributes.QuantityAttribute object at 0x7f3878bb2438>}
frame_specific_representation_info
location = None
name = 'altaz'
obstime = None
obswl = <Quantity 1. micron>
pressure = <Quantity 0. hPa>
relative_humidity = <Quantity 0.>
secz

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

temperature = <Quantity 0. deg_C>
zen

The zenith angle for this coordinate