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:

dataBaseRepresentation subclass instance

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

azAngle, optional, keyword-only

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

altAngle, optional, keyword-only

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

distanceQuantity [:ref: ‘length’], optional, keyword-only

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

pm_az_cosaltQuantity [: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_altQuantity [:ref: ‘angular speed’], optional, keyword-only

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

radial_velocityQuantity [:ref: ‘speed’], optional, keyword-only

The radial velocity of this object.

representation_typeBaseRepresentation 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_typeBaseDifferential 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.

copybool, optional

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

Other Parameters:

obstimeTime

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

locationEarthLocation

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.

pressureQuantity [: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.

temperatureQuantity [:ref: ‘temperature’]

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

relative_humidityQuantity [:ref: ‘dimensionless’] or number

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

obswlQuantity [: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
name
obstime
obswl
pressure
relative_humidity
secz	Secant of the zenith angle for this coordinate, a common estimate of the airmass.
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 = None

name = 'altaz'

obstime = None

obswl = <Quantity 1. micron>

pressure = <Quantity 0. hPa>

relative_humidity = <Quantity 0.>

secz

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

temperature = <Quantity 0. deg_C>

zen

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