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.

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, 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.

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

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>, '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

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