# AltAz¶

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

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 0x7faa1b602850>), ('location', <astropy.coordinates.baseframe.EarthLocationAttribute object at 0x7faa1b602c10>), ('pressure', <astropy.coordinates.baseframe.QuantityFrameAttribute object at 0x7faa1b602c50>), ('temperature', <astropy.coordinates.baseframe.QuantityFrameAttribute object at 0x7faa1b602c90>), ('relative_humidity', <astropy.coordinates.baseframe.FrameAttribute object at 0x7faa1b602cd0>), ('obswl', <astropy.coordinates.baseframe.QuantityFrameAttribute object at 0x7faa1b602d10>)])
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