nenupy.astro.astro_tools.altaz_to_radec

nenupy.astro.astro_tools.altaz_to_radec(altaz, fast_compute=False)[source]

Converts a celestial object horizontal coordinates to equatorial coordinates.

If fast_compute=True is selected, the computation is accelerated using Local Sidereal Time approximation (see local_sidereal_time()). The right ascension \(\alpha\) and declination \(\delta\) are computed as follows:

\[\begin{split}\cases{ \delta = \sin^(-1) \left( \sin l \sin \theta + \cos l \cos \theta \cos \varphi \right)\\ h = \cos^{-1} \left( \frac{\sin \theta - \sin l \sin \delta}{\cos l \cos \delta} \right)\\ \alpha = t_{\rm{LST}} - h }\end{split}\]

with \(\theta\) the object’s elevation, \(\varphi\) the azimuth, \(l\) the observer’s latitude and \(h\) the Local Hour Angle (see hour_angle()). If \(\sin(h^{\prime}) \geq 0\), then \(h = - (h^{\prime} - \pi)\). Otherwise, transform_to() is used.

Parameters:

altaz – Celestial object horizontal coordinates.
fast_compute (bool) – If set to True, it enables faster computation time for the conversion, mainly relying on an approximation of the local sidereal time. All other values would lead to accurate coordinates computation. Differences in coordinates values are of the order of \(10^{-2}\) degrees or less.

Returns:

Celestial object’s equatorial coordinates.

Return type:

SkyCoord

Example:

from nenupy.astro import altaz_to_radec
from nenupy import nenufar_position
from astropy.time import Time
from astropy.coordinates import SkyCoord, AltAz

radec = altaz_to_radec(
    altaz=SkyCoord(
        300, 45, unit="deg",
        frame=AltAz(
            obstime=Time("2022-01-01T12:00:00"),
            location=nenufar_position
        )
    ),
    fast_compute=True
)