63 lines
2.4 KiB
Python
63 lines
2.4 KiB
Python
"""Constellation identification.
|
|
|
|
Constellation identification is tradionally performed with:
|
|
|
|
http://cdsarc.u-strasbg.fr/viz-bin/Cat?VI/42
|
|
https://iopscience.iop.org/article/10.1086/132034/pdf
|
|
|
|
Given a position, a binary search can be used to index into its roughly
|
|
12k data table for a matching declination, but the table then needs to
|
|
be scanned linearly for the first subsequent segment that contains the
|
|
target right ascension within its bounds. The big problem for Skyfield:
|
|
that algorithm can't be constructed from vectorized NumPy primitives.
|
|
|
|
So we here take an approach that requires no linear search. The sky is
|
|
divided into a grid, with a grid line at every right ascension and
|
|
declination that is mentioned in a constellation boundary. This is a
|
|
bit wasteful, as many values are only ever used for one boundary, but
|
|
makes the search easy: do a binary search for right ascension, then for
|
|
declination, then look up the indexes in the grid. Memory requirement:
|
|
|
|
1.8k right ascension table
|
|
1.6k declination table
|
|
46.6k grid table
|
|
1k constellation name abbreviations
|
|
|
|
"""
|
|
from numpy import searchsorted
|
|
from .functions import load_bundled_npy
|
|
from .timelib import Time, julian_date_of_besselian_epoch
|
|
|
|
def load_constellation_map():
|
|
"""Load Skyfield's constellation boundaries and return a lookup function.
|
|
|
|
Skyfield carries an internal map of constellation boundaries that is
|
|
optimized for quick position lookup. Call this function to load the
|
|
map and return a function mapping position to constellation name.
|
|
|
|
>>> from skyfield.api import position_of_radec, load_constellation_map
|
|
>>> constellation_at = load_constellation_map()
|
|
>>> north_pole = position_of_radec(0, 90)
|
|
>>> constellation_at(north_pole)
|
|
'UMi'
|
|
|
|
If you pass an array of positions, you'll receive an array of names.
|
|
|
|
"""
|
|
t1875 = Time(None, julian_date_of_besselian_epoch(1875))
|
|
|
|
arrays = load_bundled_npy('constellations.npz')
|
|
sorted_ra = arrays['sorted_ra']
|
|
sorted_dec = arrays['sorted_dec']
|
|
radec_to_index = arrays['radec_to_index']
|
|
indexed_abbreviations = arrays['indexed_abbreviations']
|
|
|
|
def constellation_at(position):
|
|
ra, dec, distance = position.radec(epoch=t1875)
|
|
i = searchsorted(sorted_ra, ra.hours)
|
|
j = searchsorted(sorted_dec, dec.degrees, side='right')
|
|
k = radec_to_index[i, j]
|
|
return indexed_abbreviations[k]
|
|
|
|
return constellation_at
|