Searching for pixels around a position (cone search)

A common operation when using HEALPix maps is to try and find all pixels that lie within a certain radius of a given longitude/latitude. One way to do this would be to simply find the longitude/latitude of all pixels in the HEALPix map then find the spherical distance to the requested longitude and latitude, but in practice this would be very inefficient for high resolution HEALPix maps where the number of pixels may become arbitrarily large.

Instead, the cone_search_lonlat() method can be used to efficiently find all HEALpix pixels within a certain radius from a longitude/latitude:

>>> from astropy import units as u
>>> from astropy_healpix import HEALPix
>>> hp = HEALPix(nside=16, order='nested')
>>> print(hp.cone_search_lonlat(10 * u.deg, 30 * u.deg, radius=10 * u.deg))
[1269  160  162 1271 1270 1268 1246 1247  138  139  161 1245  136  137
  140  142  130  131 1239 1244 1238 1241 1243 1265 1267 1276 1273 1277
  168  169  163  166  164]

Likewise, if a celestial frame was specified using the frame keyword argument to HEALPix, you can use the cone_search_skycoord() method to query around specific celestial coordinates:

>>> from astropy.coordinates import Galactic
>>> hp = HEALPix(nside=16, order='nested', frame=Galactic())
>>> from astropy.coordinates import SkyCoord
>>> coord = SkyCoord('00h42m44.3503s +41d16m08.634s')
>>> print(hp.cone_search_skycoord(coord, radius=5 * u.arcmin))
[2537]