Seaching 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 :meth:`~astropy_healpix.HEALPix.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 arguent
to :class:`~astropy_healpix.HEALPix`, you can use the
:meth:`~astropy_healpix.HEALPix.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]