# Getting started¶

The cleanest way to use the functionality in healpix is to make use of the high-level HEALPix class. The HEALPix class should be initialized with the nside parameter which controls the resolution of the pixellization - it is the number of pixels on the side of each of the 12 top-level HEALPix pixels:

>>> from astropy_healpix import HEALPix
>>> hp = HEALPix(nside=16)


As described in the references above, HEALPix pixel indices can follow two different ordering conventions - the nested convention and the ring convention. By default, the HEALPix class assumes the ring ordering convention, but it is possible to explicitly specify the convention to use using the order argument, for example:

>>> hp = HEALPix(nside=16, order='ring')


or:

>>> hp = HEALPix(nside=16, order='nested')


Once this class has been set up, you can access various properties and methods related to the HEALPix pixellization. For example, you can calculate the number of pixels as well as the pixel area or resolution:

>>> hp.npix
3072
>>> hp.pixel_area
<Quantity 0.0040906154343617095 sr>
>>> hp.pixel_resolution
<Quantity 219.87113035631398 arcmin>


As you can see, when appropriate the properties and the methods on the HEALPix class return Astropy high-level classes such as Quantity, Longitude, and so on.

For example, the healpix_to_lonlat() method can be used to convert HEALPix indices to Longitude and Latitude objects:

>>> lon, lat = hp.healpix_to_lonlat([1, 442, 2200])
>>> lon
<Longitude [ 0.83448555, 1.63624617, 0.4712389 ] rad>
>>> lat

The HEALPix class includes methods that take or return SkyCoord objects (we will take a look at this in the Celestial coordinates section).