HEALPix-based maps#
This page provides examples and documentation specific to the HEALPix map
classes. All HEALPix classes inherit from Map
which provides generic
interface methods that can be be used to access or update the contents of a map
without reference to its pixelization scheme.
Warning
Gammapy uses NEST
as default pixel order scheme, while healpy
functions have RING
as the default (see https://healpy.readthedocs.io/en/1.11.0/index.html).
If you are interfacing Gammapy HEALPix maps with healpy
functions, you need to specify the pixelization scheme
either while creating the Gammapy object or when using the healpy
functions.
HEALPix geometry#
The HpxGeom
class encapsulates the geometry of a HEALPix map:
the pixel size (NSIDE), coordinate system, and definition of non-spatial axes
(e.g. energy). By default a HEALPix geometry will encompass the full sky. The
following example shows how to create an all-sky 2D HEALPix image:
from gammapy.maps import HpxGeom, HpxNDMap, HpxMap
# Create a HEALPix geometry of NSIDE=16
geom = HpxGeom(16, frame="galactic")
m = HpxNDMap(geom)
# Equivalent factory method call
m = HpxMap.create(nside=16, frame="galactic")
Partial-sky maps can be created by passing a region
argument to the map
geometry constructor or by setting the width
argument to the
create
factory method:
from gammapy.maps import HpxGeom, HpxMap, HpxNDMap
from astropy.coordinates import SkyCoord
# Create a partial-sky HEALPix geometry of NSIDE=16
geom = HpxGeom(16, region='DISK(0.0,5.0,10.0)', frame="galactic")
m = HpxNDMap(geom)
# Equivalent factory method call
position = SkyCoord(0.0, 5.0, frame='galactic', unit='deg')
m = HpxMap.create(nside=16, skydir=position, width=20.0)