How To

# Sky coordinates, sky maps and sky regions¶

## How to convert a sky coordinate from celestial to Galactic?¶

To convert from celestial to Galactic coordinates you can set a sky direction in celestial coordinates and read it back in Galactic coordinates.

Python:

>>> import gammalib
>>> dir=gammalib.GSkyDir()
>>> l=dir.l_deg()
>>> b=dir.b_deg()
>>> print(l,b)
(184.55973405309402, -5.7891829467816827)


C++:

#include "GammaLib.hpp"
GSkyDir dir;
double l = dir.l_deg();
double b = dir.b_deg();
std::cout << l << ", " << b << std::endl;


## How to convert sky map projections?¶

The following code illustrates how to convert a map in HealPix projection into a map in cartesian projection. Map conversion is performed using the += operator that adds the bilinearly interpolated intensity values from one map to another. The example code applies to any kind of map projections.

Python:

>>> import gammalib
>>> healpix = gammalib.GSkyMap("healpix.fits")
>>> map = gammalib.GSkyMap("CAR","GAL",0.0,0.0,0.5,0.5,100,100)
>>> map += healpix
>>> map.save("carmap.fits")


C++:

#include "GammaLib.hpp"
GSkyMap healpix("healpix.fits");
GSkyMap map("CAR","GAL",0.0,0.0,0.5,0.5,100,100);
map += healpix;
map.save("carmap.fits");