Coordinate system used for healpy.query_disc()

Question

The healpy.query_disc() function takes an argument vec which is a three-component unit vector defining the center of the disc. What coordinate system is being used here - why is there a third dimension for a 2-d projection? What point is the "tail" of this vector llocated at?

Answer 1

Very good you found the solution yourself, for later reference here is a full working code example:

import healpy as hp
import numpy as np


# `lonlat=True` switches `ang2vec` from requiring colatitude $\theta$ and longitude $\phi$ in radians to longitude and latitude in degrees (notice that also the order changes)

# in degrees
lon = 60
lat = 30
vec = hp.ang2vec(lon, lat, lonlat=True)


nside = 256
large_disc = hp.query_disc(nside, vec, radius=np.radians(20))
small_disc = hp.query_disc(nside, vec, radius=np.radians(8))
tiny_disc = hp.query_disc(nside, vec, radius=np.radians(2))


# `query_disc` returns a list of pixels, by default in RING ordering, let's check their length:

list(map(len, [large_disc, small_disc, tiny_disc]))


# ## Create a map and plot it in Mollweide projection

m = np.zeros(hp.nside2npix(nside))


m[large_disc] = 1
m[small_disc] = 2
m[tiny_disc] = 3


hp.mollview(m)
hp.graticule()

See the notebook with plots here: https://zonca.dev/2020/10/example-healpy-query_disc.html

Answer 2

Solved. Used the output of ang2vec() to give the vector.