Get coordinates for specific value on healpy mollview

Question

Is it possible to find the coordinates for a specific value of a hitmap from a healpy mollview plot?

For example: If I have a smoothed out hitmap and want to use the mask:

mask = (hitmap > 2.054) & (hitmap < 2.056)

to find the required values on of the hitmap.

The hitmap is used to create a mollview plot like this:

hp.mollview(hitmap, xsize=2000)

Is it then possible to find the coordinates (lon,lat) corresponding to the pixels in the mollview plot that satisfy the mask by using the mask?

Many thanks in advance!

Answer 1

The best way to do this would be via hp.pix2ang():

import healpy as hp
import numpy as hp

# Get the nside and number of pixels in your map
nside = hp.get_nside(mask)
npix = hp.nside2npix(nside)

# Use pix2ang to get the (l, b) coordinates for each pixel
glons, glats = hp.pix2ang(nside, np.arange(npix), lonlat=True)

This will give you a list of all the coordinates (l, b) for each pixel in your map. To get only the ones in the mask, simply use:

glons_eff = glons[mask]
glats_eff = glats[mask]