Get coordinates for specific value on healpy mollview

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Get coordinates for specific value on healpy mollview



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!




1 Answer
1



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


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]





Thank you. Is this a good way of overplotting the mollview plot with another plot? For example using: hp.projplot(glons_eff,glats_eff, lonlat=True) I tried using healpy.projaxes.CartesianAxes.contour() to plot a contour at the value 2.055 (between the two values in the mask) But this does not seem to work for me...
– Nanna
Aug 13 at 10:07






There is currently no built-in function to draw contours in healpy, the function you're referring to does not exist in the latest healpy version.
– Daniel Lenz
Aug 13 at 17:37






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