CODEWITHSUNDEEP
python-2.7astronomyhealpy
HBouy
HBouy

Reputation: 245

Healpy pix2ang: Convert from HEALPix index to RA,Dec or glong,glat

I am new to HEALPix and fairly new to Python as well. I try to use healpy to convert a HEALPix index to RA,Dec. I get that I have to use pix2ang, but cannot figure how to convert the output theta,phi into RA,Dec... I tried this:

import healpy as hp
import numpy as np
theta, phi = hp.pix2ang(256, 632668    ,nest=True) 
ra= phi*180./np.pi
dec = 90.-(theta*180./np.pi)

but it does not seem to give the correct result.

Hope someone can help!

Upvotes: 2

Views: 5451

Answers (2)

Conan
Conan

Reputation: 31

You can see the example below, from here.

ipix = 123
theta, phi = hp.pix2ang(nside, ipix)
ra = np.rad2deg(phi)
dec = np.rad2deg(0.5 * np.pi - theta)

Upvotes: 1

Asen Christov
Asen Christov

Reputation: 848

First of all the method pix2ang(nside,indx) gives you the coordinates of pixel with number indx. The pixel number is not directly related to a coordinate, i.e. two consecutive pixel numbers are not necessarily next to each other.

Second, as written in the manual of Healpix (which is the underlying code for healpy) (http://healpix.sourceforge.net/html/csubnode2.htm) the angle theta is defined in range [0,pi] and therefore it cannot directly represent declination [-pi/2,pi/2].

So what I do is I define a transformation and I implement it in two functions, for instance:

def IndexToDeclRa(index):
    theta,phi=hp.pixelfunc.pix2ang(NSIDE,index)
    return -np.degrees(theta-pi/2.),np.degrees(pi*2.-phi)

def DeclRaToIndex(decl,RA):
    return hp.pixelfunc.ang2pix(NSIDE,np.radians(-decl+90.),np.radians(360.-RA))

then the map itself will not be in Decl&RA but if you stick to using IndexToDeclRa and DeclRaToIndex you'll effecively have what you need.

Upvotes: 4

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