Error running healpy.sphtfunc.alm2map with alm size different from l_max required

Question

I run healpy.sphtfunc.alm2map passing alm as an array with l_max = 256 and asking as output a map with l_max = 191, but it seems that the function does not accept correctly the new l_max.

Here an example code of what I am doing, with artificially generated alms:

import healpy as hp                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               
import numpy as np

#nside and lmax                                                                                                                                                                                             
nside_high = 128 
lmax_high= 2*nside_high

nside_low = 64 
lmax_low= 3*nside_low-1

#Cl
Cl = np.ones(lmax_high)

#Alm
Alm = hp.synalm(Cl, lmax = lmax_high)

#Map
Map = hp.alm2map(Alm, nside = nside_low, lmax = lmax_low)

I get this error:

ValueError                                Traceback (most recent call last)
/var/folders/k_/2j08yy711tb17zmmsznfx1zh0000gn/T/ipykernel_9189/2207532576.py in <cell line: 18>()
     16 
     17 #Map
---> 18 Map = hp.alm2map(Alm, nside = nside_l, lmax = lmax_l)

/opt/anaconda3/lib/python3.8/site-packages/astropy/utils/decorators.py in wrapper(*args, **kwargs)
    552                     warnings.warn(msg, warning_type, stacklevel=2)
    553 
--> 554             return function(*args, **kwargs)
    555 
    556         return wrapper

/opt/anaconda3/lib/python3.8/site-packages/healpy/sphtfunc.py in alm2map(alms, nside, lmax, mmax, pixwin, fwhm, sigma, pol, inplace, verbose)
    502         mmax = -1
    503     if pol:
--> 504         output = sphtlib._alm2map(
    505             alms_new[0] if lonely else tuple(alms_new), nside, lmax=lmax, mmax=mmax
    506         )

ValueError: Wrong alm size.

Can anyone help?

Answer 1

The purpose of lmax argument in alm2map is just to handle input alms which have mmax != lmax, they cannot be used to clip alms.

Currently the easiest way to clip alms is:

alm_clipped = hp.almxfl(Alm, np.ones(lmax_low+1))
Map = hp.alm2map(alm_clipped, nside = nside_low)

The next version of healpy will have a dedicated function resize_alm:

https://github.com/healpy/healpy/pull/803

In the future it would be nice to use the function to automatically handle this, I opened an issue to track this:

https://github.com/healpy/healpy/issues/817