healpy.sphtfunc.alm2map(alms, nside, lmax=None, mmax=None, pixwin=False, fwhm=0.0, sigma=None, pol=True, inplace=False, verbose=True)

Computes a Healpix map given the alm.

The alm are given as a complex array. You can specify lmax and mmax, or they will be computed from array size (assuming lmax==mmax).

almscomplex, array or sequence of arrays

A complex array or a sequence of complex arrays. Each array must have a size of the form: mmax * (2 * lmax + 1 - mmax) / 2 + lmax + 1

nsideint, scalar

The nside of the output map.

lmaxNone or int, scalar, optional

Explicitly define lmax (needed if mmax!=lmax)

mmaxNone or int, scalar, optional

Explicitly define mmax (needed if mmax!=lmax)

pixwinbool, optional

Smooth the alm using the pixel window functions. Default: False.

fwhmfloat, scalar, optional

The fwhm of the Gaussian used to smooth the map (applied on alm) [in radians]

sigmafloat, scalar, optional

The sigma of the Gaussian used to smooth the map (applied on alm) [in radians]

polbool, optional

If True, assumes input alms are TEB. Output will be TQU maps. (input must be 1 or 3 alms) If False, apply spin 0 harmonic transform to each alm. (input can be any number of alms) If there is only one input alm, it has no effect. Default: True.

inplacebool, optional

If True, input alms may be modified by pixel window function and beam smoothing (if alm(s) are complex128 contiguous arrays). Otherwise, input alms are not modified. A copy is made if needed to apply beam smoothing or pixel window.

mapsarray or list of arrays

A Healpix map in RING scheme at nside or a list of T,Q,U maps (if polarized input)


Running map2alm then alm2map will not return exactly the same map if the discretized field you construct on the sphere is not band-limited (for example, if you have a map containing pixel-based noise rather than beam-smoothed noise). If you need a band-limited map, you have to start with random numbers in lm space and transform these via alm2map. With such an input, the accuracy of map2alm->alm2map should be quite good, depending on your choices of lmax, mmax and nside (for some typical values, see e.g., section 5.1 of https://arxiv.org/pdf/1010.2084).