healpy.sphtfunc.effective_resolution_fwhm

healpy.sphtfunc.effective_resolution_fwhm(nside, arcmin=False)

Return the Planck effective-resolution beam FWHM for a HEALPix NSIDE.

The “effective resolution beam” is a Gaussian whose full-width half-maximum (FWHM) scales with the HEALPix pixel size by a constant ratio, calibrated so that NSIDE 64 yields the 160-arcmin beam adopted by Planck for its low-resolution products (Planck Collaboration 2015 X, arXiv:1502.01588). Concretely, the FWHM returned is:

fwhm = K * nside2resol(nside)

where K = 160 arcmin / nside2resol(64) ≈ 2.91 is a fixed constant. Doubling NSIDE therefore halves the FWHM (e.g. NSIDE 128 → 80 arcmin, NSIDE 256 → 40 arcmin).

This is the same beam width that harmonic_ud_grade() applies when called with fwhm_out=None. Use this function to look up the beam width before downgrading so you know how much smoothing will be applied to your map.

Parameters:

nsideint

HEALPix NSIDE parameter.

arcminbool, optional

If True, return the FWHM in arcmin. Default: False, i.e. return radians.

Returns:

fwhmfloat

FWHM of the effective resolution beam, in radians (default) or arcmin if arcmin=True.

See also

harmonic_ud_grade

Uses this FWHM by default when fwhm_out=None.

nside2resol

Pixel angular size (radians) for a given NSIDE.

Examples

>>> import healpy as hp
>>> hp.effective_resolution_fwhm(64, arcmin=True)
160.0
>>> hp.effective_resolution_fwhm(128, arcmin=True)
80.0
>>> hp.effective_resolution_fwhm(256, arcmin=True)
40.0