This IDL facility provides the HEALPix resolution parameter Nside corresponding to Npix pixels over the full sky.

Location in HEALPix directory tree: src/idl/toolkit/npix2nside.pro

FORMAT

IDL> Nside=NPIX2NSIDE (Npix [,ERROR=])

QUALIFIERS

Npix: number of pixels over the full sky (scalar integer), should be a valid Npix ( $N_{\rm pix} = 12N_{\rm side}^2$ with $N_{\rm side}$ power of 2 in $\{1,\ldots,2^{29}\}$ )
Nside: on output: resolution parameter if Npix is valid, -1 otherwise

KEYWORDS

ERROR =: error flag, set to 1 on output if Npix is NOT valid, or stays to 0 otherwise.

DESCRIPTION

npix2nside checks that the given Npix is valid ( $N_{\rm pix} = 12N_{\rm side}^2$ with $N_{\rm side}$ a power of 2 in $\{1,\ldots,2^{29}\}$ ) and then computes the corresponding resolution parameter $N_{\rm side}$ .

RELATED ROUTINES

This section lists the routines related to npix2nside

idl: version 6.1 or more is necessary to run npix2nside .
nside2npix: computes Npix corresponding to Nside
pix2xxx, ang2xxx, vec2xxx, ...: conversion between vector or angles and pixel index and vice-versa
vec2pix, pix2vec: conversion between vector and pixel index
nest2ring, ring2nest: conversion between NESTED and RING indices

EXAMPLE:

Nside = npix2nside(49152, ERROR=error)

Nside will be 64 because 49152 is a valid pixel number (=12*64² and 64 is a power of 2), and error will be 0

EXAMPLE:

Nside = npix2nside(49151, ERROR=error)

Nside will be -1 and error: 1, because 49151 is not a valid number of HEALPix pixels over the full sky.

Eric Hivon 2010-06-18

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