Healpix / Healpy: How can I use healpy to do SHT for a patch on the sky?

Question

Healpix is a very useful software to do spherical analysis on a sphere. For example, we can use map2alm to analyze a Healpix RING ordered map and return spherical harmonics. Here the argument of map should be an array with $Npix = 12*Nside^2$. If I only want to do analysis on a patch, but not on a full-sky. Some ring-weights can be used to mask the map in Healpix. But if Npix is very big, actually the array of map will too big to be allocated in memory. Thus, how can I do the spherical harmonics transform on a sky-patch?

Answer 1

Unfortunately in healpy and HEALPix, spherical transforms are always executed full-sky.

If you are working on a small patch of sky you could use rectangular pixelization instead, see the pixell package at https://github.com/simonsobs/pixell, then you can use FFT transforms instead of spherical harmonics.

You can also checkout the CMB Analysis Summer School notebooks: https://github.com/jeffmcm1977/CMBAnalysis_SummerSchool/blob/master/CMB_School_Part_04.ipynb