python - Healpy map2alm and alm2map errors -

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i'm studying healpy. interested in map2alm , alm2map: found question example of applying functions "to both sides" (in answer) provided. can see, significant errors in comparison original map after applying map2alm , alm2map consecutively.therefore want control these errors. question is:

  1. is there way reduce error, without using higher nside? (for example if want use nside)
  2. also, maybe there way "spectral characteristic" of map, wouldn't have such big errors?

no, there no other way.

going real space harmonic space doing fast fourier transform. if don't have enough time resolution, fft not going good, there not enough information have accurate representation frequency domain.

here same, way going have low error @ low nside if have signal represented in harmonic space, example pure dipole, see notebook, @ nside 8, error @ 1/1000:

http://nbviewer.jupyter.org/gist/zonca/2f87cb19ca8293539947b0d4aeca42b2


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