Harmonic analysis of isotropic fields on the sphere with arbitrary masks

Obtaining constraints from the largest scales of a galaxy survey is challenging due to mask allowing only partial measurement large angular modes. This scatters information harmonic-space 2-point function away diagonal and introduces coupling between In this paper, we derive custom eigenbasis adapted any particular geometry so that all retained on diagonal. At expense somewhat complex pixel- selection-function-window, result with simple shot noise, covariance matrix in case Gaussian random field. We basis surface sphere, use it construct 3D spherical Fourier-Bessel power spectrum estimator assuming separable radial directions.