curved sky from the beginning.

In this paper, we rectify the shortcomings of the $k_\perp$-$k_\parallel$ plane by introducing an alternative that is well-defined in the wide-field limit. Rather than expanding sky emission in a basis of rectilinear Fourier modes, we propose a spherical Fourier-Bessel basis. In this basis, the sky brightness temperature $T(\mathbf{r})$ of a survey (where $\mathbf{r}$ is the comoving position) is expressed in terms of $\overline{T}_{\ell m} (k)$, defined as[^3] $$\label{eq:TellmEverything}
\overline{T}_{\ell m} (k) \equiv \sqrt{\frac{2}{\pi}} \int \! d\Omega dr\, r^2 j_\ell (kr) Y_{\ell m}^* ({\hat{\mathbf{r}}}) T(\mathbf{r}),$$ where $k$ is the *total* wavenumber, $\ell$ and $m$ are the spherical harmonic indices, $Y_{\ell m}$ denotes the corresponding spherical harmonic, $r \equiv | \mathbf{r}|$ is the radial distance, $\mathbf{\hat{r}} \equiv \mathbf{r} / r$ is the angular direction unit vector[^4], and $j_\ell$ is the $\ell$th order spherical Bessel function of the first kind. The quantity $P(k_\perp, k_\parallel)$ is replaced by the analogous quantity $S_\ell (k)$, the spherical harmonic power spectrum, which roughly takes the form $$\label{eq:Sellkrough}
S_\ell (k) \propto \frac{1}{2 \ell + 1} \sum_{m = -\ell}^\ell |\overline{T}_{\ell m} (k)|^2,$$ where the sum over $m$ is analogous to the binning of $k_x$ and $k_y$ into $k_\perp$, and a more rigorous definition (with 