useful intermediate quantity to compute before one performs a final binning along constant $k \equiv \sqrt{k_\perp^2 + k_\parallel^2}$ to give an isotropic power spectrum $P(k)$.

The diagnostic capability of $P(k_\perp, k_\parallel)$ is particularly apparent when considering foregrounds. Assuming that they are spectrally smooth, foregrounds preferentially contaminate low $k_\parallel$ modes, since $k_\parallel$ is the Fourier conjugate to line-of-sight distance, which is probed by the frequency spectrum in intensity mapping experiments. The situation is more complicated for the (large) subset of intensity mapping measurements that are performed on interferometers. Interferometers are inherently chromatic in nature, causing intrinsically smooth spectrum foregrounds to acquire spectral structure, which results in leakage to higher $k_\parallel$ modes. Even this leakage, however, has been shown in recent years to have a predictable “wedge" signature on the $k_\perp$-$k_\parallel$ plane, limiting the contaminated region to a triangular-shaped region at high $k_\perp$ and low $k_\parallel$ [@Datta2010; @Vedantham2012; @Morales2012; @Parsons_et_al2012b; @Trott2012; @Thyagarajan2013; @pober_et_al2013b; @dillon_et_al2014; @Hazelton2013; @Thyagarajan_et_al2015a; @Thyagarajan_et_al2015b; @liu_et_al2014a; @liu_et_al2014b; @chapman_et_al2016; @pober_et_al2016; @seo_and_hirata2016; @jensen_et_al2016; @kohn_et_al2016]. In fact, the foreground wedge is considered sufficiently robust that some instruments have been designed around it [@pober_et_al2014; @deboer_et_al2016; @dillon_et_al2016; @neben_et_al2016a; @ewall-wice_et_al2016a; @thyagarajan_et_al2016], implicitly pursuing a strategy of foreground avoidance where the power spectrum can be measured in 