2D angular matter power spectra, $C_{\delta \delta }^{(ij)}(\ell )$, projected with Levin describing the correlations between the large-scale structure within a set of 19 concentric shells spaced along the line of sight as shown in Fig. 2. The top panel shows the autocorrelations of all shells, i.e. |i − j| = 0. The bottom panel shows the correlations of each shell with its nearest neighbour, i.e. |i − j| = 1 (|i − j|> 1 are not shown as they do not have a large effect on matter fields within each shell, see Tessore et al. 2023). The colour of each line is given by the mean redshift of the i^th bin, ${\overline{z}}^{(i)}$. The underlying linear 3D matter power spectrum is based on flat ΛCDM assuming Ω_c = 0.05, Ω_b = 0.28, σ₈ = 0.79, S₈ = 0.84 and H₀ = 67 km s⁻¹ Mpc⁻¹ calculated using CAMB (Lewis et al. 2000; Lewis & Challinor 2002; Howlett et al. 2012), while the non-linear contribution is calculated using HMcode-2016 (Mead et al. 2015, 2016) assuming A_bary = 3.1. The non-Limber projection done by Levin assumes ℓ_max, nL = 1200 which causes the $C_{\delta \delta }^{(ij)}$ with |i − j|> 0 go to a value of zero for ℓ > ℓ_max, nL.