Étendue comparison for two distinct groups: optical/near-IR (filled symbols) and far-IR (open symbols) telescopes. The sample includes ground-based (orange) and space-based (blue and green) telescopes, selected to span a wide range of aperture sizes and FoVs, reflecting the diverse capabilities of current and next-generation facilities. Collecting area is evaluated assuming no field vignetting and excluding wavelength-dependent transmission, enabling a uniform, geometry-based comparison. The dotted gray contour lines show loci of constant étendue and serve as a visual reference for comparison. Standard étendue (squares) and RWE (stars) are projected onto a common FoV-based representation, enabling direct comparison on the same A–Ω plane. Specifically, the square symbols are plotted at (A, Ω_FoV), while the star symbols are plotted using an effective, resolution-weighted FoV, Ω_eff,FoV ≡ Ω_FoV/Ω_PSF,λ. The Kim TMS Type-I space telescope and the gigantic ground-based telescope E-ELT [12–14] exhibit substantially larger RWE, corresponding to greater depth and more resolved detail per pointing despite its smaller geometric FoV.