Comparison between the observed flat-field and cavity map (top row, left and center columns, respectively) and the derived flat-field (gain) and cavity map (∆a; bottom row, left and center columns, respectively). The right column in both rows showcases a zoomed-in region of each cavity map. The area corresponding to the zoomed-in region is indicated with a black square in the full cavity map to its left. The properties used to simulate SO/PHI’s etalon are R = 0.925, n = 2.29, d = 251.63 µm, f# = 60, Θ = 0.23°.