We investigate the optical properties of the tZt-penta-3,5-dieniminium cation, a simplified model for the protonated Schiff base of 11-cis retinal in rhodopsin, along the isomerization pathway by ab-initio calculations based on Many-Body Perturbation Theory using the GW method and the Bethe-Salpeter equation. Our calculations are carried out on a few significant CASSCF geometrical configurations of the isomerization minimal energy path taken from the literature. Our excitation energies are qualitatively in agreement with previous Quantum Monte Carlo and post-Hartree-Fock calculations. We also employ TDDFT based methods, and investigate the outcome of using different approximations and several exchange-correlation functionals. (C) 2011 Elsevier B.V. All rights reserved.