Effect of atmosphere refractivity for the observation T53E, calculated with the pressure and the temperature profiles retrieved by CIRS (flyby T51, 27 March 2009 at 1°S, Distance to the spacecraft : 6300 km - Table 1). Left: vertical profile of the deviation angle caused by the refraction in the atmosphere. The angular resolution of VIMS-IR (0.5 mrad) is indicated. Middle: vertical profile of refractive attenuation for three values of the distance from Cassini to Titan. Right: altitude actually probed by the light (continuous line) and the apparent altitude assuming that the light follows straight paths (dashed lines). This altitude assuming straight light is the impact parameter of the line of sight.