Extra amount of flux above the naked atmosphere needed in the system of a post-AGB star to be detected by Spitzer. We display the difference between the faint limit at 8 (black) and 24 μm (grey) and the minimal amount of flux that should be present in the system of a naked post-AGB candidate in those filters for a range of effective temperatures of the central star. The solid and dashed lines correspond to a luminosity of 1000 and 35 000 L_⊙ respectively.