The Telescope Optical Unit  BreadBoard is the first built TOU model. It was done during the PLATO Phase A Study in 2010-211.

The TOU BreadBoard  optics consist in a set of custom made lenses as close as possible to the final design lenses, differing only for some of the glasses and for the usage of a spherical lens instead of the aspherical one (L1), which is foreseen in the FMs (in addition to that, the usage of a spherical lens instead of the aspheric one imposes also the change of L2 to maintain the on-axis performances).

The lack of asphericity on L1 in the BreadBoard has no impact on the alignment procedure (only the central part of the lenses are used during the alignment).

The BreadBoard mechanical structure is equivalent to the TOU final structure, in terms of thermal behavior (to mimic the relative movements of the lenses when going from “warm” to “cold” conditions) and interfaces to the FPA.

The purpose of such a BreadBoard is to test the  alignment procedure and eventually test  the on-axis optical performance of the system – both in warm and in cold conditions – to validate the “warm” AIT in flight environment.