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We produced a "toy-model" of one Telescope Optical Unit of PLATO, the Medium sized mission selected by ESA to fly
in 2024. This is a six lenses dioptric very wide field camera with a window in front to take care of radiation impact on
the first lens whose optical glass cannot be replaced with a radiation hardened one. The main aim of this project is just to
produce a "cool" model for display purposes, in which one can "explore" the details of the inside through some openings
in the tube, in order to visually inspect some of the fine details of the opto-mechanics. While its didactic and advertising
role is out of doubt, during its construction we realized that some interesting outcome can be of some relevance for the
project itself and that some findings could be useful, in order to assess the ability of producing with the same technology
some (of course of much more modest quality) optical systems. In this context, we immediately dropped the option of
producing the lenses with opaque material painted with a color resembling a refractive material (like blue for instance)
and decided to actually produce them with transparent plastic. Furthermore the surfaces are then finely polished in order
to give them basic optical properties. Such an optical system has only very coarsely the converging properties of the
original nominal design for a number of reasons: the refractive indexes are not the nominal ones, the quality of the
surfaces and their nominal values are only roughly, within a few percent, the targeted one, and the way the surfaces are
built up makes them prone to some diffraction effects. However, the bulk of the lens and the surface roughness will give
a large magnification of the scattering effects that will be experienced, at a much lower level, on the actual flight model.
We investigated through propagation of a laser beam and by digital camera the main stray light modes that this toymodel
offers. In other words, the model amplifies, to a large extent, the negative bulk scattering and spurious reflection
just because surfaces and materials are orders of magnitude rougher that the intended ones. Even if this did not allow to
attempt to make any quantitative measurement, in order to scale down to the actual one, we used it to look
out independently for the main sources of stray light and we compared them with the one discussed by the optical design
team, obtained using professional ray tracing code. Finally, we point out some of the technicalities used in the design
to mimic the finest mechanical elements that cannot be safely incorporate in the final design and to produce pieces of
size much larger than the maximum volume allowed by our 3D printer in a single shot.
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