SPHERE (which stands for Spectro-Polarimetric High-contrast Exoplanet REsearch) is a second-generation Very Large Telescope (VLT) instrument dedicated to high-contrast direct imaging of exoplanets which firstlight is scheduled for 2011. Within this complex instrument one of the central components is the apodized Lyot coronagraph (ALC). The present paper reports on the most interesting aspects and results of the whole numerical study made during the design of the ALC for SPHERE/VLT. The method followed for this study is purely numerical, but with an end-to-end approach which is largely fed by a number of instrumental feedbacks. The results obtained and presented in this paper firstly permit to finalize the optical design before Exp Astron (2011) 30:39-58 laboratory performance testing of the ALC being built for SPHERE/VLT (see paper II "Laboratory tests and performances"), but will also hopefully help conceiving future other instruments alike, for example within the very promising extremely large telescope perspective.

Post‐asymptotic giant branch (post‐AGB) stars are key objects for the study of the dramatic morphological changes of low‐ to intermediate‐mass stars on their evolution from the AGB towards the planetary nebula stage. There is growing evidence that binary interaction processes may very well have a determining role in the shaping process of many objects, but so far direct evidence is still weak. We aim at a systematic study of the dust distribution around a large sample of post‐AGB stars as a probe of the symmetry breaking in the nebulae around these systems. We used imaging in the mid‐infrared to study the inner part of these evolved stars to probe direct emission from dusty structures in the core of post‐AGB stars in order to better understand their shaping mechanisms. We imaged a sample of 93 evolved stars and nebulae in the mid‐infrared using VLT spectrometer and imager for the mid‐infrared (VISIR)/VLT, T‐Recs/Gemini‐South and Michelle/Gemini‐North. We found that all the proto‐planetary nebulae we resolved show a clear departure from spherical symmetry. 59 out of the 93 observed targets appear to be non‐resolved. The resolved targets can be divided into two categories. (i) The nebulae with a dense central core, that are either bipolar and multipolar and (ii) the nebulae with no central core, with an elliptical morphology. The dense central torus observed likely hosts binary systems which triggered fast outflows that shaped the nebulae.

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