Optimized Cassegrain baffles provide stray light shielding over a field of view while simultaneously minimizing the pupil obstruction.  This occurs when the edge blockage from the secondary baffle equals the central blockage from the primary baffle.  There are several papers published on optimal Cassegrain baffling.  I wrote this article for Astronomy Technology Today that describes the algorithm I developed for real-ray optimization of Cassegrain baffles.  A list of articles in JOSA and Applied Optics appears at the end.  Hope this helps.

Thermal analysis must include expansion and contraction of all mechanical structures that hold the primary, secondary and instrument focal point.  Zemax thermally models mirrors by simply scaling them with temperature.  The thickness of the mirrors must be modeled by solid non-refracting materials that you create in your user glass library.  I name mine the same as the substrate (e.g., Pyrex, Zerodur, etc.).  Each user-defined “glass” has a refractive index of unity over 1E-6 to 1E+6 micron wavelength, zero absorption, the correct density for each substrate, and the correct TCE.  The reflective surface has zero thickness.  You can add backs on mirrors for drawing purposes in the DRAW options, but Zemax ignores thermal expansion.  You have to lock the reflective surface and its matching substrate together with pickups.  Zemax will scale the refractive coating and substrate together, giving a realistic thermal model for each mirror.

The newer ANSYS thermal tools may deal with this much simpler, but I haven’t used those tools yet.  Perhaps an ANSYS person can comment here on better ways to thermally model Cassegrains.