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Thermal analysis, tolerancing and straylight of Ritchey–Chrétien (RC) telescope

  • 18 March 2023
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I want to know the process of doing Thermal Analysis of RC Telescope system. Also I want to know about Tolerance analysis using MTF for this system. I also want to know how to design Baffle for this system. 

Thank you

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Best answer by Flurin Herren 24 March 2023, 12:22

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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.

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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.

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Hi @Mumlesh Sawasiya and @Mike.Jones 

Hope you are doing well.

Mike has already mentioned some very good approaches for your questions, allow to add some more comments on the thermal analysis here:

We do have some native environmental effects in OpticStudio, a good place to start in order to get an overview would be this Knowledge Base Article:

 

However, another approach would also be to use the STAR Module (OpticStudio STAR Module – Zemax) which is also included in Ansys Zemax OpticStudio Enterprise. With the STAR Module you can import, visualize and numerically fit Datasets from a FEA Software. This way you can analyze the influence of thermal and structural loads on your optical performance inside OpticStudio.

Let me know if you have any more questions here,

Happy to help!

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