Hi Annika, welcome to Zemax!

When you say you’re modeling a refraction grating, do you mean a reflective diffraction grating? This distinction might help other community members to answer your question. I’m going to answer assuming this is the case.

When you add a Diffraction Grating object to OpticStudio, the software expects you to define the amount of energy that goes into each diffraction order in reflection and transmission (you can see an example of this here). Additionally, OpticStudio will not calculate your diffraction efficiency in this case. If you know how much energy is lost, you can account for that, but OS won’t give you that answer.

If you don’t know that information for your grating, then I believe you’d need to use our RCWA DLL models to represent the actual structure of the grating. Hui makes some helpful comments in this post. We have a bounty of articles on the subject. Since your student license is the Premium edition, I believe you have access to the diffraction DLLs including the RCWA DLL.

All that said, the task you’ve described is definitely not an easy task within OpticStudio. If you have an advisor or professor who has previous experience with the software, that would be a good resource to lean on.