It's a shame that Zemax recently went down the subscription route, but I suppose most users were already getting updates every year anyway. I have a friend who

It's a shame that Zemax recently went down the subscription route, but I suppose most users were already getting updates every year anyway. I have a friend who is developing an optical train simulator in Python, but that's for a fairly niche astronomical use case (instruments for the ELT that haven't been built yet). I've always been curious about the optical stuff in Mathematica (there is also Optica) which I think you get for free. Otherwise in places I've worked we used Oslo and Fred (I think one is free if you don't save, so it's useful for quick tests). I know Zemax is used by the telescope folks in my department as well.