Dear Ethan,
I think you just served me the missing link! I just did what you recommended:
At first I got 19mm spot size, but "Ignore Lateral Color" sorted it! Of course, it is a spectrograph ...
I was not aware I needed to put the weights to zero to get a merit function which represents the RMS spot size. Now I got 5.6 micron instead of 3.9 micron which lies in the ballpark of the rms vs. wavelength plot of my first post. So I now understood what I was doing wrong!
Thank you so much for pointing this out to me - this forum is a great learning experience!
You are navigating away from the AIS Discovery experience