Hello there, 

I have an optical system of 9 lenses focusing collected light from a telescope onto an image plane. The line on the far left of the design is the focal plane from the light collected from the telescope. This imager will double as a spectrograph when a prism/grating is added into the collimated beam. Because of this, I need to know where to place the STOP in this design.

Original design: 

I understand the STOP position will be located where the image of the secondary mirror is formed.

Attempt 1 to find STOP position: 

In my attempt, I set the OBJECT to be the secondary mirror which is 3913mm before the focal plane. (Should this have infinite thickness?) The height of the secondary mirror source is set to be 118mm by adjusting the field lines. I then set the STOP surface to be where the focal plane is. (Should I set the secondary mirror to be the STOP position since that is where the light is originally focused?) Finally the aperture is set to a value so the rays reaches top and bottom of the field lens, lens following the focal plane. 

I know I am doing something wrong in this approach, what do you think I should change? Am I on the right track? I’ve tried different methods before submitting this help request, but none seem to work well either. Thank you greatly for any guidance!!