Hello there, I am still a new learner to Zemax.

I have an optical system of 7 lenses focusing collected light from a telescope onto an image plane. The goal is to make the RMS radius as small as possible. Whenever I set my parameters to variable and optimize, Zemax will change the thickness variable after my lenses to negative, rearranging the order to an impossible configuration. I am wondering if there is a way to fix the distances to be positive and keep the sequential order? Thanks for any guidance.