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Is there a technique for modelling objects in Lagrange Point positions?
I would just put the objects in an orbit of the appropriate distance, and mark them as being in the Lagrange points. Astro cant 'show' objects in fixed points. For the actual formulas to determine the positions, I'm sure they are easy enough to find online.
L4 & L5 are easy enough, just copy all of the orbital for the primary & make it 1/6 of the orbit ahead or behind. L3 is the same, just half way ahead in the orbit. L1 & L2 you just have to fake it
Is there a way to easily duplicate an orbit?
Right-Click on a body, select Properties, select the Orbital Properties tab, then copy each value to a separate document (I use Notepad). Repeat these steps to the new body, copying the values from your doc to the appropriate field. Finally add or subtract 1/6th of the Orbital Period (days) of the co-orbital body to the LaGrange object's Time past Periapsis (days) field.