Maybe add some text about the struts here in the opening text.  Something like:

You can also optionally specify that the secondary mirror (or prime focus cage or whatever) are held by some number of support struts.  These are taken to be rectangular obscurations extending from the outer edge of the pupil to the outer edge of the obscuration disk (or the center if obscuraction = 0).  You can specify how many struts there are (evenly spaced in angle), how thick they are as a fraction of the pupil diameter, and what angle they start at relative to the positive y direction.