Hi pcd109, thank you for your interest in the optical layout. Very simple. A flat 29 inch mirror reflects light down a baffle tube (looks like a refractor, but no lenses) into a 2 inch format rotatable star diagonal. No tertiary per se, well yes, since the diagonal functions as such, but the diagonal also allows what I believe, a more comfortable eyepiece viewing position, the viewer looks slightly "down" into the eyepiece. No neck strain. The maximum eyepiece height when the scope is pointed high into the sky (remember no access the zenith due to gimbal lock, or often commonly referred to as "Dobson's Hole") approx. 12 feet or so ladder height. However, for objects being viewed 45 degrees and lower, a modest step ladder, or flat-footed observing is possible. Mike Clements "If you want to reach the stars, You'll need a big stairway".....(Spanish proverb)