The following images (click on them for larger versions) show how the prism and mirror slider assembly is attached.
The compenents here are the pentaprism which redirects the light up to the observation head, a pair of mirrors at 45°, rotated 90° from each other, to select which camera port to use, and a lens that compensates for the long path length, as compared to the 160 mm tube length objectives needed. The lens is fixed, but the prism and mirrors are on linear dovetail sliders which are actuated from the exterior of the right hand side of the body
In my experience, the dovetail sliders are often frozen from years of neglect with hardened grease. In this case, I removed it entirely from the instrument and freed them with acetone, cleaning the unit thoroughly before reassembling with fresh machine oil. They work smoothly now. It is also possible, and the preferred method to free these sliders without removing it from the base. To do so, I used small amounts of acetone again, slowly breaking down the hardened grease until the full range of motion was possible, then applying fresh oil. As seen in the below photos, there are two screws that act as stops. Removing them can allow you to slide each unit out of the dovetails (after unscrewing the actuation knob) for more thorough cleaning.
The pentaprism, while being a single block which is long, due to the slider action, performs two functions. If the actuator knob is pushed in, it acts to redirect all of the light up at a 45° angle. With the knob pulled out, a second prism, bonded to the bottom, frustrates the total internal reflection and with partial silvering, redirects only 20% of the light up, with the remainging 80% now continuing down towards the two mirrors. One mirror directs the light to the front camera port and the other to the side camera port - one must choose which port.
It would be simple to install this unit if the objective focus mechanism were not in place, but then you could not install the focus mechanism...
As it is, I came in from underneath and angled it in, avoiding the front port camera lens. It was not trivial - and I do not recommend it unless there is no other choice. The alignment is difficult and critical to get the camera ports and eyepieces well centered in the field. Furthermore, it seems as though this assembly should be pinned in place, but it is not, so there is some slop in the placement with the two bolts. This has two major ramifications. If not properly positioned, the mirror slider actually hits the lens housing and cannot actuate properly; secondly, the alignment of the image up to the observation head is affected, so final alignment needs to be done with the system completely assembled. I found it difficult, if not impossible, to get it perfectly centered.
Next, I replaced the observation head assembly and covers.
for navigation!