Adding aperture disks into the optical path of a microscope-camera setup

[IronWing]

I have a stereo microscope with a camera adapter and I'm thinking I could eek out a bit more depth of field by adding an aperture disk somewhere in the optical path. This is the setup:

http://rusoptics.com/media/2010/03/mb10loaded.jpg
minus the ring light and using a Nikon D90.

The current optical path is:
specimen=>primary objective=>secondary objective=>diversion prism to send the image from the righthand optical path to the camera attachment=>mirror=>camera sensor. The limiting aperture is the diameter of the secondary objective which is about a centimeter.

I can probably fit an aperture disk in the path at the top of the tube just before the camera or at the bottom of the tube just above the mirror or possibly inside the attachment body after the prism.

Any ideas about where an aperture disk would make the most sense in this setup?

 

[IronWing]

Thinking about it some more I realized that there is no opportunity to insert an aperture disk as the scope uses "infinity" optics so that the light path is virtually parallel from the secondary objective to the sensor. Any aperture disk would have to be inserted between the primary and secondary objectives and there is no way to do that on this scope.