Success at last:Last weekend I finally had some time to open the microscope a bit more. I removed the black bottom plate with glass aperture.
At this point I could see every bit of the optical system. This proved conclusively that nothing is broken or moved.
At this point it was clear the only part of the optical system that could be giving me trouble with double image in the middle is the right angle mirrors:
Mirrors looked to be approximately at the right angle. How can we make sure? Lasers of course!
If mirror are at the right angle they will return the beam that is parallel to the source. More over if you point laser exactly at the point where mirrors meet, they will return a single dot if they are aligned and dual dots if they are not. The distance between two dots is representative of the angle error. If the mirrors are perpendicular to each other and laser beam if perpendicular to line where they intersect, the mirrors will return the beam back to the source.
In a pinch I used good quality well aligned laser from a distance meter and several 1-2-3 blocks.
Simple experiment with a target 4 feet away from the mirror revealed about 1.5" error! Then I raised the contraption by 1" to measure different section of the mirror, the error was almost the same. Set screws were user to correct the error as much as I could. Once seemingly perfect, then I repeated the same experiment at 15 feet and adjusted again and well and I could. This time the error was about 1/8".
I was not sure what is good enough here. So I assembled the microscope back to check and ... IT WORKED!!! The double vision is gone. The picture is spectacular.
The design of the Mantis is ingenious. It is almost too simple - four plastic parts, bunch of mirror held by double tape and single large lens. Yet I still do not completely understand how and why it works. There is no separate image path for every eye. All the optics on inside is "wide enough" to cover both eyes.
Taking pictures of bunch of mirrors is almost pointless. It is like Alice in Wonderland: all you see is countless reflections and not the reality.
Here is a crude diagram of what I found inside, with some basic explanations: