[mental4astro (Alexander)]

Serruier truss is more complex than being thought of as 'rigid'. It is not. What this truss system does is flex, but in a controlled manner. The flex in the secondary cage is matched by an equal flex in the primary mirror cage so that the two remain parallel, and the optical line remains exactly the same. Call it flex, deflection, flop - it is all the same thing.

Now, this flex is very small, which can confuse things with saying that the system is rigid. And when the scope is rotated in any direction, this flex is exactly the same. Very important to this too is that there are no 'appendages' cantilevering from the structure as this will fundamentally change the way the system will react to the shifting moments. This is what you are experiencing Stephen. More successful systems see the camera placed in the line of the optical axis, and the system is designed to accommodate this weight where it is located.

When you look at large professional instruments using the Serruier system, the Hale 200" as an example, the primary mirror arrangement is massive and short. The secondary end is long and light weight in comparison. But the entire system is centered around the instrument's center of gravity which is key to the truss system working successfully. The cameras are located wholly within the optical line or in the Coude focus which runs along the optical line and the centre of gravity.

This is what I believe Ken is also saying.

Where we stuff things up is by loading a massive camera off one side of the secondary cage. The system is not balanced as the scope rotates in RA and Dec., the huge camera load shifts the COG and the flex is uneven.

One simple way to control this is by adding a counterweight on the opposite side to the camera. This can add its own set of complications with over loading a mount, which you describe Stephen. To counter the need to rework the poles, loading the primary mirror end would help. Again, this adds load to the mount. And the poles being able to work properly is necessary to be determined with all this added weight.

Ultimately, a balanced system is what is needed. This will have the truss system working the way it is designed to. The truss system controls the direction and amount of deflection, but the two ends need to be matched.