Pseudo VS true serrurier truss

[coldlegs]

Recently my “pseudo” serrurier truss underwent a major re-build that included adding heim joints and turning it into a “true” serrurier truss. Then I started to discover things about it that I'm not sure I like at all. You see I normally park any scope pointing west so it fits under the closed roof of the observatory and the camera is nearest the door and easy to collimate/work on etc. So the collimation was done facing west but when I rotated the scope east it lost collimation as the cats-eye triangles were quite spread out. Think I finally worked out why. When the scope rotated the camera changed position relative to the center of mass of the front cage and the front half of the truss moved in a slightly different direction to the back half hence loss of collimation. This means that there has to be a weight equal to the camera train and focuser assembly somewhere on the opposite side of the front cage. That would be about 2.7kg. Here's where things start to get ugly. If I add the weight my balance is up the creek and the strut lengths have to be re-done or I add about 3kg to the rear making the scope around 5.7kg heavier. With the scope weight currently at 15.5kg with the new welded center and all the mods plus 2kg camera/onag etc giving 17.5kg then I'll be up to 23.2kg on a neq6pro mount. Not good for the old girl! The other choice is to re-do the struts and save 3kg which is more practical. The other big ugly is when I ditch the camera /onag and install a webcam for planetary imaging. I will need to have a weight on the camera side to make up for the weight disparity and it will need to be where the center of mass of the camera/onag setup was to work right.
At this point I'm thinking a “true” serrurier truss is a “pain in the mass”. Rolf and quite a few others do excellent imaging with “pseudo” serrurier trusses that have the struts permanently fixed and that's the way I'm heading back to as well.


Cheers
Stephen

[ZeroID]

I think I've made my 'Pseudo' Serrurier truss scope ( and am making the current RN 8F8 build ) so rigid nothing will move even if it wants to.

My primary reason for the original rebuild was to get the weight down so I could put the 10F5 up on the EQ6. My cage and strut dimensions were more to balance the weight over the OTA length so no additional weights were needed. As such it more or less accomplishes most of what a true Serrurier design sets out to do. My 10F5 build was so strong and immovable I could stand it on end and stand on top of it yet it only totalled about 17-18 kg up on the EQ6 including additional hardware.

The current 8F8 build will be even lighter although it's a longer OTA with an older and heavier mirror. I do this by minimising hardware and putting strength where it is necessary. The mirror in this case is heavy (2.45kg) but the mirror cell will be a diaphragm braced structure, almost hollow and the back plate similar. Check out progress in the DIY section.

[Merlin66]

Stephen,
Your comment:
When the scope rotated the camera changed position relative to the center of mass of the front cage and the front half of the truss moved in a slightly different direction to the back half hence loss of collimation.

This is what the Serrurier truss is all about.
The mirror cell, centre section and top ring must be rigid enough that there is no distortion or deflection.
The stiffness of the front trusses have to be matched against the back mirror cell trusses.
IMHO the problem comes down to the choice of material, section sizes and deflection calculations. Not additional weight or mass.

[coldlegs]

Ken
I should have phrased that better.
“When the scope rotated the new camera position resulted in the front cage center of gravity to be in a different position and whilst the balanced back half did its serrurier stuff and sagged straight down the unbalanced front sagged at a different angle and collimation was lost”.


I could be wrong here as I've been up the creek without a paddle before but I'm reasonably sure of the logic.


Cheers
Stephen

[Merlin66]

Stephen,
In the original Serrurier truss design/ concept the pin joints at each end of the truss connections controlled the direction and flexibility of the end ring deflections.
Even Serrurier had to compromise when they decided to add the "slot" along the tube of the 200" to allow optical access to the Coude focus.....

[mental4astro]

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.