Ok. So we are talking about a high frequency shake that happens during focusing.
The XT8 is OTA a -48” beam that weighs 20# with much of the weight concentrated in its ends. The OTA is attached to its 21# base at its balance point with two springs that pull it into a pair small diameter altitude bearings. When the OTA is disturbed by touching the focuser for example the image in the EP vibrates (shakes) in proportion to the disturbance.
Several years of experimenting leads me to believe that most of the shaking is caused by undamped ringing at the interface of the OTA and the base, which is the altitude bearings. However, I haven’t proved it to my satisfaction.
Going to a 2-speed focuser reduces the disturbance to the OTA, so reduces the shakes, but adds weight. Astro Goods (https://www.astrogoo.../bearings.shtml) sells a bolt on kit that significantly increases the size of the altitude bearings, but adds weight. When the pandemic ends, I want to take a look at these bearings and perhaps order a set to evaluate.
My original goal was to see if it’s feasible to reduce the weight of the #41 XT8 to 34#, the weight of the XT6, to combine the performance of an 8” scope with portability of a 6” scope, so adding weight is going in the wrong direction.
There’s no doubt that I can reduce the weight of the BB base from 21# to 14# by cutting enough holes in the base, which will definitely reduce the weight of the XT8 to 34#, but the precise effect on the shakes is difficult to predict, and the goal is to reduce the weight without increasing the shakes.
It seems possible that some weight can be taken out of the BB base without increasing the shakes if the primary contributors to the shakes are the altitude bearings as I suspect. For the moment, I am using the XT8 with its BB base while I decide the next step.
I wonder if the secondary is vibrating.
The thick tube would be too solid to vibrate.
Maybe the focuser is stressing the sheet metal and vibrating.
I don't think the tube would vibrate on the pads. I think it would be just up or just down.
Only other place is feet and az pads, but sort of doubt that. I think it would not reach the threshold to budge.
Now there is total mass vibration, but I think that heavy mass would not do that.
So my money is on the focuser angle changing, or the secondary vibrating.
The focuser attachment sheet metal can be braced or beefed up. And the secondary would do much better with a front counterweight, which has been tested and beatified to at least protect against collimation drift.
How you describe the OTA, sounds like it would have a big moment of inertia. Though who knows how stiff that metal really is. The problem with saving weight on metal is stiffness goes down, even if otherwise strong enough. Through some tubes have flexed under their weight, or got dented at the center rings some other how.
Edited by stargazer193857, 24 October 2020 - 08:49 PM.