Please elaborate on the altitude bearing issue. Do the 4 bearing pads fail to contact the same cylinder? Does the OTA tilt up out of the cradle when pushed? Do springs reduce this?
It would be nice to know the exact dynamics so ATMers can avoid doing the same.
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.
Edited by gwlee, 24 October 2020 - 04:52 PM.