Note: when viewing the frequency vs height plot, keep in mind that the CAD has the nominal position of the keystone at 16.5 mm below the top ring, and we'd really like to work around that range to ensure that the optic is at the right height. If absolutely necessary we could raise or lower the entire PI a few mm on the jacks, but the bellows will be under stress. That suggests an optimum compression of 23 or 23.5 mm, which would give us a load capacity of 255-257 kg and a frequency of 0.6 Hz around 16.5 mm.