Sling at perfect Centre of Gravity

This is the surface deformation for a sling supported mirror (30" diameter, 2" thick, F/5). The sling stands exactly at the centre of gravity of the mirror (22.5 mm from back side). The picture shows the residual abberations after defocus has been (rougly) removed. As seen the PTV surface z-displacements are less than 20 nm.
The following two pictures compare this case to the case of the sling standing 2.5 mm above the centre of gravity. The color scale, for comparison, is the same: -60nm to 30 nm. The conclusion is that a perfectly placed sling almost zero the residual aberrations, but this is the result of the cancellation of otherwise large deformations. Placing and keeping the sling exactly at the centre of gravity is critical. In practice any deformation of the mirror is likely to be due to "imperfections" in sling placement, friction, sling planarity (included the straight parts to the suspension points). In a word what matters is "robustness" of the design, or the sensistivity to undesired noise factors. More on this...