The EST optical configuration is Gregorian with the primary (f/1.55) is an on-axis mirror with a central obscuration. The outer diameter of the optical used area is about 4.1 m, the obscuration has a diameter of about 1.4 m. Sunlight will heat up the mirror surface without cooling. A cooling device at the back of the mirror will cool it to near ambient temperature, while the remaining warm air on the illuminated front side will be removed by a flushing system.
The primary mirror and mirror cell are mounted above the elevation axis and are directly exposed to the wind, supported by a stiff active axial support system which compensates for all static or slow changing deformations. The active system is also able to compensate for aberrations introduced by the optical elements within the remaining light path.
The mirror design is driven by the stiffness and cooling capabilities. Either a thin monolithic mirror or a thicker light-weighted mirror is possible. Investigations showed that a light weight, meniscus-shaped mirror with a moderate number of actively controlled axial actuators provides the best performance. The stiffness and cooling performance is better than for a monolithic mirror but the complexity to light weight and manufacture the mirror is much higher.
The light-weighting patterns are hexagonal cells (about 640 in total) with an inner diameter of 125 mm (72 mm cell side). The weight of the mirror is 2514 kg. The constant height of the mirror is 246 mm, the rib wall thickness 18 mm and the constant front sheet thickness 22 mm. The backs of the mirror cells have no undercuts. The mirror is supported by 96 independent axial supporting points and 30 lateral supporting points attached to the mirror cell.
The first two fundamental mirror frequencies (free-free conditions) are 33.7 Hz. Mode 3 and 4 have 90.5 Hz.
The mirror cell shall provide the high stiff support when the mirror blank faces the dynamic effect of the wind load. It shall accommodate the other subsystems: the cooling system, the suction systems and the support elements (actuators).
The M1 mirror cell is a dodecagon welded steel structure. The total weight is about 9.5 t. The diameter of the inner hole is 1300 mm; the maximum diameter is 4.9 m. The nominal rib wall has 15 mm, the inner wall 20 mm and the back plate 10 mm thickness.
The M1 cell structure attaches to the telescope structure at three main attachment points, providing an isostatic constraint.
The mean local stiffness at the actuators mounting points is 4·108 N, this is more than an order of magnitude higher than the stiffness of the actuators (107 N).
The 96 axial actuators are installed inside cylinder tubes and react against the back plate of the cell structure. They could be installed and removed without dismounting the primary mirror blank. The axial actuators are individually active controlled. The 30 lateral actuators are attached to the outer edge of the mirror cell. The lateral actuator is a passive hydraulic system. The directions of forces are according to the Schwesinger concept.