Primary Mirror

EST is a Gregorian telescope with an on-axis f/1.55 primary mirror installed in the M1 Assembly, which provides the following capabilities to ensure the best optical performance of the system: positioning, active shape correction, thermal control, and interfaces with the telescope structure.

The M1 Assembly is mounted above the telescope's elevation axis and is directly exposed to the wind, supported by a stiff active axial support system which compensates for all static and slowly changing deformations. The active system is also able to compensate for aberrations introduced by the optical elements within the remaining light path.

Figure 1. Exploded view of the EST M1 Assembly.

Characteristics and thermal control

The EST primary mirror is a solid meniscus with 70 mm thickness supported by 4 rings of tripods (see Figure 1). It has the shape of an annulus of 4.2 m external diameter and 1.3 m internal diameter.

There are 80 axial actuators with 3 N/µm stiffness, 10 mm stroke, +650/-350 N  force range, <±0.2 N force repeatability and position resolution better than 0.1 µm. In addtion, there are 20 lateral actuators in tangential configuration with the same characteristics than the axial actuators, but with 1:4 lever in order to increase the force range. These elements allow active optics corrections to be performed.

The thermal control of the M1 Assembly is based on air jet impingement applied to the backside of the mirror with a speed of 10 m/s at 13.5 ºC below ambient temperature. Sixteen sets of fans plus heat exchangers are in charge of air-cooling and recirculation, making it possible to maintain the mirror surface temperature within +0.5/-2 ºC of the ambient temperature. In addition, a passively cooled external cover is implemented in order to maintain all the external surfaces of the M1 Assembly except the mirror surface at ±2 ºC. Special efforts have been made to minimise the vibrations produced by the cooling system.