M1 subcell.

The f/0.93 elliptical segmented primary mirror has a diameter of approximately 39 m and a 11.1 m central obstruction. The filled outer radius of the primary is 18.5 m and the inner 5.5 m.

798 segments will be mounted, plus a maintainance set, for a total of 931 segments. The segments are hexagonal in shape, which permits the use of a common support structure for all segments. The maximum corner to corner dimension for the segments is about 1.45 m.

The primary mirror segments of the E-ELT can be moved in piston and tip-tilt and therefore can compensate for some deflections of the mirror cell due to temperature and gravity.

The segments are axially supported on identical 27-point whiffletrees. A lateral restraint is located in the centre of the segment using a membrane to allow limited motion in the direction orthogonal to the back surface. A clocking restraint is used to further limit the rotational freedom of the segment. The segment and supporting structure is moved in piston and tip-tilt using three position actuators. Inductive edge sensors are used to provide direct feedback to the position actuators and for global reconstruction of the mirror shape. Each segment subunit weighs approximately 365 kg including the position actuators, edge sensors etc but not necessarily their electronics.

M3 unit.


The active secondary mirror is a thin meniscus convex mirror of 4.2-m diameter. The useful area of the secondary mirror is annual with the inner radius at 545 mm and the outer at 2045 mm.

The total weight for the M2 system will be less than 12 tonnes. Given that this large weight load will be hanging above the M1 mirror obviously great care is taken about protecting the mirror from falling down.


The tertiary mirror unit is based on a thin meniscus active mirror. The tertiary mirror is a mildly aspheric concave mirror, 3.8-m across and about 100-mm thin with an hole of approximately 70-mm radius in the centre.

The tertiary mirror allows refocusing together with M2 and M4.

M4 (left) and M5 (right) units.


The quaternary mirror has an approximate diameter of 2.4-m (2380x2340mm). It is a flat adaptive mirror, with up to 8000 actuators, thereby allowing the surface to be readjusted at very high time frequencies. It is specified to be able to deliver very high Strehl ratios (over 70% in an LTAO/SCAO mode) in good conditions.

This mirror will correct in real time for high order wavefront errors (e.g. atmosphere, wind shake, low spatial frequency telescope errors) and small amplitude residual tip-tilt corrections.


The fifth mirror in the train belongs to the tip-tilt/field-stabilization unit. This mirror is a fast steering, ultra-lightweight mirror whose main purpose is to compensate for image motion at frequencies of up to a few Hz and to stabilize field. It is flat and elliptical in contour. The mirror dimensions are approximately 2.6 m by 2.1 m.