M5 is a ~3×2.4 elliptical field stabilization mirror. The tip-tit requirement due to wind shaking is the main specification in term for stroke for M5. The atmosphere is the second term but its requirement in term of stroke is an order of magnitude below the one requested for wind shaking. The PSD correction implies a stroke PV of ~360 micron. Additional stroke needed for gravity and thermal changes is completely negligible.
M5 mirror optical quality is smaller than 0.5 µm PtV for scales larger than 40mm and lower than 15 nm for scales smaller than 40mm. In passive mode under gravity and thermal loads, the M5M shall not deform more than 250nm rms for scale larger than 40 mm. Specifications on both density and first resonance frequency of M5 light weighted mirror have been indicated in M5 specifications to fully constrain the mechanical structure design. To avoid a floppy structure, position stability of both M4 and M5 is specified for the various modes to obtain a good passive stability with look-up tables.
Current design concept
The E-ELT M5 Field Stabilization Unit (M5FU) is a fast correcting optical system that shall provide tip-tilt corrections for the telescope dynamic pointing errors and for the effect of atmospheric tip-tilt and wind disturbances (0.01 to 100 Hz region). It is composed of a large (> 5 m²) light-weighted optically flat elliptical mirror, supported by a tip/tilt mechanism to actively control the mirror angle to a high precision and large bandwidth, in comparison to the size and weight of the mirror.
In the current design, the tip tilt mechanism is driven by three actuators, including relevant sensors, control local loop based on advanced algorithms and the required electronics to receive and treat the command and feed-back signals.
The M5 Mirror interfaces with the M5FU by means of 3 actuators placed quasi-symmetric and a central membrane.