The M1 cell and M3 tower

Definition

Each Unit Telescope has a primary mirror of 8.2 meter diameter supported by a structure, called M1 Cell, attached to the telescope tube. The telescope tube is closed at the bottom by the primary mirror cell, while at its top it supports the secondary mirror unit.

Physically the M1 Cell is a 3D metallic welded structure which is attached by twelve flanges to the main telescope tube. On the cell's superior side, the primary mirror is positioned and held in place by axial and lateral supports. On the bottom side of the cell there is an attachment flange for the Cassegrain adapter/rotator.

The M3 Tower is mounted in the center of the M1 Cell and can rotate along its axis (ideally coincident with the azimuth axis of the telescope). It supports the elliptical tertiary mirror. The tower extends from the M1 Cell up to the level of the altitude axis through the central hole of the primary mirror. A mechanism at the top of the M3 Tower is used to move the mirror away from the optical path when using the Cassegrain instrument. This mechanism also allows a remote, fine adjustment of the M3 position.

For maintenance, the complete M1 Cell-M3 Tower unit weighing about 48 tons is removed from the telescope and put on the fork base floor. After removal of the M3 Tower, the M1 Cell with the primary mirror is transported from the enclosure to the Mirror Maintenance Building for recoating.

Here, you can see a CAD drawing of the M1 cell and M3 tower assembly, complete with axial and lateral supports and primary mirror.

The M1 cell-M3 Tower is presently under construction by the consortium GIAT Industries, Branche GITECH and SFIM Industries (France). GIAT Industries has recently finished the assembling of the first M1 Cell. Proprietary laser cutting and welding techniques were used for cutting and welding the single assemblies of the M1 Cell. This process, completely automatized, allows to generate complex box-type structures starting from thin metal sheets. Due to the fact that no fillet material is used in the welding seams, a very limited zone of the base metal is affected by the heat and low residual stresses exists. Therefore an excellent stability is attained, low distortions are experienced during annealing and no overmass is necessary for final machining of the parts.
The final M1 Cell structure has a mass of approximately 10 tons, and exhibits a very high stiffness/mass ratio.

Here below two views of the M1 Cell are available.

Overall view

top surface

You can access larger pictures by clicking.

The two following photos show the M1 Cell during its turning over, after its assembly face down on a dedicated tool.

You can access larger pictures by clicking.

Summary of main functions

The main functions of the "M1 Cell-M3 Tower" are as follows:

• to support the primary mirror axially and laterally:
  • to apply at 150 selected actuator locations, axial forces onto the primary mirror back in order to control its optical figure. This feature is also used when changing from Nasmyth to Cassegrain focus (active optics)
  • to provide means to adjust remotely the position of the primary mirror
• to support the tertiary mirror and to select its position for focus selection
• to support the Cassegrain adapter/rotator and instrumentation adapter/rotator)
• to cool actively the primary mirror with the back plate cooling system

The M1 Cell-M3 Tower has its own Local Control Unit in order to perform the tasks and functions required by the Telescope control System and to perform its own monitoring, diagnostics and safety actions. A dedicated support system is activated in case of earthquake to protect the primary mirror.

Axial and lateral mirror supports

The axial supports of the primary mirror are constituted by a passive and an active part.

The passive axial support system of the primary mirror consists of 150 hydraulic pads connected in three independent sectors so as to form a whiffle tree. This allows to compensate for deflection of the M1 Cell under the effect of varying gravity load.

The active axial support system consists of 150 electromechanical active supports distributed in 6 concentric rings and which correct at variable time intervals the shape of the mirror by applying controlled forces on its back surface. Each active support is coupled to one hydraulic pad of the passive system so that passive and active force components are added up.

In the lateral plane the mirror is restrained by 64 hydraulic passive supports which, similarly to the passive axial supports, produce a reaction force to the portion of the mirror weight imposed on them. The total weight of the mirror is distributed between the axial and the lateral system depending on the zenithal angle (angular distance of the telescope tube from zenith). The lateral supports are connected into two circuits in order to provide equal force and not to overconstrain the mirror.

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