Antenna

The antenna consists of parabolic primary mirror (10 meters in diameter) and hyperbolic secondary mirror (Cassegrain optical scheme).

The primary mirror is too large for existing fairings hence it is has a structure of segmented deployable solid reflector. Thus, the primary mirror of Millimetron Space Observatory has the solid central part (3 meters in diameter) and 24 deployable petals, which will unfold in orbit.

The unique parameters of the antenna will be provided by the several design solutions for the primary mirror:

  • Material for the reflecting panels and the basic structure of petals is carbon fiber reinforced with plastic (CFRP). It has superior properties regarding the combination of high specific modulus and low coefficient of thermal expansion (CTE),
  • Each petal consists of a spatial framework and three independent reflecting panels,
  • Each panel will be mounted on the petal framework using three linear actuators,
  • Adjustment of panels will be performed by an active surface control system.

These solutions will allow to correct a misalignment, associated with the processes of deployment, and distortion of the surface, caused by thermo-elastic and hygroscopic effects.

Parameters

Antenna key parameters:

  • Primary mirror with 10-meter aperture;
  • The WFE of the whole antenna is 6μm RMS (goal);
  • The operating wavelength of antenna from 70 μm to 10 mm (goal);
  • An equivalent focal length is about 70m, FOV about 7 arc minutes;
  • Mass of antenna structure (including adjustment system) is about 1700 kg;
  • Operational temperature < 10K (4,5 K goal)

Folded configuration of antenna

In folded configuration the primary mirror is held in place by two fixation rings: internal and external. The internal fixation ring is located on a supporting truss of the secondary mirror. This ring contains 24 spherical tips. At the same time, each petal framework contains a conical socket, which is resting upon a matching spherical tip. The external fixation ring wraps the petal frameworks on the same level as internal. The fixation is organized by a pre-tensioned cord, which keeps all locks between petals in the latched state. Upon the activation of pyrotechnic charge, the pre-tensioned cord would be cut and release the petals for opening.

Animation of deployment strategy

The concept of the Millimetron primary mirror deployment is based on the previous successfully implemented antenna of Radioastron project. Folding and deployment of the petals is carried out in one operation – rotation along one axis through synchronous system. Such approach reduces the complexity and improves the accuracy of the deployment mechanism. In addition, that provides sufficient space, needed to ensure additional rigidity of the petal framework and a sufficient packing density of the petals in the folded position. Special latches will lock the petals to each other at their edges in operational configuration. Simultaneously, each petal is fixed with the corresponding brace strut via a lock at its end. After unfolding the primary mirror must hold a position of deployed petals with accuracy better than 1 mm from the pre-determined surface.

 

Time-lapse of the full-scale mock-up deployment

Due to the strict requirements for the form stability and the large size of the main mirror, it is important to control and check these parameters during and after the deployment process. Thus, to implement the verification plan it was necessary to produce a full-scale engineering mock-up of the primary mirror.

During the test program of this mock-up a set of crucial parameters and engineering solutions have been verified: deployment system performance in terms of deployment accuracy and repeatability, a unique zero gravity system, the latching system of petals in the operational and transport position.

Panels

The primary mirror consists of 24 panels of the solid central part and 72 panels of the deployable petals. The panels have the parabolic form with the focal length of 2400 mm.

The panels have a sandwich structure comprised of a parabolic front face sheet, rear face sheet, ribs, mounting pins and interfaces for the thermal bridges. They are made of carbon fiber reinforced with plastic (M55J/NIIKAM) based on a high modulus carbon fiber and cyanate ester resin. For the front and rear face sheets, quasi-isotropic layup is chosen whereas for ribs it is orthogonal. The material offers low areal density combined with the high specific stiffness, low coefficient of thermal expansion, low moisture absorption and increased resistance to micro cracking. To improve the longitudinal thermal conductivity of the panels, the heat-conducting layer of the pure copper foil (6 µm) on the rear surface is used. All these things allow creating a thermally stable and precise design with the minimal mass.

Parts of the panels are glued with cryo resistant adhesive which also features unique physical and mechanical properties as it has to be resistant to both high (≈ 100˚С, due to manufacturing technique) and extremely low (less than -250˚С, due to operational conditions) temperatures.

Structure of the panels (central panel is used as an example)
The photo of the central panel from the rear side

The panel manufacturing is based on the replica technique. For the demonstration of the manufacturing process, the trial central panel was produced. The measured shape accuracy of fabricated panels is 4.2 μm (estimated in RMS). The comparison of the surface accuracy of the panels with the mold indicated a possibility to produce panels with the surface accuracy better than 1 μm (RMS) and with the repeatability about 1 μm.

Trial parabolic mold from ULE glass and its SFE map
The central panel (1.3 m long) and its SFE map