I.

INTRODUCTION

As a successor to HIPPARCOS, the GAIA mission will aim at establishing a very accurate three-dimensional map of the objects of our galaxy and at mapping their motion. The GAIA payload module is developed by EADS Astrium and is built from sintered silicon carbide. It includes in particular two three-mirror anastigmatic telescopes (TMA) and a set of flat folding mirrors.

As a subcontractor of EADS Astrium, AMOS is responsible for the polishing and figuring of the secondary mirrors (M2) and of the flat mirrors (M4 and M5).

II.

MIRROR CHARACTERISTICS

The mirror blanks are built from sintered silicon carbide (SiC) by BOOSTEC. They are produced to their final geometry, the optical face lying after rectification at a few tens of microns from its wanted shape.

The mirror production steps from the blanks include:

Table 1 herebelow summarises the mirror main characteristics.

Table 1.

Mirror main characteristics

III.

M4 MIRRORS

M4 mirrors are relatively small items but require a great accuracy. Their surface figure must be better than 5 nm RMS and their residual radius of curvature has to be higher than 250 km.

For the first operation (CVD-cladding), a baffling device is installed to protect the mirror zones that have to be kept free from CVD-SiC.

A view of this mask is provided in Fig. 1 (Courtesy SCHUNK).

Fig. 1.

Baffling of M4 viewed after CVD deposition

The baffles are removed afterwards and the mirror is controlled and cleaned, the CVD layer thickness is measured and the lapping operation begins.

Conventional lapping techniques are used for those mirrors, associated with the continuous control of the CVD layer thickness, which must reach the target of 100 μm ± 50 μm.

There is also a continuous transition from lapping to polishing, the figuring phase being performed under vacuum with an ion beam.

The final step is to deposit onto the mirror an enhanced silver coating, qualified for the GAIA environment.

For all AMOS GAIA mirrors, the coating operation is performed by SAGEM-REOSC.

Fig. 2 depicts the final mirror surface figure without power measured after coating under 0-g conditions on M4-2037 mirror (Fizeau test). The last figuring operation lets a small residue of convexity on the mirror to counteract the coating tensile effect. The residual radius after coating stands around 400 km concave.

Fig. 2.

Mirro M4-2037final surface figure at 3 nm RMS

Fig. 3 shows the mirror at delivery.

Fig. 3.

Mirror M4-2037 at delivery

IV.

M5 MIRRORS

M5 mirrors are large flat mirrors that follow approximately the same general manufacturing sequence as the M4 mirrors, except that some specific steps are called for, essentially during the lapping phase.

Due to the mirror sizes, the interferometric test involves stitching of diameter 400 mm subapertures maps provided by a flat reference within a Fizeau cavity to reconstruct the full mirror aperture (Fig. 4).

Fig. 4.

Subaperture stitching principle for M5 mirror testing

The main quality requirement involves subapertures of 90 mm by 40 mm on which the surface figure must be better than 4 nm RMS and the local tilt vary less than 0.04 μrad RMS on a 18 mm shift along the long mirror size. Fig. 5 shows the surface figure values measured after coating in 0-g conditions on a Monte Carlo selection of subapertures.

Fig. 5

Surface figure values on a set of M5-2285 subapertures (mean 1.9 nm RMS)

Fig. 6 shows the mirror at delivery.

Fig. 6.

Mirror M5-2285 at delivery

V.

M2 MIRRORS

The M2 mirrors are the most challenging to face in the AMOS set.

Being convex and off-axis, they are inherently difficult to test and present in addition a high departure to the best fit sphere and an aspheric slope of some 8 mrad.

The principle of the interferometric test is to realize a null configuration in double pass on the mirror, using a computer-generated hologram (CGH) and a return sphere as auxiliary devices.

The test scheme is illustrated in Fig. 7.

Fig. 7.

Interferometric test lay-out for M2

The CGH null e-beam master on a fused silica photomask substrate is located in a diverging spherical test wavefront from the interferometer equipped with a f/3.3 reference sphere. The M2 vertex is decentered 50 mm from the CGH axis.

The minimum grating spacing within the critical null aperture is about 108 lp/mm.

The CGH null incorporates one retro-reflective (accuracy in axial positioning of about 3 microns), one autocollimation (parallel to M2 asphere axis) and one focused spot (concentric with return sphere) alignment features.

The return sphere has a radius of curvature of 1000mm. Its dimensions are 800x350mm.

The sphere is mounted in a dedicated cell that minimizes its deformation under gravity.

The test bench is aligned through a dedicated procedure in accordance with the following error budget (Table 2).

Table 2.

M2 test error budget

The measured phase map is corrected for distortion according to a dedicated software.

In order to take into account the uncertainty brought by the test set up, the figuring criterion is set at a value such that if added to this uncertainty in a root-sum-square sense, the obtained value remains at the specification.

A picture of the mirror under test is given in Fig. 8. The last manufacturing step on the mirror is performed by ion beam figuring (IBF).

Fig. 8.

M2 test set under test

VI.

SUMMARY

The steps performed by AMOS for manufacturing SiC mirrors for GAIA were here briefly described and commented.