A.

Optical design

The anamorphic telescope relies on an afocal off-axis four-mirror design. The optical layout is depicted in fig. 2. The rectangular entrance pupil with a size of 150 mm by 25 mm can be seen on the bottom left side. All mirrors are freeforms of the so-called biconic type:

The mirrors share two common planes of symmetry, the intersection of which corresponds to the optical axis in conventional systems. The entrance pupil of the telescope is shifted with respect to this axis and the viewing direction is slightly tilted. Hence, the used areas on the mirrors are far off the axis of symmetry.

Fig. 2.

Optical layout of the telescope. The entrance pupil is on the bottom left side.

The telescope images a field of ±3.22° in vertical and ±0.47° in horizontal directions, respectively. The corresponding magnifications are 0.5x and 3x, respectively. Orientations are given with respect to the field of view of the satellite.

The optical design yield diffraction-limited wavefront quality for the intended wavelength range of 6 to 13 μm. The required surface quality of the mirrors of 1.4 μm peak to valley is compatible with the intended fabrication approach by single-point diamond turning.

B.

Mechanical design

To simplify the fabrication of the mirror as well as the alignment of the telescope, the mirrors were arranged in pairs. Each pair of mirrors is fabricated in a single machining run together with the respective reference surfaces used for alignment [4]. This procedure results in an excellent knowledge of the mirror positions w.r.t. each other and to the reference features.

Fig. 3.

Mirror subassembly M1-M3. Reference features for alignment are marked red.

The housing of the telescope is based on a light weight box design made from two shells. The mirrors were attached to the housing via kinematic mounts to prevent deformations of the housing from influencing the mirror surfaces. However, the stiff substrates of the mirror pairs have some stabilizing influence on the whole system.

To simplify alignment, the housing carries reference surfaces corresponding to those on the mirror substrates. Those surfaces are finished by single point diamond fly-cutting of the assembled housing structure.

Fig. 4.

Mechanical layout of the telescope. The entrance pupil is on the back side.

In order to minimize influences of planarity tolerances of the optical bench of the instrument, the whole telescope is mounted kinematically on three bipods.

The current design results in a total mass of the telescope as low as 6.3 kg. Nevertheless, the lowest resonance frequency, which is important for the stability of the telescope against vibrations, is as high as 319 Hz (see fig. 5). The corresponding deformation pattern shows that it is limited by bending of the housing in the areas of the large optical apertures.

Fig. 5.

Lowest Eigenmode of the telescope at 319 Hz. Colours show the corresponding displacement in arbitrary units.

Since the whole telescope is made from a single material (Al 6061 T6), an athermal behavior under variations of the ambient temperature is obtained. The response to thermal gradients is low because the thermal conductivity of the telescope housing is large compared to that of the kinematic mounts (see fig. 6).

Fig. 6.

Response of the telescope to a thermal gradient of 1 K in y-direction. Colours indicate the corresponding displacement in mm. The maximum value is 1.2 μm.

A.

Manufacturing of the mirror pairs

The mirrors M1/M3 and M2/M4, respectively, are machined on a common mirror substrate to bind the position and orientation of the two elements with respect to each other. The fabrication of the freeform surfaces is based on a freeform manufacturing approach using a Fast Tool Servo (FTS) system [6]. The FTS is a redundant kinematic in addition to the in-feed axis of the ultra-precision machine. The actuator is a voice coil servo with a stroke of ±3 mm. The working principle is based on the synchronous feed of the FTS slide to the angular position of the working spindle to generate the freeform shaped surface.

To fabricate two mirrors on a common substrate in a single machining process, the two surfaces have to be described as one continuous surface in a common polar coordinate system.

The center of revolution for the mirror assemblies is shifted with respect to the axis of symmetry in order to minimize the diameter of the work piece. Furthermore, the surfaces were tilted to minimize the FTS stroke which must be smaller than 6 mm.

In the next step, the resulting surface profiles were split in a rotationally symmetric and a non-symmetric part in a first step. The symmetric part is processed like a conventional asphere, i.e. by a slow motion of the support.

Subsequent to the transformation and splitting, the surfaces are compensated for the contour radius of the diamond cutting tool to derive the tool center position. The description of the whole circular cutting surface is achieved by extrapolating the optical surfaces to a continuous surface. The freeform parts of the resulting tool paths obtained for both mirror pairs are depicted in fig. 7.

Fig. 7.

Freeform part of the calculated tool paths for both mirror pairs.

After cutting, the surface form is measured and the resulting errors are fed back into the tool path. Finally, the peak-to-valley (p.-v.) form errors of 1.2 μm and 0.6 μm could be achieved for the large mirrors M1 and M4, respectively (see fig. 8). In the case of the M1-M3 assembly, the achievable quality was limited primarily by the long processing times of 21 h. During that period, minor fluctuations of the temperature in the range of 0.2 K could not be avoided completely, which led to corresponding deviations in the positions of tool and mirror. Since such variations are not predictable, they cannot be compensated for in the correction loop.

The small mirrors M2 and M3 reached p.-v. form deviations of 0.4 μm.

Fig. 9.

Form error of the large mirrors M1 (left) and M4 (right).

The surface roughness of the mirrors is in the range of 8.3 nm root mean square (r.m.s.) on M2 up to 12.8 nm on M3. Two main sources of roughness could be identified:

Relatively high values of the cutting speed as well as the dynamics of the FTS had to be chosen in order to keep the processing time in a reasonable range (below 24 h). In general, the surface quality decreases towards the outer edges of the mirrors. A sample of such a region is depicted in fig. 10.

Fig. 10.

White light interferometry image of the micro roughness on the outer edge of mirror 1.

Grains in the Aluminum base material were identified as the second source of roughness.

The interface surfaces for the alignment on the telescope housing (reference z, tilt x, tilt y) are machined in the same setup. For this purpose, an additional milling module was integrated on the tool support (see fig. 11).

Fig. 11.

Machining setup.

B.

Manufacturing of the telescope housing

In a first step, both halves of the telescope housing were bolted together. In the next step the positions of the reference flats of the housing were measured.

Then, the assembly was placed on a precision turntable of a single point diamond milling machine. Here, the mounting surfaces for the mirrors as well as the matching reference surfaces on the housing and the optical references were reworked to the machine precision (see fig. 12).

Fig. 12.

Machining of the reference surfaces of the telescope housing.