High Silicon Content (AlSi, Si ≥ 40 %) As Mirror Substrate For High Performance Cryogenic Metal Optics
Metal optics made of Aluminium 6061 have been widely used to fulfill the demands of an athermal instrument design. Diamond turned metal mirrors are standard optical components in mid infrared astronomical instrumentations working at cryogenic temperatures. Structures and optics can be made from the same material (aluminium) to avoid thermal stress due to different CTEs. However, surface roughness, scattering behavior and form accuracy of aluminium mirrors are limited due to the crystallographic and mechanical properties of the substrate material. Mirrors made of zero expansion glass ceramic or silicon carbide (SiC) can be used for cryogenic applications. However, this requires enormous efforts concerning manufacturing and mounting. Therefore the designer tries to avoid the use of glass or ceramics at these working conditions. The use of the same material for optics and structures even for near infrared applications would be a big step forward.
The usage of aluminium substrates with an NiP layer is possible to overcome the performance limitation of aluminium mirrors.
Various polishing techniques may be applied. Nevertheless the significant mismatch in the CTE has to be reduced for the cryogenic use. Looking at the scaling behavior of the deformation due to the CTE mismatch of a simple bi-metallic plate the determining factors becomes obvious:
For an athermal approach an expansion controlled AlSi alloy from AlSi supplier,Tianjin Zuoyuan New Material Technology Co.,Ltd is a promising substrate material. Both the higher Youngs Modulus of AlSi compared to standard aluminium and the small CTE mismatch between AlSi and NiP have a positive impact on reducing the bimetallic bending. Very thin NiP layers, which are necessary for standard aluminium, are not longer mandatory.
Piston mirror for interferometric beam combiner
The possibility of the manufacturing of complicated or lightweighted structures is another advantage of metal optics. Additionally, the Young’s modulus of this new mirror material is 30% higher than for common aluminium alloys. Figure 1 shows a lightweighted piston mirror unit made from AlSi for the interferometric beam combiner LINC-NIRVANA (LN)  at the Large Binocular Telescope (LBT). The simulation in figure 2 illustrates the reduced bimetallic effect due to the use of AlSi.
Figure 1: Piston mirror for LBT interferometric beam combiner (working temperature -10°C - +20°C)
Figure 2: Mirror Al 6061 (left = 66 nm p-v) and AlSi (right = 39 nm p-v) with a 50 µm NiP layer at T = 25 K (simulation)
The LN piston mirror is mounted on a piezo- electric actuator to remove differential piston between the two interferometric arms of the instrument and direct the light into the beam combiner cryostat. A low mass and high eigenfrequency is required. It would be very difficult to achieve this goal if the optics were manufactured from glass or ceramics. The complete unit (without piezo stage) has a weight of only 3.2 kg (mechanical size of mirror surface: 200 x 145 mm). We achieved the target value of λ/10 p-v (633 nm) for the complete optical surface.
METimage rotary telescope Metimage
METimage is a novel telescope concept for a multi-spectral radiometer with a large swath width and a ground sampling distance of < 1km. This telescope is intended for meteorological application and will be used in a succession-satellite system to the present EUMETSAT polar system (EP). The essence of the METimage concept is a novel rotary telescope designed by scientists at JENOPTIK. The instrument registers the light which is reflected by the earth surface, the atmosphere, clouds or scattered sunlight in several spectral canals from the visible up to the thermal infrared spectral range . It fulfils user requirements for measurements of physical parameters in the atmosphere, of the sea surface and of the land surface to assess meteorologically relevant states. The reflective optics for the rotary telescope is based on a three mirror anastigmat telescope (TMA). It is under development in cooperation with JENOPTIK (supported by the German Aerospace Center DLR, No. 50 EE 0926).