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Lightweight optics for space & astronomy
Large mirrors need to be lightened for demanding applications like space or astronomy. Safran Reosc has made one of its specialties the design and development of precision large lightweight mirrors made from 3 types of material: glass & glass ceramics, metal or Silicon Carbide.
4 decades background in lightweight optics
Astronomy science or earth observation from space are both requiring larger and larger optical collecting area and at the same time remaining of low weight mirror and high stiffness to be easily send in space and operate efficiently.
Lightweight optics has therefore become on the specialties of Safran Reosc with the pioneering development of the glass ceramic mirror lightweighting by diamond milling.
In a second step Safran Reosc answered to more challenging demands by using Beryllium material for VLT and GTC secondary mirrors.
Today, Silicon Carbide has become quite popular in precision optical instrumentation thanks to its outstanding characteristics.
In fact Safran Reosc remains attentive to its customers key concerns and has been able to adapt its optical engineering and manufacturing skills to the high level material considerations imposed.
Safran Reosc is proud to hold a world record in the domain with the 2.7-m lightweight mirror produced by machining from a solid blank for the Stratospheric Observatory for Infrared Astronomy (SOFIA). One year milling efforts were conducted successfully to establish this record in lightweight mirror technology.
Safran Reosc pioneered in the 70's the development in Europe of the lightweighting by machining technology with Zerodur glass-ceramic and similar glassy materials. The technique consists in machining with diamond tools a solid glassy body from the rear and cut out as much material as possible. There are several challenges:
- Removing as much material to push the lightweighting ratio as close as possible to 90% or more, i.e. only 10% of the mass of the solid body is left.
- Not putting in danger the strength of the part and its capability to survive to launch or transport conditions.
- Maximizing the stiffness to weight performance of the finished piece of optics in order to minimize its sag under gravity or deformation under perturbations
- Preserving a stiff enough front optical surface in order to prevent the apparition of quilting during polishing.
- Avoid any tool failure by fatique and glass crack during the operations.
Through the years Safran Reosc mastered the technology to an higher and higher degree with precisely engineered pockets: circular, hexagonal, triangular, open-back, semi closed-back, etc. Great attention is given to the mounting points through which mechanical perturbation are introduced within the blank.
Some key reference project are:
- Many small size optics for SPOT, METEOSAT and HELIOS earth a observation programs
- 1-m ultra-lightweight main mirror of the Sunrise balloon-borne telescope and the Gemini spare Secondary mirror
- 1.8-m primary mirrors for the Sea Lite beam director
- 2.7-m Stratospheric Observatory For Ir Astronomy (SOFIA);our word record in the technology.
For metal optics, Safran Reosc worked mainly with Beryllium, the most performant metal lighter than Aluminum and stiffer than steel to produce some word reference optics :
- The four 1-m secondary mirrors of the Very Large Telescope
- The Secondary mirror of the Gran Telescopio Canarias
The material grade shall be appropriately selected for the best long term stability per the environment to which it will be submitted. The mirror design shall take into account the specific capabilities of metal cutting techniques. Thermal annealing allow to reduce all the internal stresses prior optical manufacturing. A phosphorous Nickel layer shall be applied to ease the polishing work on the aspheric profile.
Silicon Carbide is an exciting ceramic slightly more heavy than glass but 4 times stiffer. Its Coefficient of Thermal Expansion (CTE) is non zero but this drawback is balanced with a very high thermal diffusivity. Its elaboration in lightweight geometry is well mastered by companies like MERSEN-Boostec and this is why main contractors like Airbus Defense & Space (formerly Astrium) have heavily invested in space optical instrumentation made per "All-SiC" concept with mirrors and structural elements all made from the same material.
For the optical technician SiC is very hard and Safran Reosc invested in developing and mastering all the technologies to lapp, polish and figure SiC optics to the ultimate precision.
Safran Reosc supplied Airbus with all sets of ASTROTERRA optics made from SiC material;
Our master pieces are the 1500x600 mm off-axis mirrors for the GAIA astrometric instrument now orbiting in space.
A polishing layer need to be applied on the bare mirror substrate which remains porous. This is generally made from Chemical Vapor Deposition (CVD) SiC material. PVD Silicon is another candidate polishing layer but both require special installation and equipment.
More recently Safran Reosc developed its own polishing layer called R-SiC, a glassy layer deposited in-house which offer several attractive cost & schedule reduction advantages as well as lower polishing efforts.