Application of research for metal primary mirror of large-aperture infrared solar telescope

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1 pplication of research for metal primary mirror of large-aperture infrared solar telescope Meng Xiaohui a,b,c, Zhang Haiying a,b, Li Xinnan a,b a National stronomical Observatories / Nanjing Institute of stronomical Optics & Technology, Chinese cademy of Sciences, Nanjing , China; b Key Laboratory of stronomical Optics & Technology, Nanjing Institute of stronomical Optics & Technology, Chinese cademy of Sciences, Nanjing , China; c Graduate University of Chinese cademy of Sciences, Beijing , China BSTRCT Metal is an early telescope mirror material, it was later replaced by glass which has lower thermal expansion coefficient. However, for observing the sun, these glass materials in the primary mirror are affected by the sun's intense radiation, its temperature rises rapidly, but which conducts heat slowly. The temperature difference between mirror and ambient air is so large that causing the air turbulence which has affected the observation precision. While the metal material has better thermal conductivity characteristics, it can greatly improve the problems caused by air turbulence. This paper analyzes the characteristics of the various mirror materials, and then makes a rust-proof aluminum alloy 505 as the mirror substrate material. For the major deficiencies of the soft aluminum surface which is not suitable for polishing, this paper presents a method of electroless nickel plating to improve its surface properties. fter the mirror go through a thermal shock, the upper and lower levels of metal CTE don t match with each other, which leads to mirror deformation and warping. The bimetallic effect has been illustrated by the theory of beam element and give a result of elementary approximated. The analysis shows that the displacement deformation of the upper and lower layers of metal which is caused by thermal shock is smaller when the CTE is closer. In the experiments, a spherical aluminum mirrors with the substrate of 505 aluminum alloy, diameter of 110mm, the radius of curvature of mm is manufactured in classical technique. nd it ultimately achieves optical mirror-polished precision. Besides, the long-term thermal stability experimental study of the aluminum mirrors proved that l-infrared solar telescope primary mirror meets the needs of the long-term observation during use. Keywords: infrared solar telescope, metal mirror, bimetallic effect, electroless nickel plating 1.INTRODUCTION Solar is regard as one of the most important objects for the research of astronomy, which is one of the high-resolution observation fixed stars and its detailed result could help mankind to realize other stars in the universe. China solar physics instrument development planning has set the large-aperture infrared solar telescope as a goal in the next five to ten years. The primary mirror technology is one of the most key technologies for constructing telescopes in the early years, the material of metal is most widely used in the telescope mirrors, but which is replaced by glass, fused silica, zerodur, etc, because of its lower thermal expansion coefficient. But now, the main flaw of metal mirror can easily be improved with the development of active optics and adaptive optics technology, and also with the development of the research for new optical manufacture process, which then refocus attention of astronomers by virtue of its good thermal properties and forging process characteristic. For the design of infrared solar telescope, the requirements of machining precision for mirror have been reduced because of its observation focused on the infrared wave band, so it is feasible to make metal as infrared solar telescope s main mirror materials. The meal primary mirror has been gradually used by astronomical observatory and many mature telescopes with metal mirror have come into use since the nineteen sixty centuries. luminum and aluminum alloy are one kind of the most fuma2003@163.com; phone th International Symposium on dvanced Optical Manufacturing and Testing Technologies: Large Mirrors and Telescopes, edited by Wenhan Jiang, Myung K. Cho, Fan Wu, Proc. of SPIE Vol. 7654, 76540M 2010 SPIE CCC code: X/10/$18 doi: / Proc. of SPIE Vol M-1

2 widely used materials in metal, in 1968, the Merate Observatory (Italy) constructed the first telescope in the world with an aluminum primary mirror, which has a diameter size of 1.37m, and the mirror is spherical, of weigh of 900kg, with an edge thickness of 198mm, and 0.08mm nickel coating 1. The telescope was so successful that European Southern Observatory (ESO) doing a survey about the telescope when they processed with the plan of New Technology Telescope (NTT), and ESO found that the total amount of warping of the primary mirror of the telescope was equivalent to a wavelength after 10 years of use, but if a today s active support technology was used, the warping could be ignored. In the following project of Large ctive Mirrors in luminum 2 (LM), ESO intended to make metallic as a alternative to the conventional glass mirrors, and manufactured two 1.8m mirrors with aluminum alloy, which then machined in surface with an accuracy of about 7 to 10 microns rms. nd the two mirrors kept a stabilized properties after they went a series of aging test, it was obvious that aluminum-alloy blanks, polished on a thin nickel coating, with active optics to compensate its real-time deformations,could be applied to large telescope projects. 2. PRIMRY MIRROR MTERILS SELECTION Selecting the primary mirror materials needs to consider a number of characteristics related to manufacture, the criteria can be listed as follows: The mechanical property, strength, stiffness, poison s ration and etc; thermal physical property, mainly manifested in the change of thermal stress, thermal deformation and performance of the mirror materials after a series of thermal shock; optical properties, mainly contains the scattering and reflection; other factors, such as the production cost, the difficult level of processing technology, and other aspects that need practical consider. Metal materials, such as copper, aluminum, beryllium, silicon carbide, which have had a lot of applications in the design of primary mirrors, for copper, its surface scattering problems is very serious, and its mirror blank is difficult to build larger, so the actual research of copper has a little meaning to us; beryllium is the most excellent metal materials, which is widely used in the space telescope for its good thermal characteristic and the lightest structure. The 6.5m primary mirror of James Webb space telescope is made of beryllium 3, which will replace the Hubble and be expected to launch in However, the powder, which is generated in the processing of beryllium manufacturing, has toxic. The high level of protective measures requirement for process technique is so strict that it is not realistic to select beryllium as the primary mirror material. SIC has also a good mirror characteristic, the only defect is that it is a waste time and energy job for grinding and polishing because of its stiff quality and the high cost 4. luminum alloy, which has held out a promise of low cost, durability, ease of manufacture, the possibility of welding together sections to form large mirror blanks, can fully satisfy the requirement to build the infrared solar telescope primary mirror and has a great significance for research. 3. PRIMRY MIRROR TECHNOLOGY RELTED 3.1 Electroless nickel plating It is not possible to acquire high-quality machining surface through the directly polished aluminum-alloy substrate mirror because of aluminum-alloy s crystal structure, flexibility and scalability. In order to improve the poor grindability of aluminum-alloy mirror surface characteristics, thin nickel plating have shown to be able to reach such performances to put the aluminum mirror in competition versus the more traditional glass mirror. Brenner and Riddell present a method of chemical deposition nickel-phosphorus on the metal skin, namely electroless nickel plating 5. The principles is that nickel-phosphorus alloy are firstly synthesized from the ion of nickel and phosphorus, which are separated out from a solution containing nickel-phosphorus ions, and which then deposited on the metal surface with the use of reducing agent. It is common to use hypophosphite as the reducing agent in the acidic nickel plating solution, the redox process is: 2 o 3 Ni + + ( H 2PO2) + H2O Ni + ( H2PO ) + 2H + (1) It is easily to control the thickness of the nickel plating film precisely with the method of electroless nickel plating, and the film is more uniform due to nickel-phosphorus alloy s deposit layered process, the adhesion to substrate is more fastness. So high-quality glossy film can be obtained by the precise control of various parameters in the process. 3.2 Bimetallic effect s the method of nickel plating greatly improves the poor grindability of aluminum-alloy surface characteristic, it is also Proc. of SPIE Vol M-2

3 bring the bi-metallic negative effect, which leads to mirror deformation and warping because of the upper and lower levels of metal CTE don t match with each other after the mirror go through a thermal shock. Though the deformation can be estimated with the use of finite element analysis in the later stage, the CTE and Young s modulus in the two layers are not exactly anymore, the optical design process will become conservative, so it is prior to take a series of measures to reduce the error caused by bi-metallic effect. Barnes also gives a relationships for curvature of bimetallic mirror as a function of temperature 7, the result is: 2 r 6 EN 1 υ tn tn EN 1 υ { δ = Δα ΔT [1 + (4 1)]} 2 H E 1 υn H H E 1 υn Where: t t : thickness (z direction) of aluminum and EN, respectively N H = t + tn : the total thickness of bimetallic mirror r : the radius of bimetallic mirror E E : Young s modulus of aluminum and EN, respectively N Δ α = α αn : the differentials between CTE s of aluminum and EN Δ T : the change of temperature υ, υ : Poisson s ration of aluminum and EN, respectively N (2) Equation (2) gives result that the change of radius of curvature for bimetallic mirror in directly proportion to the differentials of CTE for aluminum and EN, but in inversely proportion to the total thickness of bimetallic mirror, so appropriate increasing in the thickness of the mirror body can help reduce the error caused by bi-metallic effect. The equations (2) as above illustrated is the first order solution for temperature induced bimetallic bending of plated metal mirrors, which provides a trend and not fully to guide the actual process of bimetallic mirror design. Finally, take a example of bimetallic mirror grinded in the present experimental, incorporating the physical parameters of mirror into the equations as above, the change of radius of curvature versus temperature has a linear relationship, the result is: δ Δ T (3) Where: unit is μm The deformation of bimetallic mirror surface will reach about 1micron after thirty centigrade of temperature difference according to equations(3). For the practical situation, the deformation of bimetallic mirror surface versus temperature has a complex non-linear relationship, but the equations as illustrated above still have a great guidance on the design of process within the allowed range. 3.3Residual stress During process of polishing or heat treatment, the bimetallic mirror body itself will inevitably produce various sizes of stress due to its inhomogeneity according to the available knowledge of elasticity: Where: { σ } [ D]{ ε} = (4) { σ } : stress matrix, { ε } : strain matrix [ D ]: elasticity matrix, which depends on the Young s modulus E and Poisson s ratio of the elastic material Proc. of SPIE Vol M-3

4 4. EXPERIMENTL ND ERROR NLYSIS 4.1 Material of primary mirror luminum alloy-505 is selected as the material of primary mirror based upon the principles illustrated above. This alloy is belonging to the series of high-magnesium rust-proof aluminum alloy, which has a high strength, corrosion resistance, good formability and weldability. The mechanical properties of 505 are shown in Table 1. Table 1: mechanical property of 505 Status Tensile strength Yield Stress Shearing Strength Endurance Strength HBS 10/500 M Y Y The second phase will appear in the annealed structure besides the single-phase α because the magnesium has a high proportion in the alloy of 505. There is a more electrode voltage differential between the second phase and substrate α solid solution, so the alloy has the probability of stress corrosion. The parameters of aluminum mirror grinded in the experimental are shown in Table 2. Radius of curvature (R) mm Table 2: parameters of the aluminum mirror Diameter (D) Edge thickness (H) mm (mm) Surface type Spherical 4.2 Polishing of the primary mirror The spherical aluminum mirror was to be grinded with diamantine after the preliminary molding from the mirror blanks, and then to be made the radius curvature of mirror about ΔR=±1μm by using the classical polishing method, which is a process of polishing on the single axle grind-polish machine. The manufacturing process can be divided into three stages according to the polishing powder utilized on the polishing pad. The first stage is that uses the polishing compound of chromic oxide ( Cr2O ) to polish the aluminum mirrors, and finally 3 attains the accuracy of surface type within two Newton rings. Fig.1 illustrates the aluminum mirrors be coated about 90μm of nickel-phosphorus alloy film. The concave pit caused by the release of residual stress can be seen from the pictures above, which also shows many scratches on the mirror surface. Fig.1: aluminum mirror with D=110mm, R= mm Proc. of SPIE Vol M-4

5 The second stage mainly uses the artificial diamond paste to polish the mirrors. s the artificial diamond paste has an effect of corrosion for aluminum mirrors, a polyurethane polishing disc could be used to cover the polishing pad. The aluminum mirror has smooth surface roughness as the glass blanks after this period of polishing. In order to improve the surface accuracy of mirror, a third stage is involved in. The diamond micron powder plays an important part role in this stage. The final results are shown as fig.2. Fig.2: the result of aluminum mirror (D=110mm, R=258.67mm) 4.3 ging tests series of aging test should be involved in to release the residual stress on the two aluminum-alloy mirrors 8. the mirror underwent a total of 8 thermal cycles minimum between -55 and +20. The interferometric tests were performed in each cycle which covers a whole day. The temperature variation between the tests was less than 1. Figure 3 illustrates the evolution of wavefront error for the aluminum mirror. The mirror exhibited surface deformation after the first three cycles and remained fairly stable afterwards. The conclusion shows that the mostly residual stress was cleaned out in the three cycles. s the more thermal cycles were involved in, the less content of residual stress, so the mirror surface deformation become less manifestly. The two mirrors show a well thermal stability after the thermal cycles, and the accuracy of surface for aluminum mirrors will not influence the results observed by telescope, so the aluminum mirrors can perfectly be used in the telescope number of thermal cycles wavefront error (λ) thermal cycle for aluminum mirror PV RMS Fig.3: the aluminum mirror, evolution of wavefront error Proc. of SPIE Vol M-5

6 5. CONCLUSIONS Electroless nickel (EN) can greatly improves the surface characteristics of aluminum mirrors, which then be polished by using the method of classical polishing. The finite element analysis can give the approximation result of surface deformation caused by the bi-metallic effect. Improving the thickness of mirrors or reducing the differentials between the nickel-phosphorus layer and the substrate aluminum alloy can properly relieve the error caused by the negative effect. luminum-alloy 505 was being chosen as the experimental material. The nickel-phosphorus coated mirror can be perfectly grinded and attain the surface accuracy as glass mirrors. Scratch is the most evidently thorn problem in the process of polishing, but using the suitable polishing powder can acquire a smooth mirror surface. ging tests are necessary to relieve the residual stress induced by machining process and check the stability of aluminum mirror. The aluminum mirrors will keep a stable characteristic after a series of thermal cycle. It is optimistic for aluminum-alloy to be used in the telescope project. CKNOWLEDGEMENTS This work is supported by the National Nature Science Foundation of China under Grant no The authors would like to thank Dr. Zibo Jiang and Dr.Zhichuan Dong for their fruitful discussion about the work in this paper and for their help with the preparation of this paper. REFERENCES 1. Jean P.Rozelot, jean-m. Leblanc, Metallic alternative to glass mirrors (active mirrors in aluminum). review, SPIE, Vol P , J. P. Rozelot, The L..M. (Large ctive Mirrors in luminum) programme, SPIE, Vol.1931.P33-36,1992 Robert Hardesty, The design and producibility of precision Beryllium structures for Spacecraft applications, SPIE, Vol, 2543 P , Robert Hardesty, The design and producibility of precision beryllium structures for spacecraft applications, SPIE,Vol P , Brian Catanzaro, etc. C/SIC advanced mirror system demonstrator design, SPIE Vol. 4013,P , Massimo Pasquett, Electroless nickel coating applying to large aluminum mirrors: main critical aspects and their solution, SPIE Vol. 1931,P , Steven L. Folkman, Michael S. Stevens,Characterization of electroless nickel plating on aluminum mirrors SPIE Vol. 4771,P , Daniel Vukobratovich and llen Gerzoff, Therm-optic analysis of bi-metallic mirrors, SPIE Vol P.Dierickx,Optical quality and stability of I.8-M aluminum mirrors, SPIE Vol. 1931,P78-84,1992. Proc. of SPIE Vol M-6