Gran Telescopio Canarias optics manufacture : Final Report

Transcription

1 Gran Telescopio Canarias optics manufacture : Final Report Roland GEYL, Marc CAYREL, Michel TARREAU SAGEM Aerospace & Defence - REOSC High Performance Optics Avenue de la Tour Maury Saint Pierre du Perray, France roland.geyl@sagem.com ABSTRACT This paper is intended to establish, after delivery of the last batch of segments, the final progress report of the manufacturing and testing of the Gran Telescopio Canarias optics, Keywords : GTC, large optics, aspheric, active optics, polishing, testing, lightweight, Beryllium, SiC. 1. INTRODUCTION The REOSC High Performance Optics team of SAGEM has been selected by GRANTECAN SA for the optical manufacturing of the Gran Telescopio Canarias (GTC) segmented 10,4-m Zerodur primary mirror and the 1,18-m lightweight Beryllium secondary mirror. The so-called master segment has been produced in September 2003 and was used to test the first set of 6 primary mirror segments in March The last batch of segment was delivered in December 2005 and the Secondary Mirror in March This paper aiming to be the synthesis of the project will therefore briefly present the project and its optics specifications. Developments conducted to set up our production facility within our giant optics laboratory of Saint Pierre du Perray will be presented. The final performance results of both primary mirror segments and secondary mirror, of unprecedented quality through the world, will be reported. 2. THE GRAN TELESCOPIO CANARIAS PROJECT SPAIN, in partnership with Mexico and the University of Florida (USA), has decided to set up a state of the art large segmented aperture telescope : the Gran Telescopio Canarias (GTC) in the Canaries islands, on top the Roque del Muchachos observatory, La Palma island. The GTC project office has been set up as an independent body with the status of a non-profit enterprise called GRANTECAN S.A. owned in share between the Canarian Regional Government and the Spanish National Government. The telescope is inspired from the Keck project but is designed for improved performances. The near 10,4-m equivalent diameter (11,4-m from edge to edge) primary mirror is split in 36 hexagonal segments as shown on the sketch at right. Each segment contour is inscribed in an 1.87-m circle. The substrate of the segments is Zerodur material from Schott in the form of a thin meniscus of 80 mm thickness. Each segment is axially supported on a 36 point whiffle tree support system as shown on the CAD view at left. Six of these support points are active in order to ensure the capability to generate some low order optical surface deformation thanks to bending moments induced in the mirror. The primary mirror area is paved in a six-fold symmetry with six different types of segments. Six additional segments bring the total number of segments to be manufactured to 42 and are dedicated to be used to ease the periodic maintenance and act as spares for the project. GRANTECAN SA selected SAGEM-REOSC for the polishing of the 42 primary mirror segments and to integrate them on their whiffle tree support system. In addition, SAGEM-REOSC got the contract for the GTC beryllium secondary mirror design and manufacture.

2 3. THE GTC PRIMARY MIRROR SEGMENTS SPECIFICATIONS The optical figuring specifications of the GTC primary mirror segments take into account the presence of the active support, the effect of atmospheric turbulence but are also driven by the goal of high quality imagery despite the segmented nature of the pupil of the telescope. The segments shall be submitted to final optical performance verification while integrated on their support with the possibility for SAGEM-REOSC to use this active support to remove some low spatial frequency figuring errors, but within a limit of ± 5 N.m max bending moment. The high spatial frequency errors are specified by the Central Intensity Ratio (CIR) as per the ESO VLT project in order to take into account an atmospheric free-air seeing of 0.4 arcsec at a wavelength of 500 nm corresponding to a Fried parameter of 0,2516 meter. A min CIR performance level is requested for each segment. A mean CIR level is then required for any set of 36 segments within the 42 to be produced. The contribution of local edge deformations (generally edge turn down) are also to be taken into account in the calculation of each segment CIR. The segments are aspheric off-axis mirrors. Their relative position and orientation with respect to the vertex of the parent mirror has also to be achieved and known with a high accuracy. The main optical specifications of the primary mirror segments are summarized here : Radius : R = ± 10mm Inter-segment radius variation : ± 0.2 mm, 0.1 mm RMS Conic constant : ε = -1,00225 ± 0,0005 Active correction : < 5 N.m bending moment Individual CIR : > 0,887 (λ = 500nm, r 0 = 252mm) including edge effects Global CIR : > 0,90 Relative position : ± 0,25 mm / ± 0,5 mrad Micro roughness : < 20 Angstrom RMS Cosmetic defects : scratch dig 80/60 The six types of segments represent six different aspheric surfaces to be polished with different sag with respect to the best fit sphere. The figure below and at right show these types with their aspheric departure contour level with 10,6 µm iso-level line spacing. As explained below in a next paragraph, one of the segments of type 2 is used as a reference piece with respect to which all other segments will we checked in term of radius of curvature. We call it the Master Segment. The CIR specification can be transformed, as an indication to near 30 nm RMS reflected wavefront residual figuring error and a λ/4 edge turn down over the last 2 mm, if it is present along all the edges. These values correspond to very high level of optical performances for capability of diffraction limited imagery in vacuum. In clear, the GTC telescope will never be limited in optical resolution by its primary mirror segments. The above specifications combined with the schedule constraints, lead to seriously approach the requirements for the coming generation of Extremely Large Telescopes which are : Accurate profile generation into a rough blank and associate metrology Rapid polishing without creation of irreversible edge effect Accuate figuring down to nanometer level for reaching the final specification Precise radius, conic constant, off axis distance, figure error and edge residual evaluation The notion of speed and cost-effectiveness of the processes being developed is crucial for meeting the schedule requirements which will be even more stringent for the ELT s. In this sense SAGEM has developed a set of world unique equipment and processes that have now been fully validated in real scale with the successful production of the first set of 6 segments for the Gran Telescopio Canarias. The Master Segment was the path finder during the last two years of activity at SAGEM REOSC and was declared in September 2003, finished and ready to hold its function of long term optical reference through the project.

3 4. MAIN MANUFACTURING STEPS The production of the GTC segments has been organized within our giant mirror optical shop in a mode of serial production with several units progressively reducing the figure error from several hundreds of µm to the last nanometers. In the same time the surface roughness and subsurface damage level has to be reduced from the rough milling state provided by the glass ceramic manufacturer down to fine optical polish and low cosmetic defects. This is done with new processes developed and equipment set up and validated specifically for the project. An aspheric generator machine, produces, in a highly stable thermal environment, the ground surface of each segment to sub µm accuracy with low sub-surface damage, ready to be rapidly polished. Fine lapping and polishing is done with a battery of robot systems fitted with a whole set of specific tools. They allow parallel production of smoothly polished surfaces up to the last edge in a timely manner. Final figuring to the exact shape is done with Ion Beam Figuring technology (IBF) in a large chamber installed within the 8-m optical shop capable to process components up to 2.5-m in size.. The main difficulty of the whole development has been to continuously prevent the apparition of edge defects (turn down or turn up) that always lead to difficult and time consuming corrective runs, when not impossible to do so. All these equipment represent major capital and human investments in large free form optics manufacturing. 5. MAIN METROLOGY STEPS Associated to the various manufacturing steps described above, metrology means and procedures have been developed in order to be able to feed the optical machinery with accurate measurement. Again the considerations of production rate have led to push towards rapidity of measurements without sacrifice in accuracy and density of the data. The six different types of segments complicate the tasks with an additional requirement for rapid reconfiguration of the test benches from one segment type to any other. A large 3D coordinates measuring machine is the first tool for acquisition of rough data of the generated aspheric shape and bonded pads. A 2D spherometer, extrapolated from the VLT one is used to generate sub µm accurate, relatively dense, map of the optical surface, even at the lapping stage when specular reflexion is still not reached. He-Ne interferometry with CGH corrector is performed at center of curvature at the top of the VLT, 29-m high test tower. Finally, the whole batch of segment is assembled on a global test bench in order to precisely evaluate figure and inter-segment radius match.

4 At the top of the test tower we installed a sophisticated metrology system combining the individual segment test tools using carefully designed and manufactured Computer Generated Holograms (CGH) and the global test mean based on the use of an Offner type null lens. Rapid switch from one configuration to the other is provided to the metrology engineer who can immediately process the acquired data to determine mirror performance status and generate the prescription for the next figuring run. Specific care has been taken and demonstrated to our customer that pupil distortion is taken into account and corrected for each segment and that the required 5 mm resolution at segment level is obtained. Also the WFE error budget through the null lens, and more generally, the CIR measurement accuracy for full pupil or edge effect has been carefully evaluated. At the base of the test tower, mirror segment supports have been installed on the 8-m table of the VLT mirror polishing machine. Up to four segments can be placed simultaneously on the table for CGH testing. In parallel, a global support structure for a full set of 6 segments, plus the master segment, is also installed to conduct the final evaluation of performances, especially the inter-segment radius error of 0.1 mm. Each segment, installed on its 36 wiffle tree support device delivered by GTC, can be moved along the 6 degrees of freedom in order to precisely place them at their theoretical transverse position, to phase them with an accuracy better than 10 µm and orient them precisely to acquire individual or global fringe patterns through the individual CGH or Offner corrector lens. The photo at left shows the arrangement viewed from the top of the tower. 6. THE MASTER SEGMENT In September 2003 the master segment passed successfully the entire cycle of manufacturing and testing and demonstrated that the various difficulties have been overcome. Its absolute certification has been performed to determine its actual off axis distance and residual figure errors. This has been done according to an special procedure combining several measurements. A summary of its performances of the master segment is : Radius of curvature : ,2 ± 1,5 mm Conic Constant : ± 0,0001 Surface figure errors : 17 nm RMS CIR (global WFE) : 0,946 (overall figure contribution) CIR (edge effect) : 0,955 (edge effect contribution) CIR (total) : 0,903 (product of the above values) Active support : not used for the Master Segment. The master segment appears then as a component of very high quality with very smooth residual figure errors as shown on the interferogram at right obtained synthetically from the phase map at left resulting from the processing of the many individual measurements involved in the metrology process. To our knowledge such a quality of an optics of this shape and this nature has never been obtained through the world and constitute a major achievement of SAGEM- REOSC.

5 7. EDGE EFFECT EVALUATION As explained above, edge effects are a serious contributor to light diffusion around the central sport of the image delivered by a segmented aperture telescope. This would make much more difficult the detection of faint objects, i.e. exoplanets, close to bright stars. Knowing that such type of discovery is one the main motivation to construct 10-m and more telescope, it is therefore clear that great attention shall be given to such edge effect. To evaluate it, SAGEM-REOSC has set up additional metrology means. This consists in a 200 mm diameter subaperture interferometer moved on 12 locations around each segment periphery. The obtained interferograms are processed to remove their low order content corresponding to the aspheric shape and figure segment errors evaluated through the full aperture measurement. Some internal and external zones are removed due to gauge residual figure errors. Finally, two edge portions are processed to determine an average edge profile for the segment. The average edge effect amounts to 50 nm wavefront error only up to a few mm from the edge. Worst cases remain below 200 nm. From these data, calculation of the CIR degradationis done according to various simulations performed by GTC. 8. PERFORMANCE OF THE VARIOUS BATCHES DELIVERED In December 2005, the last batch of segment was delivered fully within specification. A summary table of the figure error, the most easily understandable performance criterion within the optical community, for the various batches of segments produced : Batch # Date Figure error nm RMS nm RMS nm RMS nm RMS nm RMS nm RMS nm RMS The average performance over the full aperture is 13 nm RMS, corresponding to a state of the art monolithic primary of a 4 to 8-m class telescope. The interferogram at right shows the outstanding performance level reached by this primary mirror with no noticeable edge effect along the segment s contour. After the period of process development, the total optical area, amounting to 2.6 times the one of the James Webb Space Telescope (JWST), has been polished in less that 2 years. The inter segment radius error is below 0.05 mm over the 33 meters baseline radius of curvature. Active forces were rarely used to improve segment performance, and mainly for the first batches, within the 5 N.m allowed range of torques. Batch 1 Reconstructed fringes for batches 2-7 and 1 ( in Actve Mode) The team and some segments

6 9. SECONDARY MIRROR The Secondary Mirror design has been described in several other papers and we will not present this here but only indicate the final performance results obtained for this mirror. These are summarized as follow : Item Spec Tol Performance Radius mm +/- 5 mm / mm Conic const / / WFE passive < 381 nm RMS NA 74 nm RMS CIR active > NA WFE active per CIR NA 8.9 nm RMS Mass 46.4 kg +/- 2 kg kg +/ kg GTC M2 on its metrology plate Again, this mirror has reached a level of very high quality thanks to optimized polishing and figuring and metrology technology. The resulting wavefront is well smooth as shown on the reconstructed interferogram corresponding to the active mode performance, i.e. 8.9 nm RMS. Zoom on one edge Edge effect has been subject to particular attention as each millimeter lost on its edge contour will mean loosing 10 mm on the primary mirror. Some zoomed interferogram like the one shown at left have been used to assess the mirror quality up to the very edge. GTC M2 OPD The mirror has been carefully mounted on its support triangle that interfaces with the chopping mechanism. The photo below shows the mirror from its rear face, in the hand of Javier CASTRO leader of GTC optics team. 10. CONCLUSION Mirror integrated SAGEM-REOSC is very proud to have contributed to leverage the state of the art of optical manufacturing technology through its successful delivery of the 42 primary mirror segments and the lightweight secondary mirror to the Gran Telescopio Canarias project. This is the result of major effort and dedication of all our team of technicians and engineers over quite 5 years and major capital investment in new and innovative optical manufacturing process, production machinery and metrology tools. 10 years after VLT and GEMINI 8-m monolithic mirrors, a significant step has been made with the first ultra high quality large segmented aspheric optics delivered for GRANTECAN. The lessons learned, and there are numerous, will be more than useful to assess and prepare the next steps in giant telescopes of 30 to 50 meter primary mirror aperture. SAGEM wants to thank its customer GRANTECAN SA for the confidence placed in the company and thank all the team that contributed with courage and dedication to the success of this very challenging project. A special thank is addressed to Javier CASTRO who contributed greatly to our success with its patience, acute mind and energy to push us beyond our limit.