MANUFACTURING OF PYRAMID WAVEFRONT SENSORS FOR ASTRONOMICAL ADAPTIVE OPTICS BY DEEP X-RAY LITHOGRAPHY |
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F. Pérennès1, M. Ghigo2 and S. Cabrini1
Recently a new kind of wavefront sensor for astronomical adaptive optics systems has been tested in laboratory and on the field by a group of Italian researchers [1]. This system allows gain and sampling to be changed in a continuous way, thus enabling a better match of the system performance with variable wavefront aberration than those obtained with the classical Shack-Hartman set-up. The wavefront sensor consists of a four face flat pyramid (i.e with vertex angle slightly smaller than 180°) made of transparent material acting as an image splitter. The dimensions and manufacturing tolerances of this component are dictated mainly by the scale of the focal plane re-imaged on the pyramid tip. In order to minimise the loss of the star light, manufacturing errors like turned edges between the faces and pyramid face surface roughness have to be kept as low as possible. Narrow turned edges are very difficult to obtain using the classical figuring and polishing techniques on glass. This manufacturing process is also time consuming and offers a low repeatability of the desired optical characteristics because every pyramid is "hand made". Moreover this technique often creates a roof on the tip of the pyramid, due to the fact that the lapping procedure is not interrupted at the correct moment on the last face. Pupil plane wavefront sensors like the pyramid are key components for the multi-conjugated tomographic systems that are currently under development for the future large telescopes. Preliminary experimentation of such a correction system requires the production of several rigorously identical pyramids. For example the Multi Conjugate Adaptive Optics (MCAO) prototypes in test for the Telescopio Nazionale Galileo look simultaneously at four references, hence requiring four pyramids with a vertex angle of 178.256° (+/- 0.058°) and a repeatability among them of 0.007° (26 arcsec). We have developed a new manufacturing method (Fig.1) for the pyramid sensor based on the Deep X-ray Lithography (DXRL) process which allows the etching of polymethylmethacrylate (PMMA i.e Plexiglas) in depth of several millimetres with optical quality surface finish [2, 3]. To produce a single pyramid, we built a special holder that was mounted on the Jenoptik scanner scanning stage (Fig.2). A pre-cut cylinder of PMMA is blocked on a holder using a vacuum system. One motor controlled rotating table is used to give the desired fixed inclination of the PMMA piece versus the beam propagation direction. A knife edge masking the bottom part of the PMMA is used to create the first pyramid face. The whole assembly is scanned into the X-ray beam until the required dose is deposited at a depth equal to the radius r of the cylinder. The exposure is then repeated 3 more times after performing 90° rotation around the z axis using a second rotating table (Fig.2). At an electron energy of 2.4 GeV and with the right set of filters we have been able to etch pyramids with 12.7 mm diameter. After development, the irradiated zones of the PMMA cube above the 4 faces of the pyramid are etched away and a pyramid with a vertex angle of 180°-2a and a tip at the centre of the initial circular base is obtained. Four pyramids of 6 mm diameter base, with a point-like tip and a vertex angle of 178.273° were fabricated. They all showed a microroughness of 5 nm rms and turned-edges of 35 µm with an angle repeatability among them of 25 arcsec. These were successfully tested on the MCAO bread-board prototype of the Telescopio Nazionale Galileo [4]. The European Southern Observatory Very Large Telescope in Chile is also developing its own MCAO system requiring eight pyramids with diameter 12.7 mm. We show that it was possible to manufacture such a large base pyramid at ELETTRA (Fig.3) but the exposure time required was around 4 hours per face. Given the limited beam time available we are considering to fabricate a metallic master from this pyramid and reproduce it by hot embossing process. This will ensure a faster and cost effective production method of virtually identical copies of this optical component (enabling also the use of other plastic materials other than PMMA). The properties of this new kind of wavefront sensor can also be useful for practical applications of adaptive optics in the eye. Recently some Spanish researchers have successfully used it to measure aberration in a living human eye [5]. Some pyramids fabricated with the DXRL process are currently tested on their adaptive optic bench. ACKNOWLEDGMENTSThe authors wish to thank the CNAA (Consorzio Nazionale per l'Astronomia e l'Astrofisica) for partly financing the project. 
Fig 1: Pyramid fabrication process using DXRL 
Fig 2: Special holder used to fabricate the pyramid wavefront sensor 
Fig 3: 12.7 mm diameter pyramid wavefront sensor fabricated at ELETTRA References [1] R. Ragazzoni, Pupil plane wavefront sensing with an oscillating prism, J. of Mod. Optics 43, pp 189-193(1996). [2] M. Ghigo, F. Pérennès, and R. Ragazzoni, Manufacturing by deep x-ray lithography of pyramid wavefront sensors for astronomical adaptive optics, Astronomical Telescopes and Instrumentation conference, 22-28 August 2002,Waikoloa, Hawaii, USA, to appear in Proceedings of SPIE Vol. #4839-29. [3] F. Pérennès, M. Ghigo, S. Cabrini, Characterisation of adaptive optic pyramid wavefront sensors fabricated by deep X-ray lithography, Micro and Nano Engineering Conference 2002 (MNE02), Lugano 17-19 Sept, submited to Microsystems Technologies. [4] E. Diolaiti et al., Some novel concepts in multipyramid wavefront sensing, Astronomical Telescopes and Instrumentation conference, 22-28 August 2002,Waikoloa, Hawaii, USA, to appear in Proceedings of SPIE Vol. #4839-36. [5] I. Iglesias, R. Ragazzoni, Y. Julien and P. Artal, Extended source pyramid wave-front sensor for the human eye, Optics Express, Vol. 10, No.9, p419-428, (2002). Requests for more information should be addressed to F. Pérennès
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