ESPRESSO Coudé-Train: complexities of a simultaneous optical feeding from the four VLT unit telescopes

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  ESPRESSO Coudé-Train: complexities of a simultaneous optical feeding from the four VLT unit telescopes
    ESPRESSO Coudé-Train: complexities of a simultaneous optical feeding from the four VLT unit telescopes Alexandre Cabral *1 , Manuel Abreu 1 , João Coelho 1 , Ricardo Gomes 1 , Manuel Monteiro 2 , António Oliveira 3 , Pedro Santos 3 , Gerardo Ávila 4 , Bernard-Alexis Delabre 4 , Marco Riva 5 , Paolo Di Marcantonio 6 , Igor Coretti 6 , Nuno C. Santos 3,7 , Filippo Zerbi 5 , Denis Mégevand 8 1  Centro de Astronomia e Astrofísica da Universidade de Lisboa, Faculdade de Ciências, Universidade de Lisboa, 1749-016 Lisboa, Portugal 2  Centro de Astrofísica da Universidade do Porto, Rua das Estrelas, 4150-762 Porto, Portugal 3  Laboratório de Óptica, Lasers e Sistemas, Faculdade de Ciências, Universidade de Lisboa, 1749-016 Lisboa, Portugal 4  European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Muenchen, Germany 5 INAF - Osservatorio Astronomico di Brera, Via Bianchi 46, 23807 Merate, Italy 6  INAF - Osservatorio Astronomico di Trieste, Via Tiepolo 11, I - 34143, Trieste, Italy 7  Departamento de Física e Astronomia, Faculdade de Ciências, Universidade do Porto, Portugal 8  Observatoire de l’Université de Genève, ch. des Mailettes, 21, CH - 1290 Sauverny, Genève, Switzerland ABSTRACT   ESPRESSO is a fibre-fed, cross-dispersed, high-resolution, echelle spectrograph. Being the first purpose of ESPRESSO to develop a competitive and innovative high-resolution spectrograph to fully exploit the VLT (Very Large Telescope), and allow new science, it is important to develop the VLT array concept bearing in mind the need to obtain the highest stability, while preserving its best efficiency. This high-resolution ultra-stable spectrograph will be installed in the VLT at the Combined Coudé Laboratory (CCL), fed by four Coudé Trains, which brings the light from the Nasmyth platforms of the four VLT Unit Telescopes to the CCL. ESPRESSO will combine the efficiency of modern echelle spectrograph with extreme radial-velocity precision. It will achieve a gain of two magnitudes with respect to its predecessor HARPS, and the instrumental radial-velocity precision will be improved to reach cm/s level. Thanks to its ability of combining incoherently the light of the 4 UTs, ESPRESSO will offer new possibilities in various fields of astronomy. The Coudé Train is composed of a set of prisms, mirrors and lenses to deliver a pupil and an image in the CCL, including an Atmospheric Dispersion Compensator. The use of mainly refractive optics, and Total Internal Reflection, has the advantage of the inherent higher throughput, especially in the blue region of the spectrum. In this paper, we present the design of the Coudé Train, the evolution of the concept towards the manufacturing phase, its main characteristics and performances, and detail its subsystems: optical, mechanical and control electronics and software. Keywords:  Coudé optics, optical design, telescopes 1.   INTRODUCTION ESPRESSO, the Echelle SPectrograph for Rocky Exoplanet and Stable Spectroscopic Observation, will be installed in the VLT at the Combined Coudé Laboratory (CCL) fed by the Coudé Train (CT) which brings the light from the  Nasmyth platforms to the CCL (Figure 1), where it feeds the spectrograph 1-4 . *; phone 351 217 500 753; Ground-based and Airborne Instrumentation for Astronomy V, edited by Suzanne K. Ramsay, Ian S. McLean, Hideki Takami, Proc. of SPIE Vol. 9147, 91478Q · © 2014 SPIE · CCC code: 0277-786X/14/$18 · doi: 10.1117/12.2055876Proc. of SPIE Vol. 9147 91478Q-1 Downloaded From: on 08/19/2014 Terms of Use:  Distances from telescope azimuth to c, onvergence point in CCL   Figure 1. VLT Unit telescopes (UT) and the Incoherent CCL where the beams converge in a point. The spectrograph only needs to capture the light from the target star and from one equivalent part of the sky in the border of the Field of View (FOV), the sky fiber. The CT relay to the CCL a corrected and unvignetted FOV (by design) of 17 arcsec (diameter). The required wavelength coverage range goes from 380 nm to 780 nm. The requirements on the photon detection efficiency are very tight. If, for the one hand, the use of fibers in the CT would simplify the design, for the other hand, the inevitable losses impose a limit on the maximum length of fiber we can consider. Even using a fiber optimized for the blue region, it will not be acceptable to consider lengths larger than 20 m. This eliminates the possibility to use a fiber to link directly the Nasmyth (B) focus to the CCL. In addition of having a reduced FOV, the lengths of the CT optical systems for the four VLT Unit Telescopes (UT) is substantial and different for all of them, as shown in Figure 2. Figure 2. Distances for the Coudé Train light path: left, from VLT Nasmyth focus to Coudé Room; right, form telescope Coudé Room to the Combined Coudé Lab trough the incoherent light ducts. 1.1   Coudé Train Concept Selection The selection of the ESPRESSO Coudé Train concept results from a trade-off of several designs developed in the last 7 years. From the beginning it was identified that a full fiber solution was not acceptable due to the distances involved and the inevitable losses in the blue region of the spectrum. The first design, presented by G.Ávila (ESO) in 2008 5 , comprised the use of refractive optics (total internal reflection  prisms and pentaprisms) from telescope focus to Coudé Room (CR), following the path shown in Figure 2 left. In the CR, a folding mirror with an off axis parabola collimates the beam to be sent trough the light duct. In the CCL an Proc. of SPIE Vol. 9147 91478Q-2 Downloaded From: on 08/19/2014 Terms of Use:  P6P7 VLTi Intrumentation P4 P5 - g Gerardo Periscope R9 CoudéRoom I Between 32 m and 55 in L10 ADC12 Focal Plane (CT -FEU interfrace) Pupil (CT -FEU interfrace) I Convergence point at CCL   Atmospheric Dispersion Compensator (ADC) compensates the atmospheric dispersion and a lens injects the light into the FEU fiber(s). With the first design as a starting point, several concepts were analyzed considering the use of mirrors, prisms, lenses or fibers or any of the possible combinations of them. In 2010, a trade-off analysis was presented 6 , considering several concepts from where two possible approaches emerged: a Full Optics (FO) or a Long Fiber (LF) concept. The first uses only conventional optics to launch the light from the telescope into the FEU. The LF concept basically used a prism to collimate the light at the VLT Nasmyth focus while a fiber injection lens system injected the light in a long (16 m fiber), in the other extreme of the fiber an off-axis parabola collimates the light transmitted by the fiber and sends it to the CCL. Although the LF concept would be less complex to implement, and with less impact on the existing VLT infrastructure and other instrumentation, the final selection went onto the FO, as this design being based mainly on refractive elements, takes the advantage from the inherent higher throughput (more than 25% in the blue region of the spectrum compared to the use of a 16 m fiber option). The final selected concept, presented initially in 2012 7 , is sketched in Figure 3. A set of 4 prisms (with power) convey the light of the telescope from the Nasmyth Focus (B), by insertion prism P4, down to CR below each UT with prisms P5 to P7. In the CR, the light is directed towards the CCL using a 2 mirror periscope (R8 and R9), required to circumvent existing instrumentation, and 2 lenses (L10 and L11) to image field and pupil in the CCL. In the design presented in 2012 6 , the Gerardo periscope was made from two TIR prisms, replaced in this final design by two mirrors to avoid the possibility of having pupil ghosts in the image plane.  In the CCL, an Atmospheric Dispersion Compensator (ADC) corrects the effect of atmospheric dispersion on the optical path. After the ADC, a focal plane materializes the interface  between the CT and the FEU. To direct the light towards ESPRESSO spectrograph, prisms P4, P6 and P7 must be inserted. Prisms P6 and P7 are removable in order to allow the VLTi usage. P5 will be fixed in the telescope structure but its mechanics include an automated cover to protect the prism while not in use and an alignment source to be inserted for telescope calibration. With the exception of the ADC, all the other items only require an on/off control. Figure 3. The ESPRESSO Coudé Train concept. 2.   COUDÉ TRAIN IMPLEMENTATION 2.1   Optical design The Coudé Train concept is based upon the incoherent combination of beams from the 4 Unit Telescopes (UT), or only a single UT, in the visible domain. The optical design uses only conventional optics to launch the light from the telescope into the FEU. Proc. of SPIE Vol. 9147 91478Q-3 Downloaded From: on 08/19/2014 Terms of Use:  P4 - - M 1 P5 P4P7 /. L Gerardo Periscope .. r . : I . . GoudeRoom 15 Bodega Li O illEgairLight ducts ,to C.CL1S Corridor L10 :10 \R8 Gerardo Periscope R9N L11 ADC12 _11111 V Ll 1 Combined Coudé Lab ADC12   The optical setups (for all the UTs) consider a tilt in P4 to remove P5 from the path of the VLT alignment tool. The different Back Focal Plane of UT4 is also considered. The design is made in order to avoid any “collision” with any of the VLTi instrumentation, by keeping the beam above the CR roof (90° bend in P7) and to use two separated flat mirrors in a periscope configuration. Figure 4 illustrates the Coudé Train implementation and the location of each element in the VLT UT. The concept is illustrated in Figure 4 and the location of each element in the VLT UT in Figure 5. Figure 4. Location of the components of the ESPRESSO Coudé Train in the VLT infrastructure. Following is a description of all the elements and their role in the CT. P4 – The Total Internal Reflection (TIR) prism will bend the beam by 92.5°, and the reflection surface will be located about 35 mm closer to M3 with respect to a 90° configuration. Besides the clearance of the VLT alignment tool path, this angle also helps on the TIR. P4 has power to bring the focus into the vicinity of P5. UT1 to UT3 has equal Prisms but in UT4, due to a different Back Focal Length, P4 will have different characteristics. P5 – The TIR prism will bend the beam by 135.5°, and the reflection surface will be located about 170 mm away from the vertical axis for a 90° configuration (the 135.5° angle is the result of subtracting 2.5° from 138° between the tubes of the Coudé path). P5 has power on both entrances. Essentially, the first lens is used to help on the achromatization and the second to image the pupil in the vicinity of P6. The central part of the prism is always the same for all UTs, while the attached lenses will all be different. P6 – Similar to P5, but in this case bends the beam by 138°. P6 (with P7) has power to put the focus in-between the Gerardo Prisms (P8 and P9) and L10. As in P5, the central part of the prism is always the same and the attached lens all different. Proc. of SPIE Vol. 9147 91478Q-4 Downloaded From: on 08/19/2014 Terms of Use:  -+7 5 arcsec- 0.0 arcsec--7.5 aresec P4(@ UT1) -+7 5 arcsec- 0.0 arcsec--7.5 aresec P5 (@ UT1) -+7 5 arcsec - 0.0 arcsec - -7.5 arcsec P6 (@ UT1) - +7 5 arcsec - 0.0 arcsec - -7.5 arcsec 79 mm P7 (@ UT1) - +7.5 arcsec - 0.0 aresec - -7.5 arcsec I MI MNlJ MI EN =NM \ M MINE ,IMIIME ,.. 140 mm R8&R9 (@ UT1) - +7 5 arcsec - 0 0 arcsec - -7 5 arcsec T 7- EE N J_ L10 (@ UT1) - +7.5 arcsec - 0.0 aresecu-7.5 arcsecEE N L11 (@ UT1) EE - +7.5 arcsec - 0.0 aresec - -7.5 arcsec ADC12 (@ UT1)   P7 – The TIR prism bends the beam by 90° (towards the light duct) above the CR ceiling. The central part of the prism is always the same. The attached lens will all be different. R8 and R9 – These two flat mirrors bend the beam by 90°. The second mirror (R9) also bends the beam horizontally by about 7.3°. The mirrors will be equal for all UTs. L10 – Located in the middle of the wall between the bodega and the corridor, for all UTs. It images the pupil near the light duct exit. The lenses will be different for all UTs. L11 – Will be located differently for all the UTs, in the vicinity of the light duct exit at the CCL. Some will be inside some outside (Figure 6). It creates (in combination with the ADC lenses) a focal plane in the CT/FEU interface. The lenses will be different for all UTs. ADC12 – Located inside the CCL, in a converging beam, will have the pupil in the middle and will be at the same distance to the convergence point for all UTs. The lens before the first ADC prism collimates the beam and the one after the second prism creates the desired converging beam. The full ADC (lenses and prisms) is equal for all UTs. Figure 5. Layout of all the optical elements in the CT of UT1. Figure 6 shows the location of L11, ADC and image plane (interface between CT and FE) in the Combined Coudé Lab. Proc. of SPIE Vol. 9147 91478Q-5 Downloaded From: on 08/19/2014 Terms of Use:
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