ESPRESSO: design and analysis of a Coudé-train for a stable and efficient simultaneous optical feeding from the four VLT unit telescopes

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  ESPRESSO: design and analysis of a Coudé-train for a stable and efficient simultaneous optical feeding from the four VLT unit telescopes
    ESPRESSO - Design and analysis of a Coudé-Train for a stable and efficient simultaneous optical feeding from the four VLT unit telescopes Alexandre Cabral* 1 , André Moitinho 2 , João Coelho 1 , Jorge Lima 2 , Gerardo Ávila 3 , Bernard-Alexis Delabre 3 , Ricardo Gomes 1 , Denis Mégevand 4 , Filippo Zerbi 5 , Paolo Di Marcantonio 6 , Christophe Lovis 4 , Nuno C. Santos   7,8 1  CAAUL, Faculty of Sciences, University of Lisbon, Estrada do Paço do Lumiar, 22, 1649-038 Lisboa, Portugal 2  SIM, Faculty of Sciences, University of Lisbon, Campo Grande 1749-016 Lisboa, Portugal 3  European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Muenchen, Germany 4  Observatoire de l’Université de Genève, ch. des Mailettes, 21, CH - 1290 Sauverny, Genève, Switzerland 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  Centro de Astrofísica da Universidade do Porto, Rua das Estrelas, 4150-762 Porto, Portugal 8 ABSTRACT   Departamento de Física e Astronomia, Faculdade de Ciências, Universidade do Porto, Portugal ESPRESSO is a fiber-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. A previous trade-off analysis, considering the use of mirrors, prisms, lenses or fibers and several possible combinations of them, pointed towards a Full Optics solution, using only conventional optics to launch the light from the telescope into the front-end unit. In this case, the system is composed of a set of prisms and lenses to deliver a pupil and an image in the CCL, including an Atmospheric Dispersion Compensator. In this paper, we present the optical design of the Coudé Trains, the opto-mechanical concept, the main characteristics and expected performances. 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 1-3 In a preliminary phase, several concepts were analyzed considering the use of mirrors, lenses or fibers or any of the possible combinations of them. 4,5 * After a trade-off analysis, considering several concepts, two possible advantageous 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 front-end unit. The LF concept basically uses a prism to collimate the light from the telescope main mirrors while a fiber injection lens system injects 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. ; phone 351 217 500 753;    Figure 1. VLT Unit telescopes (UT) and the Incoherent CCL where the beams converge in a point. Although the LF concept would be less complex to implement, and will less impact on the VLT infrastructure and other instrumentation, the final selection fall onto the FO. After surpassing some of the mechanical constrains coming from the fact that a considerable volume in the Coudé Room near the azimuth axis is already occupied with VLTi instrumentation (that initially pointed the LF as the only possible solution 5 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). For the spectrograph to be able to reach the desired performances, the Front End of the instrument performs the acquisition and guiding. The required wavelength coverage range is within the 380 nm and 780 nm. ), it was possible to design a CT based only on refractive elements, taking advantage from the inherent higher throughput (more than 25% in the blue region of the spectrum). The requirements on the photon detection efficiency are very tight. If, for the one hand, the use of fibers in the CT can 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. Although having a reduced FOV, the lengths of the CT optical systems for the four VLT Unit Telescopes (UT) is considerable and different for all of them, as shown in Figure 2. Figure 2. Distances for the Coudé Train light path.    2.   COUDÉ TRAIN CONCEPT 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, using a set of 6 prisms (some with power) to convey the light of the telescope from the Nasmyth Focus (B), by insertion of a prism (P4) , to the entrance of the light duct in the Coudé Room (CR) below each UT unit (Figure 3). There, the light is directed towards the CCL using 2 lenses. 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 Front End (FE). Figure 3. The Full Optics concept. The FO concept uses only conventional optics to launch the light from the telescope into the front-end unit. The system is composed of a set of relay optics to comply with the existing VLT mechanical structure upstream the CR (Figure 4). The srcinal M4 Nasmyth folded mirror is replaced by a prism (P4) that bends the light towards the Coudé tubes. The optical setups (for all the UTs) consider the inclination 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. As it is impossible to avoid the collision, the solution adopted for all the UTs is to keep the beam near the CR roof (90° bend in P7) and to use two separated prisms near the light duct entrance (P8 and P9, the Gerardo prisms), in a periscope configuration. 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. P7 – The TIR prism bends the beam by 90° (towards the light duct) as close as possible to the ceiling. The central part of the prism is always the same. The attached lens will all be different. P8 and P9– These two TIR prisms bend the beam by 90°, being the centre of the (vertical) beam between prisms at 150 mm from the light duct entrance. The second prism also bends the beam horizontally by approximately 7° to redirect the beam towards the VLT Coudé light ducts. The Prisms will be equal for all UTs.    Figure 4. Location of the components of the Full Optics Concept in the VLT infrastructure. The mechanical support zone for the components are also shown (SZ). 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/FE 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 lenses will be different for all UTs. But the ADC prisms are the same. 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. Figure 6. Location of L11, ADC and image plane (interface between CT and FE) in the Combined Coudé. 2.2   Atmospheric dispersion compensation Even if telescope pointing and guiding will be perfect at a given reference wavelength, atmospheric dispersion will shift the image centroid at different wavelengths. Moreover such effect will vary during acquisition, and between different exposures, with the observation zenith angle. An Atmospheric Dispersion Compensator (ADC) is thus mandatory to avoid wavelength dependent injection losses. The effect of differential atmospheric refraction is a consequence of the wavelength-dependent index of refraction of the atmosphere. As an example, the differential refraction at 60° zenith angle over the instrument wavelength range for Paranal with an altitude of 2635.43 m, temperature of 12°C and humidity of 15% is approximately 2.2 arcsec. The ADC will be located in a convergent beam, but the design will be done considering a collimated beam (less critical in terms of aberrations). For that, as described previously, the CT optical system uses two lenses collimate and focus the beams before and after the ADC. The selected ADC configuration comprises a set of four prisms paired, with the ability to counter rotate them in order to vary the dispersion to compensate that of the atmosphere at a given zenith angle. Rotation ability on both prisms must be performed to match the telescope parallactic and zenith distance angles. When the two prisms are in opposite orientations they give null dispersion, while they give maximum dispersion when their orientation is the same (Figure 7). Figure 7. ADC with counter rotation prisms, for a maximum zenith (left) and 0° zenith (right) configuration.
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