Optical design of the ESPRESSO spectrograph at VLT P. Spanò*a, D. Mégevandb, J.M. Herrerosc, F.M. Zerbia, A. Cabrald, P. Di Marcantonioe, C. Lovisb, S. Cristianie, R. Reboloc, N. Santosf, F. Pepeb a
INAF - Osservatorio Astronomico di Brera, V. Bianchi 46, I-23807 Merate, Italy; Univ. de Genève, Obs. Astronomique, 51 Ch. Maillettes, 1290 Versoix, Switzerland; c Istituto de Astrofisica de Canarias, Via Lactea s/n, 38200 La Laguna, Tenerife, Spain; d Univ. de Lisboa, Estr. do Paço do Lumiar 22, 1649-038 Lisboa, Portugal; e INAF - Osservatorio Astronomico di Trieste, V. Tiepolo 11, I-34143 Trieste, Italy; f Univ. do Porto, Centro de Astrofisica, Rua das Estrelas, 4150-72 Porto, Portugal; b
ABSTRACT ESPRESSO, a very high-resolution, high-efficiency, ultra-high stability, fiber-fed, cross-dispersed echelle spectrograph located in the Combined-Coudè focus of the VLT, has been designed to detect exo-planets with unprecedented radial velocity accuracies of 10 cm/sec over 20 years period. To increase spectral resolution, an innovative pupil slicing technique has been adopted, based onto free-form optics. Anamorphism has been added to increase resolution while keeping the physical size of the echelle grating within reasonable limits. Anamorphic VPH grisms will help to decrease detector size, while maximizing efficiency and inter-order separation. Here we present a summary of the optical design of the spectrograph and of expected performances. Keywords: High-resolution spectrograph, echelle grating, volume-phase holographic grating, anamorphism, pupilslicer, free-form optics, exo-planets
1.
INTRODUCTION
Since '90, when very large telescopes (diameters >8m) like Keck and VLT became operative, many efforts have been put to fully exploit their large collecting area to resolve tiny spectral features onto faint, deep objects. HIRES1 on the Keck telescope and UVES2 on the VLT Kueyen unit telescope are worldwide recognized as the two best examples of possible solutions, even if they differ in many aspects: the former maximized the area of the dispersive element with a 300mm collimated beam dispersed by a 3x1 mosaiced R-2.8 echelle replica, the latter, while keeping the collimated beam at a lower 200 mm value, increased the blaze angle of the 2x1 mosaiced, monolithic, echelle grating at the maximum available value (76 deg, corresponding to a R4 echelle) in order to maximize the spectral resolution. This asked for an innovative white-pupil configuration to minimize shadowing effects and large anamorphism due to the steep angle hitting the echelle grating. Moreover, it helped to keep the size of both the cross-disperser and the all-dioptric camera optics within reasonable size. Upcoming 30-40m class ELT telescope size fight against scaling laws for spectrographs3, asking for collimated beams larger than 60cm. Then, echelle grating area must exceed 1m2, making them extremely heavy, complex to be replicated or aligned, and expensive. Moreover, all the collimation optics start to be comparable to 1-2m telescope optics. Even on smaller 8-10m class telescopes, spectral resolution cannot be increased, without breaking the same scaling laws. In order to circumvent such a hard limit, new approaches must be found, keeping into account state-of-the-art technologies, like free-form optics and newly available dispersive devices, like slanted volume-phase holographic (VPH) gratings. ESPRESSO spectrograph optical design has been derived from ideas developed for CODEX4, the Cosmic Dynamic EXperiment (now re-called "COsmic Dynamics and EXo-earth experiment") by Delabre and Dekker5 (ESO). It cleverly combine pupil slicing technique with anamorphism and slanted-VPH gratings to achieve very high resolving powers on the 42m European Extremely Large Telescope. *
[email protected]; phone +39 039 5971 063; fax +39 039 5971 001; www.brera.inaf.it/utenti/spano
Here we will give an overview of the optical design choices selected to match scientific and technical requirements. More details about the ESPRESSO overall system and the project status can be found in other paper within this same proceeding6,7,8. 1.1
Science drivers
As defined in the name, the Echelle SPectrograph for Rocky Exoplanet and Stable Spectroscopic Observation, ESPRESSO, will aim to: (a) measure high precision radial velocities (RV) for search for rocky planets; (b) measure the variation of physical constants; (c) analyze the chemical composition of stars in nearby galaxies. Mainly, the first two scientific requirements can be translated into a quest for extreme spectroscopic and radial-velocity precision, approximately improving by one order of magnitude current state-of-the-art instruments, like HARPS spectrograph9, mounted on the ESO 3.6m telescope in La Silla. A stability budget has been set up, taking into account all possible sources of RV drifts, from the telescope and the Coudè train to the fibers, the scrambler, the calibration sources, and the spectrograph itself. On the spectrograph side, some important technical drivers have been fixed. The spectrograph will have no moving functions, placed inside a vacuum chamber within a many-layers thermally controlled room. Monolithic devices, like the echelle 1x3 mosaic replica of R4 echelle grating onto a common Zerodur blank substrate has been preferred. The last main scientific driver will ask for maximum efficiency compatible with all other constraints. Generally speaking, stability fights against efficiency, so a careful trade-off must be made. The VLT telescopes offer the possibility to feed the spectrograph with light coming from anyone of the four UTs, or to simultaneously inject light coming from all telescopes together, thus allowing to operate ESPRESSO with a 16m-equivalent telescope, enabling high-resolutions spectroscopy of very faint sources. A much larger science case has been studied for ESPRESSO, properly described by Pepe6 et al. (2010). 2.
OPTICAL DESIGN
Both high spectral resolution and efficiency requirements can be met despite the large size of the telescope and the 1 arcscec field of the instrument. At the entrance of the spectrograph an anamorphic pupil slicing unit (APSU) shapes the beam in order to compress the beam in cross-dispersion direction but not main-dispersion direction, where resolving power is achieved. In the latter direction, however, the pupil is sliced and superimposed on the echelle grating to minimize its size. The rectangular white-pupil is then re-imaged and compressed by the anamorphic VPH grism. Given the wide spectral range and the required efficiency, two large 90x90 mm2 CCD detectors are required to record the ful spectrum. Therefore, a dichroic beam splitter separates the beam in a blue and a red arm which in turn allows to optimize each arm for image quality and optical efficiency. The cross-disperser has the function of separating the dispersed spectrum in all its spectral orders. In addition, an anamorphism is re-introduced to make the pupil square and to compress the order width in cross-dispersion direction, such that the inter-order space is maximized.
Table 1. Main spectrograph parameters in the three observing modes. Parameter Telescope aperture Spectral Resolution Wavelength Range Aperture on sky Sampling (average) Spatial sampling Simultaneous reference Sky subtraction Total Efficiency Instrumental RV precision Detector area
Standard 1-UT 8m 140’000 380-790 nn 1.0 arcsec 3.5 pixels 7.0 pixels Yes (no sky) Yes (no sim. ref.) 12%
