PEPSI: The high‐resolution échelle spectrograph and polarimeter for the Large Binocular Telescope

Abstract

PEPSI is the bench-mounted, two-arm, fibre-fed and stabilized Potsdam Echelle Polarimetric and Spectroscopic Instrument for the 2×8.4 m Large Binocular Telescope (LBT). Three spectral resolutions of either 43 000, 120 000 or 270 000 can cover the entire optical/red wavelength range from 383 to 907 nm in three exposures. Two 10.3k×10.3k CCDs with 9-μm pixels and peak quantum efficiencies of 94-96 % record a total of 92échelle orders. We introduce a new variant of a wave-guide image slicer with 3, 5, and 7 slices and peak efficiencies between 92-96 %. A total of six cross dispersers cover the six wavelength settings of the spectrograph, two of them always simultaneously. These are made of a VPH-grating sandwiched by two prisms. The peak efficiency of the system, including the telescope, is 15 % at 650 nm, and still 11 % and 10 % at 390 nm and 900 nm, respectively. In combination with the 110 m 2 light-collecting capability of the LBT, we expect a limiting magnitude of ≈ 20th mag in V in the low-resolution mode. The R = 120 000 mode can also be used with two, dual-beam Stokes IQUV polarimeters. The 270 000-mode is made possible with the 7-slice image slicer and a 100-μm fibre through a projected sky aperture of 0.74 , comparable to the median seeing of the LBT site. The 43 000-mode with 12-pixel sampling per resolution element is our bad seeing or faint-object mode. Any of the three resolution modes can either be used with sky fibers for simultaneous sky exposures or with light from a stabilized Fabry-Pérotétalon for ultra-precise radial velocities. CCD-image processing is performed with the dedicated data-reduction and analysis package PEPSI-S4S. Its full error propagation through all image-processing steps allows an adaptive selection of parameters by using statistical inferences and robust estimators. A solar feed makes use of PEPSI during day time and a 500-m feed from the 1.8 m VATT can be used when the LBT is busy otherwise. In this paper, we present the basic instrument design, its realization, and its characteristics. Some pre-commissioning first-light spectra shall demonstrate the basic functionality.

Comparing modal noise and FRD of circular and non‐circular cross‐section fibres

“…3 with the long science fibres for PEPSI. These fibres are also from Polymicro's FBP family and have a length of 44 m (Strassmeier et al 2015). If we compare these with the ones shown in Fig.…”

“…In this paper, we present characterizations of a series of different types of fibres with particular focus on modal noise and focal-ratio degradation. Our direct aim is to compare the fibres of the new high-resolution spectrograph PEPSI for the Large Binocular Telescope (Strassmeier et al 2015) with other solutions.…”

Solar physics at the Einstein Tower

“…Electric dehumidifiers remove atmospheric moisture and keep the humidity at a constant level in the gallery. The concept of a nested spectrograph chamber was also included in the much more elaborate design of PEPSI (Strassmeier et al 2015), however, with a much higher stability for temperature and humidity. The interior of the spectrograph chamber is painted in a matt black finish to minimize stray light (see Fig.…”

Astronomical photonics in the context of infrared interferometry and high-resolution spectroscopy

“…Multimode fibers in fiber-fed spectrographs efficiently enable multiplexed observations of few hundreds objects per pointing. Current examples are the visible high-resolution spectrograph PEPSI on the LBT 74 or the near-IR fiber multi-object spectrograph FMOS on Subaru 75 .…”