Multi-channel aperiodic fiber Bragg grating (FBG) based hydroxyl (OH) line filters have attracted significant interest in ground-based near-infrared (NIR) astronomical observations. In this paper, we present the performance of what we believe to be a new self-compensating enclosure for the filters, that can be used in non-temperature-controlled environments. Our prototype encloses a 110 mm long single-mode photosensitive optical fiber with three ∼10 mm filter gratings. A fourth grating was used as a reference outside the package to measure the uncompensated wavelength shift. The prototype was tested over three thermal cycles, and showed a maximum wavelength deviation of 12 pm, a wavelength drift of only 0.37 pm/°C, over the temperature range of -17°C to 15°C. The athermalization factor, i.e., the ratio of the maximum wavelength shift of the compensated grating to the uncompensated reference filter grating was 1/22. The results demonstrate the capability of the prototype for stabilizing multi-channel long-length FBGs or chirped FBGs, particularly for astronomical applications that require sub-picometer stability.

The first generation of ELT instruments includes an optical-infrared high resolution spectrograph, indicated as ELT-HIRES and recently christened ANDES (ArmazoNes high Dispersion Echelle Spectrograph). ANDES consists of three fibre-fed spectrographs ([U]BV, RIZ, YJH) providing a spectral resolution of similar to 100,000 with a minimum simultaneous wavelength coverage of 0.4-1.8 mu m with the goal of extending it to 0.35-2.4 mu m with the addition of an U arm to the BV spectrograph and a separate K band spectrograph. It operates both in seeing- and diffraction-limited conditions and the fibre-feeding allows several, interchangeable observing modes including a single conjugated adaptive optics module and a small diffraction-limited integral field unit in the NIR. Modularity and fibre-feeding allows ANDES to be placed partly on the ELT Nasmyth platform and partly in the Coude room. ANDES has a wide range of groundbreaking science cases spanning nearly all areas of research in astrophysics and even fundamental physics. Among the top science cases there are the detection of biosignatures from exoplanet atmospheres, finding the fingerprints of the first generation of stars, tests on the stability of Nature's fundamental couplings, and the direct detection of the cosmic acceleration. The ANDES project is carried forward by a large international consortium, composed of 35 Institutes from 13 countries, forming a team of almost 300 scientists and engineers which include the majority of the scientific and technical expertise in the field that can be found in ESO member states.

We present here the preliminary design of the RIZ module, one of the visible spectrographs of the ANDES instrument 1. It is a fiber-fed high-resolution, high-stability spectrograph. Its design follows the guidelines of successful predecessors such as HARPS and ESPRESSO. In this paper we present the status of the spectrograph at the preliminary design stage. The spectrograph will be a warm, vacuum-operated, thermally controlled and fiber-fed echelle spectrograph. Following the phase A design, the huge etendue of the telescope will be reformed in the instrument with a long slit made of smaller fibers. We discuss the system design of the spectrographs system.

We introduce a new solar feed for the PEPSI nighttime spectrograph of the LBT. It enables spectroscopy of the Sun-as-a-star in circular polarization (CP) and linear polarization (LP) with a spectral resolution of 250,000 (approximate to 0.025 angstrom or 600ms(-1)) for the wavelength range 383-907 nm. The polarimeter is a dual-beam design with a modified Wollaston prism as beam splitter and linear polarizer combined with a retractable super-achromatic lambda/4 retarder. The Wollaston beam diameter is 14 mm and large enough that it does not require a classical telescopic feed anymore. Both polarimetric beams are re-imaged into respective integration spheres from which two fibers feed the scrambled light to the spectrograph. The system is fully automated in the sense that it finds the Sun in the morning, closes the guider loop, observes a predefined number of individual spectra, and moves to a home position at the end of the day. Among the scientific aims is Zeeman-Doppler imaging of the Sun as a star over the next activity cycle. Our first-light application detects a clear Stokes-V/I profile with a full amplitude of 1x10(-4) on, for example, October 13, 2023, suggesting a solar disk-averaged line-of-sight net magnetic field of +0.37 +/- 0.02 G. Comparison of this value with a contemporary full-disk line-of-sight magnetogram suggests an unsigned mean field of about approximate to 13 G.

Arrayed waveguide gratings (AWG) have gained attention as promising integrated spectrographs for ground-based telescopes, airborne applications, and spaceborne instrumentation due to their low mass, diffraction limit characteristics, thermal stability, and robustness against vibrations and misalignment. The Potsdam Arrayed Waveguide Spectrograph (PAWS) is a cross-dispersive instrument based on an integrated photonic spectrograph (IPS) that is optimized for the astronomical H-Band and was designed and developed by innoFSPEC at the Leibniz Institute for Astrophysics Potsdam (AIP). The main element is a second-generation AWG that is fibre coupled and works as a first dispersive element. To work as an IPS, the dispersed light of the AWG is sampled at the output facet and magnified by a microscope objective. The light is then fed into a free-space optical system housed in a cryostat working at 140 K. An afocal relay re-images the exit pupil of the microscope into the plane of a cross-dispersive element containing a diffractive grating. Subsequently, an objective focuses the resulting echellogram on a Teledyne 2k x 2k H2RG near-infrared array working at 80 K. To validate the functionality of the system, different light sources have been used. A tunable laser source generated an echellogram through frame stacking. Furthermore, the cross-dispersed output of a supercontinuum source and of an in-house developed frequency comb system were recorded under varying input conditions of the AWG, successfully achieving first light laboratory measurements. Throughout multiple cycles and measurements of the instrument, several parameters and characteristics were identified, providing opportunities for optimization to enhance the instrument's performance and facilitate the miniaturization of future iterations. In this manuscript, we will provide a concise overview of the PAWS instrument, the preliminary results of laboratory measurements, and lessons learned to improve the future iterations of the next generation near-infrared cross-dispersed integrated photonic spectrograph. PAWS represents a pioneering demonstration of an astronomy optimized AWG chip, showcasing the advantageous capabilities of integrated photonic spectrograph.

We present the design of an athermal package for fiber Bragg grating (FBG) filters fabricated at our Institute for use in ground-based near-infrared (NIR) telescopes. Aperiodic multichannel FBG filters combined with photonic lanterns can effectively filter out extremely bright atmospheric hydroxyl (OH) emission lines that severely hinder ground-based NIR observations. While FBGs have the capability of filtering specific wavelengths with high precision, due to their sensitivity to temperature variations, the success in their performance as OH suppression filters depends on a suitable athermal package that can maintain the deviations of the FBG wavelengths from that of the OH emission lines within sub-picometer accuracy over a temperature range of about 40K (i.e. 263K to 303 K). We aim to develop an athermal package over the aforementioned temperature range for an optical fiber consisting of multichannel FBGs for a maximum filter length of 110 mm. In this work, we demonstrate the complete design methodology of such a package. First, we developed a custom-built test rig to study a wide range of critical physical properties of the fiber, such as strain and temperature sensitivities, elastic modulus, optimum fiber pre-tension, and adhesion performance. Next, we used these data to confirm the athermal response of an FBG bonded on the test rig from room temperature to 313 K. Based on this study, we developed a computer-aided design (CAD) model of the package and analyzed its athermal characteristics with a suitable selection of materials and their nominal dimensions using finite element analysis (FEA). We finally discuss the novel aspects of the design to achieve high-precision thermal stabilization of these filters in the temperature range of interest. Our design is scalable to longer filter lengths and can also be used for other in-fiber devices for example, FBG-based frequency combs for their stable operations in astronomical applications.

The Potsdam Arrayed Waveguide Spectrograph (PAWS) is based on an integrated photonic spectrograph designed and developed by innoFSPEC to work in the astronomical H‐band. PAWS demonstrates how a traditional bulk optics spectrograph can be miniaturized and additionally serves as a tool for testing in‐house developed astrophotonical devices. The main element is a second‐generation Arrayed Waveguide Grating (AWG) with unprecedented performance in terms of spectral resolution and throughput that is fiber‐coupled and works as a first dispersive component. The pre‐dispersed light is fed into a free‐space optical system located in a cryostat, where the overlapping spectral orders are separated by cross‐dispersion. The resulting echellogram is recorded by a Teledyne 2k×2k H2RG near‐infrared array. Once the cryogenic cross‐dispersion optics inside the vacuum chamber were appropriately aligned to the AWG coupling optics outside of the chamber, different light sources were used to test the functionality of the complete system. A tunable laser source was used to create an echellogram through frame stacking. Additionally, the cross‐dispersed output of a supercontinuum source was recorded using different input waveguides of the AWG, and first light was achieved with laboratory measurements. PAWS represents a pioneering demonstration of an astronomy‐optimized AWG chip, exhibiting the advantageous capabilities of integrated photonic spectrographs, in particular in the context of space applications.

The improved High-resolution Fast Imager (HiFI+) is a multiwavelength imaging filtergraph, which was commissioned at the GREGOR solar telescope at Observatorio del Teide, Izana, Tenerife, Spain, in March 2022 - followed by science verification in April 2022, after which it entered routine observations. Three camera control computers with two synchronized sCMOS and CMOS cameras each provide near diffraction-limited imaging at high cadence in six wavelength bands (Ca II H at 396.8 nm, G-band at 430.7 nm, blue continuum at 450.6 nm, narrow- and broad-band Ha at 656.3 nm, and TiO bandhead at 705.8 nm). This unique combination of photospheric and chromospheric images provides "tomographic" access to the dynamic Sun and complements spectropolarimetric observations at the GREGOR telescope. High image acquisition rates of 50 and 100 Hz facilitate image restoration, where time series of restored images have a typical cadence of 6 and 12 s, which is sufficient to resolve the dynamics of the solar photosphere and chromosphere. In principle, all imaging channels can be restored individually using the speckle masking technique or multiframe blind deconvolution (MFBD). However, images recorded strictly simultaneously in the narrow-/broad-band Ha and the G-band/blue continuum channels can be pairwise subjected to multiobject multiframe deconvolution (MOMFBD) expanding the science capabilities of HiFI+. For example, the narrowband (FWHM= 60 nm) Halle Ha Lyot filter isolates the Ha line core, which facilitates matching chromospheric fibrils and filamentary structures to photospheric bright points. Likewise, dividing G-band by blue continuum images enhances small-scale brightenings, which are often related to small-scale magnetic fields so that their evolution can be tracked in time. A detailed description of the improved high- cadence, large-format imaging system is presented and its performance is assessed based on first-light observations. (c) The Authors. Published by SPIE under a Creative Commons Attribution 4.0 International License. Distribution or reproduction of this work in whole or in part requires full attribution of the original publication, including its

The Potsdam Arrayed Waveguide Spectrograph (PAWS) is built upon an integrated photonic spectrograph designed for astronomy. Similar to integrated optic beam combiners for interferometry, PAWS is intended to demonstrate on sky how a traditional bulk optics spectrograph with R = 15.000 in the H band can be miniaturized to fit on a chip. The integrated photonic spectrograph is based on second-generation Arrayed Waveguide Gratings (AWG) with unprecedented performance in terms of spectral resolution and throughput. The fibre-coupled AWG serves as a first dispersive element. The pre-dispersed light is fed into a free-space optical system located in a cryostat. Here the overlapping spectral orders are separated by cross-dispersion. The resulting echellogram is recorded by a Teledyne 2kx2k H2RG near-infrared array. Locally controlled constant cryogenic temperatures are required for the operation of the cryostat. This was achieved by fine-tuning and optimizing the original cryostat design using experimental data from multiple cryogenic cycles. These steps included the optimization of thermal interfaces, gold coating of the radiation shield, and an appropriate cooling sequence using the constraints of the allowed cooling rate for H2RG focal plane arrays. Using the readout electronics and GEIRS software provided by the Max Planck Institute for Astronomy (MPIA), frames of the H2RG were obtained, allowing performance calculations and dark pre-characterization of the system. For the optimum alignment of the optical system, the coefficient of thermal expansion (CTE) was measured with an interferometric set-up that recorded the spatial displacements of two reflecting optical elements within the cryostat during a cryogenic cycle. An appropriate strategy was developed to adjust the cryogenic cross dispersion optics inside the vacuum chamber to the AWG coupling optics outside the chamber.

The first generation of ELT instruments includes an optical-infrared high resolution spectrograph, indicated as ELT-HIRES and recently christened ANDES (ArmazoNes high Dispersion Echelle Spectrograph). ANDES consists of three fibre-fed spectrographs (UBV, RIZ, YJH) providing a spectral resolution of ∼100,000 with a minimum simultaneous wavelength coverage of 0.4-1.8 µm with the goal of extending it to 0.35-2.4 µm with the addition of a K band spectrograph. It operates both in seeing- and diffraction-limited conditions and the fibre-feeding allows several, interchangeable observing modes including a single conjugated adaptive optics module and a small diffraction-limited integral field unit in the NIR. Its modularity will ensure that ANDES can be placed entirely on the ELT Nasmyth platform, if enough mass and volume is available, or partly in the Coudé room. ANDES has a wide range of groundbreaking science cases spanning nearly all areas of research in astrophysics and even fundamental physics. Among the top science cases there are the detection of biosignatures from exoplanet atmospheres, finding the fingerprints of the first generation of stars, tests on the stability of Nature's fundamental couplings, and the direct detection of the cosmic acceleration. The ANDES project is carried forward by a large international consortium, composed of 35 Institutes from 13 countries, forming a team of more than 200 scientists and engineers which represent the majority of the scientific and technical expertise in the field among ESO member states.

We present the optomechanical design of the Potsdam Arrayed Waveguide Spectrograph (PAWS), which is the first on-sky demonstrator of an integrated photonic spectrograph specifically designed and optimized for astronomy. The instrument is based on an arrayed waveguide grating (AWG) that was designed by and custom fabricated for the innovation center innoFSPEC Potsdam. The commissioning of the instrument is planned at the Calar Alto 2:2m Telescope in southern Spain. The core of the instrument is the AWG-chip as the primary dispersive element. The AWG device is coupled to the telescope module via a single-mode fibre (SMF). The spectral image on the output facet of the AWG is a superposition of multiple spectral orders due to the cyclic dispersive behavior of the waveguide array. The output of the AWG is fed into a free-space optical system housed inside a cryostat via an infinity-corrected microscope objective. The overlapping spectral orders are separated by a second dispersion stage using a ruled grating as a cross-dispersive element, and the resulting echellogram is projected onto a Teledyne 2k x 2k H2RG near-infrared array. The requirement of sub-micron accuracy of the fibre-chip alignment has led to an advanced photonic packaging method. In order to avoid on-site alignment procedures during the on-sky testing, the AWG mount, fibre-support, and microscope objective were integrated into a single monolithic module. Optical and thermal simulations and the design of the cryostat were realized by Andes Scientific. The read-out electronics and the compatible operating software for the detector was provided by the Max Planck Institute for Astronomy (MPIA). Data analysis is performed using the open-source data reduction software P3D, which provides functionality for the removal of the instrument signature, extraction of the spectra, correction for the blaze function, wavelength calibration, and processed data file export.

Ballistische Messkammer with 10k CCD's (BMK10k) central goal is supporting the upcoming PLAnetary Transits and Oscillation of stars (PLATO) mission by reducing its number of false positives (planet-compatible transit candidates but not planets at the end). For Kepler, the number of false positives had been estimated to still be about 10%. Understanding the PLATO false alarm rate is not as straightforward, and preparatory data from the ground will dramatically improve the target selection and minimize the number of false positives. We aim to identify the target contamination for every PLATO target and initially provide well-sampled light curves of all detectable contaminants in the long-duration southern PLATO field. Thereby, it will be possible to identify background eclipsing binaries and rotating spotted stars with periods between 1 hr and 100 days down to 17-18th mag. This will be achieved with a dedicated robotic telescope with one of the world's largest CCDs with the smallest pixels, enabling a field of view of 52.5 square degrees with a sampling of 2.5 ''/pix. Auxiliary science is also foreseen and will focus on the period-mass relation of Ptolemy's cluster (M7). In this paper, we describe the entire telescope system and show initial commissioning data.

GREGOR at night spectrograph (GANS) is a high-resolution thermally-stabilised vacuum-enclosed fixed-format fiber-fed Echelle spectrograph. GANS will be installed starting 2018 alongside the daytime instrumentation in the building of the 1,5m Gregor Solar Telescope at the Observatorio del Teide at Izan˜a, Tenerife. Specified resolving power is R~55k with wavelength coverage from 470 to 680 nm in single shot on 2k 2k CCD with 3”, 50μm fiber on sky, and with space between orders for simultaneous calibration light in the form of a Fabry-Perot Etalon or a Laser-comb spectrum. The end-to-end simulated radial velocity precision performance estimate is 2 ms−1. The main observing project of GANS will be the ground-based follow-up survey of TESS and PLATO2.0 exoplanet candidates. GANS will observe its targets in autonomous operation without human intervention using the normally human-operated day-time observatory. Limited operations will begin in first half of 2019 with first science-light planned for summer 2019.

The problem of atmospheric emission from OH molecules is a long standing problem for near-infrared astronomy. PRAXIS is a unique spectrograph which is fed by fibres that remove the OH background and is optimised specifically to benefit from OH-Suppression. The OH suppression is achieved with fibre Bragg gratings, which were tested successfully on the GNOSIS instrument. PRAXIS uses the same fibre Bragg gratings as GNOSIS in its first implementation, and will exploit new, cheaper and more efficient, multicore fibre Bragg gratings in the second implementation. The OH lines are suppressed by a factor of ∼ 1000, and the expected increase in the signal-to-noise in the interline regions compared to GNOSIS is a factor of ∼ 9 with the GNOSIS gratings and a factor of ∼ 17 with the new gratings. PRAXIS will enable the full exploitation of OH suppression for the first time, which was not achieved by GNOSIS (a retrofit to an existing instrument that was not OH-Suppression optimised) due to high thermal emission, low spectrograph transmission and detector noise. PRAXIS has extremely low thermal emission, through the cooling of all significantly emitting parts, including the fore-optics, the fibre Bragg gratings, a long length of fibre, and the fibre slit, and an optical design that minimises leaks of thermal emission from outside the spectrograph. PRAXIS has low detector noise through the use of a Hawaii-2RG detector, and a high throughput through a efficient VPH based spectrograph. PRAXIS will determine the absolute level of the interline continuum and enable observations of individual objects via an IFU. In this paper we give a status update and report on acceptance tests.

The 4MOST instrument is a multi-object-spectrograph for the ESO-VISTA telescope. The 4MOST fiber feed subsystem is composed of a fiber positioner (AESOP) holding 2436 science fibers based on the Echidna tilting spine concept, and the fiber cable, which feeds two low-resolution spectrographs (1624 fibers) and one high-resolution spectrograph (812 fibers). In order to optimize the fiber feed subsystem design and provide essential information required for the spectrograph design, prototyping and testing has been undertaken. In this paper we give an overview of the current fiber feed subsystem design and present the preliminary FRD, scrambling, throughput and system performance impact results for: maximum and minimum spine tilt, fiber connectors, cable de-rotator simulator for fiber cable lifetime tests.

PEPSI is the bench-mounted, two-arm, fibre-fed and stabilized Potsdam Echelle Polarimetric and Spectroscopic Instrument for the 2x8.4 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 10.3kx10.3k CCDs with 9-mu m pixels and peak quantum efficiencies of 94-96 % record a total of 92 echelle 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 approximate to 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-mu m 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-Perot etalon 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. ((c) 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

VIRUS is the visible, integral-field replicable unit spectrograph for the Hobby-Eberly-Telescope (HET) consisting of 75 integral-field-units that feed 150 spectrographs. The full VIRUS instrument features over 33,000 fibres, each projecting to 1.5 arcseconds diameter on sky, deployed at the prime focus of the upgraded 10m HET. The assembly and acceptance testing for all IFUs includes microscopic surface quality inspections, astrometry of fibre positions, relative throughput measurements, focal-ratio-degradation evaluation, and system acceptance using a VIRUS reference spectrograph to verify the image quality, spectral transmission, stability, or to detect any stray light issues.

4MOST, the 4m Multi-Object Spectroscopic Telescope, features a 2.5 degree diameter field-of-view with ~2400 fibers in the focal plane that are configured by a fiber positioner based on the tilting spine principle (Echidna/FMOS) arranged in a hexagonal pattern. The fibers feed two types of spectrographs; ~1600 fibers go to two spectrographs with resolution R>5000 and ~800 fibers to a spectrograph with R>18,000. Part of the ongoing optimization of the fiber feed subsystem design includes early prototyping and testing of key components such as fiber connectors and fiber cable management. Performance data from this testing will be used in the 4MOST instrument simulator (TOAD) and 4MOST system design optimization. In this paper we give an overview of the current fiber feed subsystem design, simulations and prototyping plans.