Volume-phase holographic gratings (VPHGs) are widely used in astronomical spectrographs due to their adaptability and high diffraction efficiency. Most VPHGs in operation use dichromated gelatin as a recording material, the performance of which is sensitive to the coating and development process, especially in the near-ultraviolet (UV). In this letter, we present the characterization of two UV-blue VPHG prototypes for the BlueMUSE integral field spectrograph on the Very Large Telescope, based on dichromated gelatin and the Bayfol®HX photopolymer film as recording materials. Our measurements show that both prototypes meet the required diffraction efficiency and exhibit similar performance with a wavelength-average exceeding 70% in the 350 to 580 nm range. Deviations from theoretical models increase toward 350 nm, consistent with previous studies on similar gratings. We also report similar performances in terms of spatial uniformity and grating-to-grating consistency. Likewise, no significant differences in wavefront error or scattered light are observed between the prototypes.
BIFROST, an upcoming instrument for the VLTI, is part of the Asgard Suite, a VLTI visitor instrument. It comprises two spectrograph arms that are optimised for wavelength range of 1- 1.75μm (fed by a fiber that is placed on-axis for fringe tracking/monitoring) and 1-1.3μm (fed by a fiber that can positioned either on-axis or off-axis to observe a faint target) wavelength range, respectively. Volume phase holographic gratings (VPHGs) are employed to achieve high spectral resolving power up to 25,000 and a throughput above 75% for all dispersing elements. In this contribution, we discuss the optical and optomechanical design of the spectrographs, as well as a new relay optics design that minimizes the thermal background, with a factor 4 reduction in thermal background compared to the non-relay optics design. We will also present the first lab results obtained with the YJH spectrograph.
MIRC-X and MYSTIC are six-telescope near-infrared beam (1.08-2.38μm) combiners at the CHARA Array on Mt Wilson CA, USA. Ever since the commissioning of MIRC-X (J and H bands) in 2018 and MYSTIC (K bands) in 2021, they have been the most popular and over-subscribed instruments at the array. Observers have been able to image stellar objects with sensitivity down to 8.1mag in H and 7.8mag in K-band under the very best conditions. In 2022 MYSTIC was upgraded with a new ABCD mode using the VLTI/GRAVITY 4-beam integrated optics chip, with the goal of improving the sensitivity and calibration. The ABCD mode has been used to observe more than 20T Tauri stars; however, the data pipeline is still being developed. Alongside software upgrades, we detail planned upgrades to both instruments in this paper. The main upgrades are: 1) Adding a motorized filter wheel to MIRC-X along with new high spectral resolution modes 2) Updating MIRC-X optics to allow for simultaneous 6T J+H observations 3) Removing the warm window between the spectrograph and the warm optics in MYSTIC 4) Adding a 6T ABCD mode to MIRC-X in collaboration with CHARA/SPICA 5) Updating the MIRC-X CRED-ONE camera funded by Prof. Kraus from U. Exeter 6) Carrying out science verification of the MIRC-X polarization mode 7) Developing new software for ABCD-mode data reduction and more efficient calibration routines. We expect these upgrades to not only improve the observing experience, but also increase the sensitivity by 0.4mag in J+H-bands, and 1mag in K-band.
BIFROST is the short-wavelength, high-spectral resolution instrument in the Asgard Suite of VLTI visitor instruments. It will be optimized for spectral line studies in the Y, J, and H bands (1.05-1.75 μm) that include many strong lines & molecular features. In this presentation, we outline the BIFROST science drivers that have guided our design choices and map them against the operational modes that are being implemented. We give an overview about the status of the project and the milestones from the ongoing integration & testing phase in Exeter to shipping & commissioning on Paranal, scheduled for 2025 and 2026. We review the BIFROST subsystems and discuss how they interface with the broader Asgard Suite. Finally, we outline other BIFROST-related activities pursued by our group that are intended for implementation in BIFROST as part of future upgrades.
Volume Phase Holographic Gratings (VPHGs) have introduced, in recent years, a significant improvement in low and medium resolution spectrographs with an apparent throughput step up. They are still considered the baseline for the DE in low and medium resolution spectrographs. At INAF-Osservatorio Astronomico di Brera, we have been developing VPHGs based on an innovative approach for some years. Almost 15 produced VPHGs are operating in spectroscopic facilities and some prototypes are under production for several spectrographs. VPHGs up to 190 mm in diameter can be manufactured in a wide range of line density. Moreover, innovative configurations, such as multiorder grating and stacked grating, have been developed to increase the spectral range and resolution at the same time.
The next steps will be the improvement of the VPHG performances especially in the diffraction efficiency across the bandwidth and the transparency in the UV. Concerning the size of the diffraction elements, a new facility is under building that will be capable to produce VPHGs up to 450 mm in diameter.
The Center for High Angular Resolution Astronomy (CHARA) has developed and continues to manage an optical interferometric array of six telescopes located on Mount Wilson, California. This interferometer is particularly suited to stellar astrophysics. MIRC-X and MYSTIC beam combiners are two instrumentations of the array. The former is designed to work in the J and H band wavelengths, while the latter is designed to work in the K band. To enhance the spectroscopic capabilities, new GRISMs based on volume phase holographic (VPH) gratings have been designed and manufactured. Two dispersing elements operate in the J band, offering a resolving power of R = 6000 at 1.09 μm and 1.27 μm, while the other two elements work in the K band, providing R = 4000 at 2.17 μm and 2.33 μm. The VPHGs are manufactured employing an innovative process developed at Istituto Nazionale di Astro Fisica (INAF). The prisms for the GRISMs are in CLEARTRAN. The design, manufacturing and assembly of the devices are presented together with the measured performances in terms of diffraction efficiency and diffracted wavefront error.
Volume Phase Holographic Gratings (VPHGs) have proved to be of interest in astronomical fields because they show large diffraction efficiency and they can be tailored according to the spectrograph requirements. Different low dispersion VPHGs working in the UV-vis spectral region have been manufactured and fully characterized at INAF-Osservatorio Astronomico di Brera. Furthermore, such show a peak efficiency that reaches 90% and that remain large across the bandwidth. Dual order low dispersion VPHG has been developed covering the whole optical spectrum (from 350 nm to 1000 nm) in one exposure. The main features of these recent VPHGs are reported highlighting the efficiency performances.
Volume-phase holographic gratings (VPHGs) are widely used in astronomical spectrographs due to their adaptability and high diffraction efficiency. Most VPHGs in operation use dichromated gelatin as a recording material, whose performance is sensitive to the coating and development process, especially in the near-UV. In this letter, we present the characterization of two UV-blue VPHG prototypes for the BlueMUSE integral field spectrograph on the VLT, based on dichromated gelatin and the Bayfol®HX photopolymer film as recording materials. Our measurements show that both prototypes meet the required diffraction efficiency and exhibit similar performance with a wavelength-average exceeding 70% in the 350-580 nm range. Deviations from theoretical models increase towards 350 nm, consistently with previous studies on similar gratings. We also report similar performances in terms spatial uniformity and grating-to-grating consistency. Likewise, no significant differences in wavefront error or scattered light are observed between the prototypes.
This paper presents the design and technical progress of a precision X-Y stage for detector dithering and flexure compensation being developed for the Magellan InfraRed Multi-Object Spectrograph, MIRMOS. Designed to operate at 80 K, the stage will accurately control detector position in two orthogonal degrees of freedom. The piezo-driven flexure stage is very compact providing high-resolution backlash-free motion of the detector. A magneto resistive bridge provides position feedback in each degree of freedom, greatly reducing hysteresis, which is common in piezoelectric actuators. The system is designed to operate in open loop using a lookup table keyed to the Nasmyth rotator angle for flexure control. Here, the optomechanical design of the stage, electrical control system, and performance results from early prototype efforts are presented and discussed.
We present an update to the 2024 Magellan Infrared Multi-Object Spectrograph (MIRMOS). MIRMOS is a wide-field (13′ × 3′) near-infrared (0.89-2.4 μm) spectrograph with either a multiobject or integral-field mode. MIRMOS is designed to perform either faint-object distant-object spectroscopy, or high-signal-to-noise transmission spectroscopy of exoplanet atmosphere. MIRMOS is designed to deliver a spectral resolution of > 3, 400, delivering the Y, J, H, and K bands over four Hawaii H2RG detectors. The slit robot allows the observer to create 92 slits or a 26′′ ×20′′ integral-field unit can be swapped in. In this proceeding, we describe changes since the last SPIE conference.
The Magellan InfraRed Multi-Object Spectrograph (MIRMOS) is a planned next generation multi-object and integral field spectrograph for the 6.5m Magellan telescopes at Las Campanas Observatory in Chile. MIRMOS will perform R∼3700 spectroscopy over a simultaneous wavelength range of 0.886 - 2.404μm (Y,J,H,K bands) in addition to imaging over the range of 0.7 - 0.886μm. The integral field mode of operation for MIRMOS will be achieved via an image slicer style integral field unit (IFU) located on a linear stage to facilitate movement into the beam during use or storage while operating in multi-object mode. The IFU will provide a ∼ 20′′×26′′ field of view (FoV) made up of 0.84′′ ×26′′ slices. This will be the largest FoV IFS operating at these wavelengths from either the ground or space, making MIRMOS an ideal instrument for a wide range of science cases including studying the high redshift circumgalactic medium and emission line tracers from ionized and molecular gas in nearby galaxies. In order to achieve the desired image quality and FoV while matching the focal ratio to the multi-object mode, our slicer design makes use of novel freeform surfaces for the pupil mirrors, which require the use of high precision multi-axis diamond milling to manufacture. We present here the optical design and predicted performance of the MIRMOS IFU along with a conceptual design for the opto-mechanical system.
Volume Phase Holographic Gratings (VPHGs) are optical dispersing element widely used in astronomical spectrographs. In the last years, the availability of high performance photopolymers allowed for the development of innovative approaches to produce these dispersing elements. The key activities that have been carried out are: i) a production process based on photopolymeric holographic materials (in particular Bayfol®HX by COVESTRO AG) was defined; ii) high quality VPHGs > 170 mm in diameter were manufactured; iii) innovative configurations to increase the dispersion/spectral range were implemented. The effectiveness of these activities is confirmed by the fact that more than 10 devices are mounted on observing facilities and several more are in development or planned. In this paper, we present the VPH technology based on photopolymers.
AFOSC is a versatile instrument mounted at the Copernico 1.82 meter telescope in Asiago (Italy) and it is equipped with a suite of dispersing elements covering the optical range. Two innovative dispersing elements in GRISM configuration are designed in order to boost the observing performances. One GRISM works in the Ha and it is characterized by a resolving power of 3600. Its peak diffraction efficiency reaches 90% and it remains high in the spectral window. The other GRISM is a multiorder element that allows for the recording of a low-resolution spectrum in the 0.35 – 1.00 um in a single exposure, by projecting two diffraction orders on the detector. The VPHG is optimized in both first and second orders and a combination of two prisms (forming an Amici prism) is used to achieve the cross-dispersion. The diffraction efficiency of the VPHG is well balanced across the spectral range with <80% of peak efficiency in the 1st order and <60% in the 2nd.
Volume Phase Holographic Gratings (VPHGs) are optical element widely used in astronomical spectrographs as main disperser or cross-disperser in high resolution echelle instruments. In spite of the fact that other technologies are available on the market, the VPH technology remain a key one. In the EU funded OPTICON project, different activities were carried out in order to consolidate the design and production of VPHGs for astronomy. In particular: i) a production process based on innovative high performance holographic materials (by COVESTRO AG) has been defined; ii) high quality VPHGs ⪆ 170 mm in diameter were manufactured; iii) innovative configurations, such as multiorder and multiplexed gratings were proposed and the devices realized. Now, more than 10 devices based on this technology are mounted on observing facilities and several more are in development or planned. Here, we retrace the achievement of the VPH activities in the last years and we propose our roadmap for future improvements in the VPHG design and production for supporting the requests of the astronomical community starting from the ORP EU project.
The FORS Upgrade project (FORS-Up) aims at bringing a new life to FORS (the visual and near UV FOcal Reducer and low dispersion Spectrograph for the VLT). One of the activity focuses on the installation of three new GRISMs. One at low-resolution covering the 0.33 to 0.62 um spectral range (centered at) and two at higher resolution targeting the Sodium and Potassium signals, which are centered at 0.58 um and 0.77 um respectively. The three VPHGs have been designed by means of an RCWA (rigorous coupled-wave analysis) approach and the feasibility of using the innovative photopolymers developed by COVESTRO AG has been evaluated. Thanks to the new high performances of the mentioned photosensitive materials, it has been possible to ensure a high peak efficiency together with a large efficiency over the bandwidth. In parallel, the GRISM parameters have been set and the overall efficiency of the dispersing element has been evaluated.
Within the framework of any Space Surveillance and Tracking activities, the capability of optical sensors to perform spectroscopic observations would add a unique value to acquiring supplementary information on any extemporary object crossing the telescope field of view. In addition to the astrometric information, probing the body’s albedo via low-resolution spectroscopy would constrain the geometry and physical nature of the target and discriminate among spent rocket parts, coarse debris and intact spacecraft. The EU-SST project SuperFOSC, currently in progress at INAF-OAS, is a one square degree wide-field camera that will equip the “G.D. Cassini’” telescope, located close to Bologna (Italy). Besides the imaging capabilities, we also envisage an option to secure one-shot slit-less spectroscopy of all the (censed and uncensed) objects crossing the telescope field of view along with the observations of the bonafide planned targets. This will be done by placing a diffractive grism on a pupil image inside the camera. This paper investigates the system performance in terms of spectral resolution as a function of the intruding object(s) path for a grism with a fixed or variable position angle. In addition, as an innovative concept, we also discuss a three-faces grism (Trism) solution to catch any source three spectra at a time, each 120 deg apart in position angle on the sky, to maximise spectral resolution disregarding object trajectory. An added value of our Trism solution would be avoiding moving optical parts inside the instrument with a significantly improved image quality.
KEYWORDS: Optical spheres, Sensors, Planets, Spectrographs, Iterated function systems, Stars, Spectral resolution, Coronagraphy, Adaptive optics, Signal to noise ratio
MedRes is a proposed MEDium RESolution integral field spectrograph for upgrading SPHERE, the high contrast instrument for the ESO VLT telescope. MedRes is actually thought of as a potential Visitor Instrument with the scope to provide high contrast diffraction limited medium-high resolution spectra (R ≥ 1000) over a reasonably large field of view (a square with a side of at least 0.4) and across the spectral region 1.2-1.65 microns. Two main science objectives are driving the proposition for such an instrument on SPHERE. First of all, MedRes shall improve the detection of previously unknown giant planets (contrast 10−5 , goal 10−6 ), in particular accreting planets, at small separation from the star (< 0.2”, goal, 0.1”). And second, MedRes will boost the characterisation of known (faint) planets at a spectral resolution substantially higher than currently possible with SPHERE IFS (R ~ 35 − 50) and for contrasts much better than achievable with IRDIS Long Slit Spectroscopy (LSS) at small separations. The design will be optimised for SPHERE, fully exploiting the capabilities offered by a second stage Adaptive Optics (SAXO+) and complementing the niches of IRDIS, IFS and HiRise in the near IR channel. A preliminary optomechanical design and simulations of performance will be presented.
SPHERE+ is a proposed upgrade of the SPHERE instrument at the VLT, which is intended to boost the current performances of detection and characterization for exoplanets and disks. SPHERE+ will also serve as a demonstrator for the future planet finder (PCS) of the European ELT. The main science drivers for SPHERE+ are 1/ to access the bulk of the young giant planet population down to the snow line (3 − 10 au), to bridge the gap with complementary techniques (radial velocity, astrometry); 2/ to observe fainter and redder targets in the youngest (1 − 10 Myr) associations compared to those observed with SPHERE to directly study the formation of giant planets in their birth environment; 3/ to improve the level of characterization of exoplanetary atmospheres by increasing the spectral resolution in order to break degeneracies in giant planet atmosphere models. Achieving these objectives requires to increase the bandwidth of the xAO system (from ~1 to 3 kHz) as well as the sensitivity in the infrared (2 to 3 mag). These features will be brought by a second stage AO system optimized in the infrared with a pyramid wavefront sensor. As a new science instrument, a medium resolution integral field spectrograph will provide a spectral resolution from 1000 to 5000 in the J and H bands. This paper gives an overview of the science drivers, requirements and key instrumental tradeoff that were done for SPHERE+ to reach the final selected baseline concept.
The Exoplanets at LBT with a Visible IFS for SHARK-VIS (ELVIS) is an add-on imaging spectrograph to be integrated in the new LBT high-contrast high-resolution AO-assisted imager SHARK-VIS. ELVIS is optimized for a medium/high spectral resolution of 10-20k with a limited bandwidth around the Hα, and it is planned fed by a small core (10-20 ⊘ µm) multi mode fiber bundle providing about 140 spaxels on a field of view around 300×300 sqmas. This instrument has a very compact design based on a VPH dispersing element to allow its installation within a standard 19” rack mount. As shown in the literature, young accreting sub-stellar and planetary companions are better detected and analyzed by these instruments allowing to reach contrast at least ten times fainter (in their Hα emission) with respect to standard imagers.
The BIFROST instrument will be the first VLTI instrument optimised for high spectral resolution up to R=25,000 and operate between 1.05 and 1.7 μm. A key component of the instrument will be the spectrograph, where we require a high throughput over a broad bandwidth. In this contribution, we discuss the four planned spectral modes (R=50, R=1000, R=5000, and R=25,000), the key spectral windows that we need to cover, and the technology choices that we have considered. We present our plan to use Volume Phase Holographic Gratings (VPHGs) to achieve a high efficiency > 85%. We present our preliminary optical design and our strategies for wavelength calibration.
Compressive sensing (CS) is a new acquisition technique that can potentially open the way to multi- and hyper-spectral imaging in wide spectral regions with a simplified optical scheme. In this framework, we are studying different approaches applied to Earth observation in different contexts. We describe the design of a vis-NIR telescope, where the image sampling is performed in its focal plane with a Digital Micromirror Device (DMD), and the hyperspectral acquisition is made possible by a compact spectrometer with a multispectral detector. We show the approach for a pushbroom multispectral acquisition from space, working in both the visible and thermal bands, to monitor the status of the cryosphere. Finally, we describe a compact wideband system working in the 0.4- 2.5 micron spectral range, based on three parallel spectrometers, where the acquisition is performed with a pushbroom scanning and a rotating coding mask. Opportunities and constraints given by the CS approach in these different contexts are highlighted.
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