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.
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.
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.
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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