HARMONI is a visible and near-infrared integral field spectrograph equipped with two complementary adaptive optics systems, fully integrated within the instrument. A Single Conjugate AO (SCAO) system offers high performance for a limited sky coverage and a Laser Tomographic AO (LTAO) system provides AO correction with a very high sky-coverage. While the deformable mirror performing real-time correction of the atmospheric disturbances is located within the telescope itself, the instrument contains a suite of state-of-the-art and innovative wavefront sensor systems. Laser guide star sensors (LGSS) are located at the entrance of the instrument and fed by a dichroic beam splitter, while the various natural guide star sensors for LTAO and SCAO are located close to the science focal plane. We present opto-mechanical architecture and design at PDR level for these wavefront sensor systems.
MOSAIC is a mixed-mode multiple object spectrograph planned for the ELT that uses a tiled focal plane to support a variety of observing modes. The MOSAIC AO system uses 4 LGS WFS and up to 4 NGS WFS positioned anywhere within the full 10 arcminute ELT field of view to control either the ELT M4/5 alone for GLAO operation feeding up to 200 targets in the focal plane, or M4/5 in conjunction with 10 open-loop DMs for MOAO correction. In this paper we present the overall design and performance of the MOSAIC GLAO and MOAO systems.
With the recent development of new ultra fine aluminium alloys and progress in the field of directly machined freeform surfaces, diamond machined freeform gratings could play an important part in future spectrographs or integral field units, particularly at SWIR and LWIR wavelengths where the improved thermal performance of metal optics at cryogenic temperatures is well established. Freeform diamond machined gratings can offer a cost-effective, compact, and flexible alternative to gratings fabricated by other methods such as ion beam etching or complement these technologies. In this paper, both the advantages and limitations of 5 axis diamond machined freeform gratings are presented and potential applications are discussed.
The Optical Relay Module of the MOSAIC multiple-object spectrograph is used to relay 400-1800nm light picked off from the ELT focal plane to either a fibre-based integral field unit or a natural guide star wavefront sensor. Here we present the preliminary optical design offering a telecentric exit beam with a focal-ratio of F/17.718 and the opto-mechanical analysis of flexures with a study of the impact in the optical layout performances such as: deviation of the PSF centroid, tip-tilt of the image focal plane, variations of the wavefront error, optical quality and pupil wandering at the deformable mirror position.
Spectroscopy is a key technique in astronomy and nowadays most major telescopes include at least one spectrograph in their instrument suite. The dispersive element is one of the most important components and it defines the pupil size, spectral resolution and efficiency. Different types of dispersive elements have been developed including prisms, grisms, VPH and echelle gratings. In this paper, we investigate the design and optimization possibilities offered by metallic freeform gratings using diamond machining techniques. The incorporation of power in a diffraction grating enables several functionalities within the same optical component, such as the combination of dispersion, focusing and field reformat. The resulting benefit is a reduction in the number of surfaces and therefore, an improvement in the throughput. Freeform surfaces are also interesting for their enhanced optical performance by allowing extra degree of freedom in the optimization. These degrees of freedom include the shape of the substrate but also additional parameters such as the pitch or the number of blaze angle. Freeform gratings used as single optical component systems also present some limitations such as the trade-off between optical quality versus field of view or the spectral range versus spectral resolution. This paper discusses the possibility offered by the design of freeform gratings for low to medium spectral resolution, in the visible and near-infrared, for potential applications in ultra-compact integral field spectrographs.
A novel concept for the calibration of multi object fiber-fed spectrographs is described for the 4MOST instrument. The 4MOST facility is foreseen to start science operations in 2022 at the ESO VISTA telescope. The calibration system provides intensity, wavelength and resolution calibrations for the 4MOST spectrographs. The heart of the system is a combination of a bright broad band lamp and a Fabry-Perot etalon. The lamp is able to provide sufficient flux to illuminate the VISTA focal plane and the Fabry-Perot etalon provides a regular comb of spectral lines. The Fabry-Perot etalon can be moved in and out of the optical beam to choose between intensity and spectral calibrations. A fiber bundle of 156 fibers is guided to the VISTA spider arms where each fiber is connected to a small integrating sphere. The integrating spheres are attached to the bottom side of the four VISTA telescope spider struts and provide unvignetted illumination of the telescope. The exit port of the integrating spheres is projected on the VISTA focal plane with a small collimator lens. The integrating spheres assure a uniform illumination of the focal plane and are insensitive to FRD effects of the input fibers due to motion and stress during telescope movements. The calibration system illumination only originates from the telescope spiders and therefore the telescope pupil is not fully filled. The calibration system uses the azimuthal scrambling properties of the fibers that connect the telescope focal plane and the spectrometers to completely fill the spectrograph pupil.
In recent decades, spectroscopic capabilities have been significantly enhanced by new technological developments, in particular spatial reformatting. Spatial reformatting allows multiple functionalities: the observation of a larger area of sky, obtaining the spectra of all spatial elements under the same atmospheric conditions; modification of the shape and size of the field of view; focal-ratio conversion for the optimized coupling between the telescope and the spectrograph; increase in the spatial and spectral resolving power; the observation of multiple objects; homogeneity in the illumination; scrambling of spatial and/or phase induced structure with the instrument, thus improving the system stability; relocation of the exit pupil, especially important for telecentric systems. The impact of reformatting and the breadth of science cases is so great that many alternative methods and technologies have been proposed: image slicers using refractive or reflective solutions; optical fibers with different core sizes and geometries; microlenses used in isolation or combined with fibers and more recently, photonic devices such as Photonic lanterns to produce modal decomposition. In this paper, a comparison between all currently available options is presented, with a detailed analysis of their advantages and limitations and a proposal for a new reformatter combining slicers and photonic devices. This proposal presents the advantages of the other alternatives and additionally offers: minimization of focal-ratio degradation; produces image and modal decomposition; improves the throughput along the spectral range, increases the spectral resolving power and adds the functionality of scrambling. All of these advantages are combined in a system where photonic and astronomical instrumentation capabilities are joined in an innovative solution with many applications, like for example, the Extremely Large Telescope.
The continuous strive for increased sensitiv ity and higher resolution of space based telescopes can only be satisfied with larger primary mirrors. There are quite a few challenges in launching large mirrors in space such as surviving the stress created from the launch acceleration, deployment, thermoelastic deformations, the gravity release etc. Major constraint to space based application is weight which drives the development of thin, extremely lightweight mirrors. Such mirrors are prone for stress based deformations and need active optics correction chain (AOCC) in order to be operated at their full potential. An AOCC for large monolithic mirrors consists of three key active optics components: corrective element (e.g. deformable mirror or DM), wavefront sensor (WFS) and correction algorithm. In order to assess the feasibility of such a system we have developed an AOCC test stand in a collaboration with the European Space Agency (ESA) a nd Netherlands Organisation for Applied Scientific Research (TNO). With this development we aim to measure the performance and the long-term reliability of an AOCC in controlled laboratory conditions. Our design consists of two separate parts, one where the expected aberrations are generated and another where they are measured and corrected. Two deformable mirrors of 37.5 mm and 116 mm are used, the smallest mirror to generate aberrations and the largest to correct them. For wavefront sensing we are using two different wavefront sensors, an 11x11 Shack-Hartmann as well as phase diversity based at the science sensor. We are able to emulate the conditions for both, astronomy related, and Earth observations. Here, we present the design of the system, including the test stand and the correction algorithms, the performance expected from simulations, and the results from the latest lab tests.
HARMONI is the E-ELT’s first light visible and near-infrared integral field spectrograph. It will provide four different spatial scales, ranging from coarse spaxels of 60 × 30 mas best suited for seeing limited observations, to 4 mas spaxels that Nyquist sample the diffraction limited point spread function of the E-ELT at near-infrared wavelengths. Each spaxel scale may be combined with eleven spectral settings, that provide a range of spectral resolving powers (R ~3500, 7500 and 20000) and instantaneous wavelength coverage spanning the 0.5 – 2.4 μm wavelength range of the instrument. In autumn 2015, the HARMONI project started the Preliminary Design Phase, following signature of the contract to design, build, test and commission the instrument, signed between the European Southern Observatory and the UK Science and Technology Facilities Council. Crucially, the contract also includes the preliminary design of the HARMONI Laser Tomographic Adaptive Optics system. The instrument’s technical specifications were finalized in the period leading up to contract signature. In this paper, we report on the first activity carried out during preliminary design, defining the baseline architecture for the system, and the trade-off studies leading up to the choice of baseline.
We present here a four mirror anastigmatic optical collimator design intended for the calibration of an earth observation
satellite instrument. Specifically, the collimator is to be applied to the ground based calibration of the Sentinel-4/UVN
instrument. This imaging spectrometer instrument itself is expected to be deployed in 2019 in a geostationary orbit and
will make spatially resolved spectroscopic measurements of atmospheric contaminants.
The collimator is to be deployed during the ground based calibration only and does not form part of the instrument itself.
The purpose of the collimator is to provide collimated light within the two instrument passbands in the UV-VIS (305 –
500 nm) and the NIR (750 – 775 nm). Moreover, that collimated light will be derived from a variety of slit like objects
located at the input focal (object) plane of the collimator which is uniformly illuminated by a number of light sources.
The collimator must relay these objects with exceptionally high fidelity. To this end, the wavefront error of the
collimator should be less than 30 nm rms across the collimator field of view. This field is determined by the largest
object which is a large rectangular slit, 4.4° x 0.25°. Other important considerations affecting the optical design are the
requirements for input telecentricity and the size (85 mm) and location (2500 mm ‘back focal distance’) of the exit pupil.
The design of the instrument against these basic requirements is discussed in detail. In addition an analysis of the
straylight and tolerancing is presented in detail.
We describe here an optical polariser module intended to deliver well characterised polarised light to an imaging spectrometer instrument. The instrument in question is the Sentinel-4/UVN Earth observation imaging spectrometer due to be deployed in 2019 in a geostationary orbit. The polariser module described here will be used in the ground based calibration campaign for this instrument. One critical task of the calibration campaign will be the highly accurate characterisation of the polarisation sensitivity of instrument. The polariser module provides a constant, uniform source of linearly polarised light whose direction can be adjusted without changing the output level or uniformity of the illumination. <p> </p>A critical requirement of the polariser module is that the illumination is uniform across the exit pupil. Unfortunately, a conventional Glan-Taylor arrangement cannot provide this uniformity due to the strong variation in transmission at a refractive surface for angles close to the critical angle. Therefore a modified prism arrangement is proposed and this is described in detail. Detailed tolerance modelling and straylight modelling is also reported here.
This communication presents the feasibility study of an image slicer for future space missions, especially for the integral field unit (IFU) of the SUVIT (Solar UV-Visible-IR telescope) spectro-polarimeter on board the Japanese-led solar space mission Solar-C as a backup option. The MuSICa (Multi-Slit Image slicer based on collimator-Camera) image slicer concept, originally developed for the European Solar Telescope, has been adapted to the SUVIT requirements. The IFU will reorganizes a 2-D field of view of 10 x 10 arcsec<sup>2</sup> into three slits of 0.18 arcsec width by 185.12 arcsec length using flat slicer mirrors of 100 μm width. The layout of MuSICa for Solar-C is telecentric and offers an optical quality limited by diffraction. The entrance for the SUVIT spectro-polarimeter is composed by the three IFU slits and one ordinal long slit to study, using high resolution spectro-polarimetry, the solar atmosphere (Photosphere and Chromosphere) within a spectral range between 520 nm (optionally 280 nm) and 1,100 nm.
Integral Field Spectroscopy is an innovative technique that is being implemented in the state-of-the-art instruments of the
largest night-time telescopes, however, it is still a novelty for solar instrumentation. A new concept of image slicer,
called MuSICa (Multi-Slit Image slicer based on collimator-Camera), has been designed for the integral field
spectrograph of the 4-m European Solar Telescope. This communication presents an image slicer prototype of MuSICa
for GRIS, the spectrograph of the 1.5-m GREGOR solar telescope located at the Observatory of El Teide. MuSICa at
GRIS reorganizes a 2-D field of view of 24.5 arcsec into a slit of 0.367 arcsec width by 66.76 arcsec length distributed
horizontally. It will operate together with the TIP-II polarimeter to offer high resolution integral field spectropolarimetry.
It will also have a bidimensional field of view scanning system to cover a field of view up to 1 by 1 arcmin.
This communication presents a family of spectrographs designed for the European Solar Telescope. They can operate in
four different configurations: a long slit standard spectrograph (LsSS), two devices based on subtractive double pass
(TUNIS and MSDP) and one based on an integral field, multi-slit, multi-wavelength configuration. The combination of
them composes the multi-purpose grating spectrograph of EST, focused on supporting the different science cases of the
solar photosphere and chromosphere in the spectral range from 3900 Å to 23000 Å. The different alternatives are made
compatible by using the same base spectrographs and different selectable optical elements corresponding to specific
subsystems of each configuration.
Integral field spectroscopy is a modern technique used in Astronomy to obtain simultaneous spectral information of all
points in a bidimensional field of view. This communication presents the preliminary design of a multi-slit image slicer
to be coupled to the spectrographs of the 4 meters aperture European Solar Telescope. This integral field unit will
provide the observation of an 80 arcsec<sup>2</sup> field of view, rearranged into 8 slits of 200 arcsec length by 0.05 arcsec width.
Different optical design alternatives with diffraction limited optical quality, as well as the design of a prototype for the
GREGOR solar telescope, are presented.
EST is a project for a 4-meter class telescope to be located in the Canary Islands. EST will be optimized for studies of
the magnetic coupling between the photosphere and the chromosphere. This requires high spatial and temporal resolution
diagnostics tools of properties of the plasma, by using multiple wavelength spectropolarimetry. To achieve these goals,
visible and near-IR multi-purpose spectrographs are being designed to be compatible with different modes of use: LsSS
(Long-slit Standard Spectrograph), multi-slit multi-wavelength spectrograph with an integral field unit, TUNIS (Tunable
Universal Narrow-band Imaging Spectrograph), and new generation MSDP (Multi-channel Subtractive Double-pass
Spectrograph). In this contribution, these different instrumental configurations are described.
This communication shows the feasibility study of a new instrument designed for the 4 meter European Solar Telescope
(EST) for high resolution spectro-polarimetric observations. This paper is specifically focused on the spectrographs that
allow the simultaneous observation of 5 visible and 4 near-infrared wavelengths (complying with the science
requirements), with 8 entrance slits of 200arcsec each fed by an integral field unit covering an area on the solar surface
of 9 x 9 arcsec<sup>2</sup>.
This communication reviews the participation of the Instituto de Astrofísica de Canarias (IAC) in the design of the
European Solar Telescope. Apart of being the coordinator institution of the whole project, and, as such, responsible for
the project managing, the IAC leads several tasks like overall instrument definition or characterization of the
atmospheric turbulence profile with height or the definition of adequate detectors. More in particular, the IAC will
design and build two long-base SHABAR (SHAdow BAnd Ranger), instruments to measure medium-altitude seeing.
The IAC is also responsible for the design, together with other institutions, of the design of grating spectropolarimeters
suitable for multiwavelength high spatial and spectral resolution.
In this contribution an Atmospheric Dispersion Corrector (ADC) developed for the FastCam project is presented.
FastCam is a system based on 'lucky imaging' techniques for high spatial resolution, developed at the Instituto de
Astrofisica de Canarias (IAC). The use of a system to correct the atmosphere's effects is necessary to obtain a good
optics quality in order to satisfy science requirements.
Two alternatives for an atmospheric dispersion corrector for the instrument FastCam have been studied. One ADC has
been designed to be implemented and intended to correct that atmospherical effect at the 4.2m William Herschel
Telescope (Roque de los Muchachos Observatory, La Palma) for zenithal distances larger than 15 degrees, mainly in I
band. The excellent resolution reached with FastCam makes this effect to distort the images. This work presents the
design of an ADC as a pre-focus system of two prisms with variable separation in order to compensate the dispersion
until zenithal distances of 50°. Both are placed in a rotator to align the orientation of zenithal distorsion. The results for
this ADC has been tested in lab, and in May 2008, at the WHT; our next step will be to use it at the 10.4m Gran
Telescopio Canarias (GTC). Although the system is still under the final stages of its development, we want to implement
it to the new instruments for high spatial resolution.
FastCam is an instrument jointly developed by the Spanish Instituto de Astrofísica de Canarias and the Universidad Politécnica de Cartagena designed to obtain high spatial resolution images in the optical wavelength range from ground-based telescopes.
The instrument consists of a very low noise and very fast readout speed EMCCD camera capable of reaching the diffraction limit of medium-sized telescopes from 500 to 850 nm. FastCam incorporates a FPGAs-based device to save and evaluate those images minimally disturbed by atmospheric turbulence in real time. The undisturbed images represent a small fraction of the observations. Therefore, a special software package has been developed to extract, from cubes of tens of thousands of images, those with better quality than a given level. This is done in parallel with the data acquisition at the telescope.
After the first tests in the laboratory, FastCam has been successfully tested in three telescopes: the 1.52-meter TCS (Teide Observatory), the 2.5-meter NOT, and the 4.2-meter WHT (Roque de los Muchachos Observatory). The theoretical diffraction limit of each telescope has been reached in the I band (850 nm) -0.15, 0.08 and 0.05 arcsec, respectively-, and similar resolutions have been also obtained in the V and R bands.
Future work will include the development of a new instrument for the 10.4-meter GTC telescope on La Palma.
This communication shows the design, layout, mounting and start-up of a high-resolution grating spectrograph for VIS-NIR
at GREGOR 1.5m Solar Telescope (Observatorio del Teide, Tenerife, Canary Islands). The instrument will be used
together with the Tenerife Infrared Polarimeter (TIP-II). As special characteristics of the design, the following can be
mentioned: The first folding mirror of the spectrograph can be placed in two positions to take into account the change of the
optical axis introduced by the polarizing beamsplitter of TIP-II. This way the instrument is optimally aligned when used
in situations with and without polarimeter. The second and third mirrors rotate the image of the entrance slit, making it parallel to the grating grooves. A system of prisms are used to adequately fit onto the detector the two orthogonal polarized beams generated by the
polarimeter. Two output beams are possible, to make feasible simultaneous visible and near-infrared observations.