The construction of the next generation of 40 m-class astronomical telescopes poses an enormous challenge for the design of their instruments and the manufacture of their optics. Optical elements typically increase in both size and number, placing ever more demands on the system manufacturing and alignment tolerances. This challenge can be met by using the wider design space offered by freeform optics, by for instance allowing highly aspherical surfaces. Optical designs incorporating freeform optics can achieve a better performance with fewer components. This also leads to savings in volume and mass and, potentially, cost.
This paper describes the characterization of the FAME system (freeform active mirror experiment). The system consists of a thin hydroformed face sheet that is produced to be close to the required surface shape, a highly controllable active array that provides support and the ability to set local curvature of the optical surface and the actuator layout with control electronics that drives the active array.
A detailed characterisation of the fully-assembled freeform mirror was carried out with the physical and optical properties determined by coordinate measurements (CMM), laser scanning, spherometry and Fizeau interferometry. The numerical model of the mirror was refined to match the as-built features and to predict the performance more accurately.
Each of the 18 actuators was tested individually and the results allow the generation of look-up tables providing the force on the mirror for each actuator setting. The actuators were modelled with finite element analysis and compared to the detailed measurements to develop a closed-loop system simulation. After assembling the actuators in an array, the mirror surface was measured again using interferometry. The influence functions and Eigen-modes were also determined by interferometry and compared to the FEA results.
Immersed gratings offer several advantages over conventional gratings: more compact spectrograph designs, and by using standard semiconductor industry techniques, higher diffraction-efficiency and lower stray-light can be achieved. We present the optical tests of the silicon immersed grating demonstrator for the Mid-infrared E-ELT Imager and Spectrograph, METIS. We detail the interferometric tests that were done to measure the wavefront-error and present the results of the throughput and stray-light measurements. We also elaborate on the challenges encountered and lessons learned during the immersed grating demonstrator test campaign that helped us to improve the fabrication processes of the grating patterning on the wafer.
Immersed gratings offer several advantages over conventional gratings: more compact spectrograph designs, and by using standard semiconductor industry techniques, higher diffraction-efficiency and lower stray-light can be achieved. We present the optical tests of the silicon immersed grating demonstrator for the Mid-infrared E-ELT Imager and Spectrograph, METIS. We detail the interferometric tests that were done to measure the wavefront-error and present the results of the throughput and stray-light measurements. We also elaborate on the challenges encountered and lessons learnt during the immersed grating demonstrator test campaign that helped us to improve the fabrication processes of the grating patterning on the wafer.
This paper discusses the development, realization and initial characterization of a demonstrator for a cryogenic 'set and forget' deformable mirror. Many optical and cryogenic infrared instruments on modern very and extremely large telescopes aim at diffraction-limited performance and require total wave front errors in the order of 50 nanometers or less. At the same time, their complex optical functionality requires either a large number of spherical mirrors or several complex free-form mirrors. Due to manufacturing and alignment tolerances, each mirror contributes static aberrations to the wave front. Many of these aberrations are not known in the design phase and can only be measured once the system has been assembled. A 'set-and-forget' deformable mirror can be used to compensate for these aberrations, making it especially interesting for systems with complex free-form mirrors or cryogenic systems where access to iterative realignment is very difficult or time consuming.
The mirror with an optical diameter of 200 mm is designed to correct wave front aberrations of up to 2 μm root-mean square (rms). The shape of the wave front is approximated by the first 15 Zernike modes. Finite element analysis of the mirror shows a theoretically possible reduction of the wave front error from 2 μm to 53 nm rms. To produce the desired shapes, the mirror surface is controlled by 19 identical actuator modules at the back of the mirror.
The actuator modules use commercially available Piezo-Knob actuators with a high technology readiness level (TRL). These provide nanometer resolution at cryogenic temperatures combined with high positional stability, and allow for the system to be powered off once the desired shape is obtained. The stiff design provides a high resonance frequency (>200 Hz) to suppress external disturbances.
A full-size demonstrator of the deformable mirror containing 6 actuators and 13 dummy actuators is realized and characterized. Measurement results show that the actuators can provide sufficient stroke to correct the 2 μm rms WFE. The resolution of the actuator influence functions is found to be 0.24 nm rms or better depending on the position of the actuator within the grid. Superposition of the actuator influence functions shows that a 2 μm rms WFE can be accurately corrected with a 38 nm fitting error. Due to the manufacturing method of the demonstrator an artificially large print-through error of 182 nm is observed. The main cause of this print-through error has been identified and will be reduced in future design iterations. After these design changes the system is expected to have a total residual error of less than 70 nm and offer diffraction limited performance (λ14) for wavelengths of 1 μm and above.
The use of Immersed Gratings offers advantages for both space- and ground-based spectrographs. As diffraction takes place inside the high-index medium, the optical path difference and angular dispersion are boosted proportionally, thereby allowing a smaller grating area and a smaller spectrometer size. Short-wave infrared (SWIR) spectroscopy is used in space-based monitoring of greenhouse and pollution gases in the Earth atmosphere. On the extremely large telescopes currently under development, mid-infrared high-resolution spectrographs will, among other things, be used to characterize exo-planet atmospheres. At infrared wavelengths, Silicon is transparent. This means that production methods used in the semiconductor industry can be applied to the fabrication of immersed gratings. Using such methods, we have designed and built immersed gratings for both space- and ground-based instruments, examples being the TROPOMI instrument for the European Space Agency Sentinel-5 precursor mission, Sentinel-5 (ESA) and the METIS (Mid-infrared E-ELT Imager and Spectrograph) instrument for the European Extremely Large Telescope. Three key parameters govern the performance of such gratings: The efficiency, the level of scattered light and the wavefront error induced. In this paper we describe how we can optimize these parameters during the design and manufacturing phase. We focus on the tools and methods used to measure the actual performance realized and present the results. In this paper, the bread-board model (BBM) immersed grating developed for the SWIR-1 channel of Sentinel-5 is used to illustrate this process. Stringent requirements were specified for this grating for the three performance criteria. We will show that –with some margin– the performance requirements have all been met.
We present implementations of optical instrumentation that records five dimensions of light: polarization state as a function of wavelength, two spatial dimensions, and time. We focus on the optimal integration of polarimetry within microlens-based integral-field spectroscopy. The polarimetric analyzer (or beam-splitter) and dispersing element could be implemented separately, but also amalgamated in the form of a polarization grating. We present optimizations for stacking the polarization-split spectra on a 2D detector. The polarimetric modulation can be performed in the temporal, the spatial or the spectral domain. Temporal modulation could be set up with achromatic optics conform the Stokes definition scheme, but a wide wavelength range generally demands a “polychromatic” modulation approach for which the modulation efficiency for all or some of the Stokes parameters is optimized at every wavelength. Spectral modulation (full-Stokes or optimized for linear polarization) yields instruments without any moving parts, for which all polarization information is obtained in one shot. We present first results from two polarimetric IFU instruments; the ExPo pIFU and LOUPE. The first is based on a rapid polychromatic modulator consisting of two FLCs and two fixed retarders, while the latter is based on spectral modulation for linear polarization. In addition to applications within astronomy and planetary science, we discuss remote-sensing applications for such instruments.
Well over 700 exoplanets have been detected to date. Only a handful of these have been observed directly. Direct observation is extremely challenging due to the small separation and very large contrast involved. Imaging polarimetry offers a way to decrease the contrast between the unpolarized starlight and the light that has become linearly polarized after scattering by circumstellar material. This material can be the dust and debris found in circumstellar disks, but also the atmosphere or surface of an exoplanet.
We present the design, calibration approach, polarimetric performance and sample observation results of the Extreme Polarimeter, an imaging polarimeter for the study of circumstellar environments in scattered light at visible wavelengths.
The polarimeter uses the beam-exchange technique, in which the two orthogonal polarization states are imaged simultaneously and a polarization modulator is swaps the polarization states of the two beams before the next image is taken. The instrument currently operates without the aid of Adaptive Optics. To reduce the effects of atmospheric seeing on the polarimetry, the images are taken at a frame rate of 35 fps, and large numbers of frames are combined to obtain the polarization images.
Four successful observing runs have been performed using this instrument at the 4.2 m William Herschel Telescope on La Palma, targeting young stars with protoplanetary disks as well as evolved stars surrounded by dusty envelopes. In terms of fractional polarization, the instrument sensitivity is better than 10−4. The contrast achieved between the central star and the circumstellar source is of the order 10−6. We show that our calibration approach yields absolute polarization errors below 1%.
Imaging polarimetry offers a way to increase the contrast of light scattered from circumstellar material, enabling
direct observation of exoplanets –possibly rocky– with the E-ELT. To actually characterize these planets, some
spectral resolution is essential. With sufficient resolution –both spectral and spatial– the spectral differential
imaging technique can be used in addition to the polarimetry to detect circumstellar point sources. We present
the concept for a spectro-polarimetric integral field spectrograph for the EPICS-EPOL instrument and our
current efforts to demonstrate this concept with our existing imaging polarimeter ExPo.
Control software for adaptive optics systems is mostly custom built and very specific in nature. We have developed
FOAM, a modular adaptive optics framework for controlling and simulating adaptive optics systems in various
environments. Portability is provided both for different control hardware and adaptive optics setups. To achieve
this, FOAM is written in C++ and runs on standard CPUs. Furthermore we use standard Unix libraries
and compilation procedures and implemented a hardware abstraction layer in FOAM. We have successfully
implemented FOAM on the adaptive optics system of ExPo - a high-contrast imaging polarimeter developed at
our institute - in the lab and will test it on-sky late June 2012. We also plan to implement FOAM on adaptive
optics systems for microscopy and solar adaptive optics. FOAM is available* under the GNU GPL license and
is free to be used by anyone.
EPOL is the imaging polarimeter part of EPICS (Exoplanet Imaging Camera and Spectrograph) for the 42-m E-ELT. It
is based on sensitive imaging polarimetry to differentiate between linearly polarized light from exoplanets and
unpolarized, scattered starlight and to characterize properties of exoplanet atmospheres and surfaces that cannot be
determined from intensity observations alone. EPOL consists of a coronagraph and a dual-beam polarimeter with a
liquid-crystal retarder to exchange the polarization of the two beams. The polarimetry thereby increases the contrast
between star and exoplanet by 3 to 5 orders of magnitude over what the extreme adaptive optics and the EPOL
coronagraph alone can achieve. EPOL operates between 600 and 900 nm, can select more specific wavelength bands
with filters and aims at having an integral field unit to obtain linearly polarized spectra of known exoplanets. We present
the conceptual design of EPOL along with an analysis of its performance.
Research on extrasolar planets is one of the most rapidly advancing fields of astrophysics. In just over a decade
since the discovery of the first extra-solar planet orbiting around 51 Pegasi, 289 extrasolar planets have been
discovered. This breakthrough is the result of the development of a wide range of new observational techniques
and facilities for the detection and characterisation of extrasolar planets. In Utrecht we are building the Extreme
Polarimeter (ExPo) to image extra-solar planets and circumstellar environments using polarimetry at contrast
ratio of 10-9.
To test and calibrate ExPo, we have built a laboratory-based simulator that mimics a star with a Jupiter-like
exoplanet as seen by the 4.2m William Herschel Telescope. The star and planet are simulated using two single-mode
fibres in close proximity that are fed with a broadband arc lamp with a contrast ratio down to 10-9. The
planet is partially linearly polarized. The telescope is simulated with two lenses, and seeing can be included
with a rotating glass plate covered with hairspray. In this paper we present the scientific requirements and the
The Extreme Polarimeter (ExPo) is approaching its first deployment at the 4.2 m William Herschel Telescope at La
Palma. This imaging polarimeter, developed at the Astronomical Institute of Utrecht University, aims to study
circumstellar material at a contrast ratio with the central star of 10-9. Working at visible wavelengths, it will provide an
inner working angle down to 0.5 arcsec and a field of view of 20 arcsec diameter. ExPo employs a dual beam-exchange
technique based on polarimeter designs for solar studies. A partially transmitting coronagraph mask placed in the first
focus reduces the light of the star. The beam is modulated using three ferro-electric liquid crystals in a Pancharatnam
configuration, then split in a polarizing beamsplitter. Both beams are re-imaged onto the same Electron-Multiplying
We present the design of the ExPo instrument, highlighting the elements that are critical to the polarimetric performance.
Some prototype laboratory experiments demonstrating the instrument concept are discussed. These have been performed
using our realistic exoplanet laboratory simulator.