Gabby Kroes

Design and development of a freeform active mirror for an astronomy application

Abstract. The advent of extremely large telescopes will bring unprecedented light-collecting power and spatial resolution, but it will also lead to a significant increase in the size and complexity of focal-plane instruments. The use of freeform mirrors could drastically reduce the number of components in optical systems. Currently, manufacturing issues limit the common use of freeform mirrors at short wavelengths. This article outlines the use of freeform mirrors in astronomical instruments with a description of two efficient freeform optical systems. A new manufacturing method is presented which seeks to overcome the manufacturing issues through hydroforming of thin polished substrates. A specific design of an active array is detailed, which will compensate for residual manufacturing errors, thermoelastic deformation, and gravity-induced errors during observations. The combined hydroformed mirror and the active array comprise the Freeform Active Mirror Experiment, which will produce an accurate, compact, and stable freeform optics dedicated to visible and near-infrared observations.

Final design and progress of WEAVE: the next generation wide-field spectroscopy facility for the William Herschel Telescope

We present the Final Design of the WEAVE next-generation spectroscopy facility for the William Herschel Telescope (WHT), together with a status update on the details of manufacturing, integration and the overall project schedule now that all the major fabrication contracts are in place. We also present a summary of the current planning behind the 5-year initial phase of survey operations. WEAVE will provide optical ground-based follow up of ground-based (LOFAR) and space-based (Gaia) surveys. WEAVE is a multi-object and multi-IFU facility utilizing a new 2-degree prime focus field of view at the WHT, with a buffered pick-and-place positioner system hosting 1000 multi-object (MOS) fibres, 20 integral field units, or a single large IFU for each observation. The fibres are fed to a single (dual-beam) spectrograph, with total of 16k spectral pixels, located within the WHT GHRIL enclosure on the telescope Nasmyth platform, supporting observations at R~5000 over the full 370-1000nm wavelength range in a single exposure, or a high resolution mode with limited coverage in each arm at R~20000. The project is now in the manufacturing and integration phase with first light expected for early of 2018.

Cryomechanisms for positioning the optical components of the mid-infrared instrument (MIRI) for NGST

Mechanisms operating in the cryovacuum are required to rotate filter and dichroic wheels, to tilt gratings and to flip in the beam of an internal calibration source. The design proposed here is based on similar mechanisms flown successfully on the liquid helium cooled European ISO-satellite and being presently under qualification for ESAs cooled HERSCHEL-satellite. Their main characteristics are high reliability during the 10 year lifetime in space, high precision and low heat dissipation in the cryovacuum.

Freeform mirror based optical systems for FAME

In this paper we present the design of freeform mirror based optical systems that have the potential to be used in future astronomical instrumentation in the era of extremely large ground based telescopes. Firstly we describe the optical requirements followed by a summary of the optimization methodology used to design the freeform surface. The intention is to create optical architectures, which not only have the numerous advantages of freeform based systems (increased optical performance and/or reduction of mass and volume), but also can be manufactured and tested with today’s manufacturing techniques and technologies. The team plans to build a demonstrator based on one of the optical design examples presented in this paper. The demonstrator will be built and tested as part of the OPTICON FP7 Freeform Active Mirror Experiment (FAME) project. A hydroforming technique developed as part of the previous OPTICON FP7 project will be used to produce an accurate, compact and stable freeform mirror. The manufacturing issues normally experienced in the production of freeform mirrors are solved through the hydroforming of thin polished substrates, which then will be supported with an active array structure. The active array will be used to compensate for residual manufacturing errors, thermo-elastic deformation and gravity-induced errors.

A new generation active arrays for optical flexibility in astronomical instrumentation

Throughout the history of telescopes and astronomical instrumentation, new ways were found to open up unexplored possibilities in fundamental astronomical research by increasing the telescope size and instrumentation complexity. The ever demanding requirements on instrument performance pushes instrument complexity to the edge. In order to take the next leap forward in instrument development the optical design freedom needs to be increased drastically. The use of more complex and more accurate optics allows for shorter optical trains with smaller sizes, smaller number of components and reduced fabrication and alignment verification time and costs. Current optics fabrication is limited in surface form complexity and/or accuracy. Traditional active and adaptive optics lack the needed intrinsic long term stability and simplicity in design, manufacturing, verification and control. This paper explains how and why active arrays literally provide a flexible but stable basis for the next generation optical instruments. Combing active arrays with optically high quality face sheets more complex and accurate optical surface forms can be provided including extreme a-spherical (freeform) surfaces and thus allow for optical train optimization and even instrument reconfiguration. A zero based design strategy is adopted for the development of the active arrays addressing fundamental issues in opto-mechanical engineering. The various choices are investigated by prototypes and Finite Element Analysis. Finally an engineering concept will be presented following a highly stable adjustment strategy allowing simple verification and control. The Optimization metrology is described in an additional paper for this conference by T. Agócs et al.

MATISSE cold optics opto-mechanical design

MATISSE is a mid-infrared spectro-interferometer combining beams of up to four telescopes of the ESO VLTI providing phase closure and image reconstruction using interferometric spectra in the LM and N band. This paper presents the opto-mechanical design of the two cold benches containing several types of cryogenic mechanisms (shutter, Tip/Tilt) used for cryogenic alignment. Key aspects are detailed such as the highly integrated opto-mechanical approach of the design in order to guarantee component stability and accuracy specifications in the order of nanometers and arcseconds.

Optimizing optical systems with active components

The increasing requirement on the performance of optical instruments leads to more complex optical systems including active optical components. The role of these components is to correct for environmental influences on the instrument and reduce manufacturing and alignment residuals. We describe a method that can be used to design and operate instruments with active components that are not necessarily located in the pupil. After the optical system is designed, the next step is to analyse the available degrees of freedom (DOF), select the best set and include them in the active component. By performing singular value decomposition (SVD) and regularization of the sensitivity matrix, the most efficient DOF for the active component can be calculated. In operation of the instrument, the wavefront at the pupil plane is reconstructed from phase diversity (PD); a metrology having minimal impact on instrument design. Information from SVD, forward and reverse optimization are used to model the process, explore the parameter space and acquire knowledge on convergence. The results are presented for a specific problem.

Perspective of imaging in the mid-infrared at the Very Large Telescope Interferometer

MATISSE is a mid-infrared spectro-interferometer combining the beams of up to four Unit Telescopes or Auxiliary Telescopes of the Very Large Telescope Interferometer (VLTI) of the European Southern Observatory. MATISSE will constitute an evolution of the two-beam interferometric instrument MIDI. New characteristics present in MATISSE will give access to the mapping and the distribution of the material, the gas and essentially the dust, in the circumstellar environments by using the mid-infrared band coverage extended to L, M and N spectral bands. The four beam combination of MATISSE provides an efficient uv-coverage: 6 visibility points are measured in one set and 4 closure phase relations which can provide aperture synthesis images in the mid-infrared spectral regime. We give an overview of the instrument including the expected performances and a view of the Science Case. We present how the instrument would be operated. The project involves the collaborations of several agencies and institutes: the Observatoire de la Côte d’Azur of Nice and the INSU-CNRS in Paris, the Max Planck Institut für Astronomie of Heidelberg; the University of Leiden and the NOVA-ASTRON Institute of Dwingeloo, the Max Planck Institut für Radioastronomie of Bonn, the Institut für Theoretische Physik und Astrophysik of Kiel, the Vienna University and the Konkoly Observatory.

Optimizing an active extreme asphere based optical system

Methods are presented that can be used to design and operate optical systems with actively controlled components. Optical systems based on extreme aspheres and freeform surfaces have been investigated. Existing three mirror anastigmat (TMA) designs have been re-optimized in order to achieve two spherical and one challenging (extreme asphere or freeform) mirror surface. We foresee a manufacturing method, where the mirror substrate is plasticised by cold hydro-forming and its surface shape can be controlled via actuators to remove residual errors. Based on singular value decomposition (SVD) and regularization of the sensitivity matrix, the degrees of freedom (DOF) of the active surface can be analysed. Phase diversity (PD) is used as a wavefront retrieval process, to measure the performance metric and determine the sensitivity matrix thus correlating the performance metric of the system and the DOF of the active component.

Piezo-driven adjustment of a cryogenic detector

This paper presents the specifications, design, construction and evaluation of a piezo-driven tip/tilt/focus mechanism which can align a detector or any other optical component in a cryogenic environment. Even with a no-adjustment design philosophy, usually one or two components have to be adjusted in order to compensate for the total of optical and mechanical tolerances in an optical cryogenic instrument. Normally these adjustments are made by means of shims or stiff screw mechanisms and are applied at room temperature. In order to adjust the particular component(s), mostly by just a few microns, the high-risk and time-consuming operation of opening a cryostat is required. For a large cryostat the typical cycle of cooling, testing, warm-up, opening, adjustment, closing and cooling again, takes roughly two weeks. Often the cycle needs to be repeated a few times before the required position is obtained. ASTRON developed a piezo driven tip/tilt/focus mechanism which can adjust a detector or any other optical component in both the ambient and cryogenic (<100 K, vacuum) environment. Only during adjustment the system is active, for the rest of time it is a passive robust system with a high stability. The main specifications are a stroke of ± 0,6 mm and tip/tilt of ±1,2 mrad.