UK OPTICAL DESIGN MEETING
Thursday 15th September 2016
Rutherford Appleton Laboratory
Harwell, Oxfordshire
Abstracts
The Giant Magellan Telescope: A 21st Century Mega-Project and
its integrated Optical Systems
Andrew Rakich
Giant Magellan Telescope Organization (GMTO) 265 N. Halstead St., Pasadena, CA 91107, USA
The Giant Magellan Telescope is currently on track to be the first of the new generation of Extremely Large Telescopes to see starlight. The telescope is an aplanatic Gregorian of 24.5 m aperture. The primary mirror consists of seven 8.4 m diameter mirror segments; six arranged in a symmetrical pattern about a central segment. Each primary mirror segment will illuminate a conjugated secondary mirror segment of 1 m diameter. With this massive aperture, 368 square meters of light collecting area will concentrate starlight to a diffraction disk of ~ 1/10th of the angular diameter delivered by the Hubble Space Telescope. In other words, with the GMT, nearly 100 times the mirror surface area will deliver light to a diffraction disc subtending 100th of the solid angle of that of Hubble.
In this paper I will be discussing the optical technologies that together play a critical role in realizing the GMT. The main optical train of primary and secondary mirrors will be discussed in detail. The task of maintaining the extraordinarily tight tolerances required for the seven “unit telescopes” to operate as a phased array will be considered, along with the various tools such as ADM and DMI interferometric trusses, phasing cameras, wavefront sensors, guide cameras etc. that will be employed to achieve and maintain co-alignment and phasing.
In summary, this paper will not only provide an interesting and informative overview of the various optical components and systems that go into the GMT, but will convey the sense of the excitement the GMT team experience, and the technical progress they are delivering, on the path to producing a world-beating optical telescope.
Effect on optical properties of coatings and glass exposed to space environment. Modelling of optical properties behaviour
M. Fernández-Rodríguez, C. Pastor, C. G. Alvarado, A. Núñez,
A. Álvarez-Herrero and L. M. GonzálezLaboratorio de Instrumentación Espacial, LINES, Instituto Nacional de Técnica Aeroespacial, INTA, 28850 Torrejón de Ardoz, Madrid, Spain
Hostile conditions of the space environment affect the optical performance of instruments on board of satellites. Radiation, vacuum, high temperature differences, mechanical loads and atomic oxygen corrosion are the main factors responsible for the loss in the optical performance of the space instrumentation. Particularly, those environmental conditions can induce changes of glass and coatings properties such as refractive index, density, spectral transmission and structural variations.
In this study, the optical properties of different types of glass and coatings commonly used in optical instrumentation have been analyzed after they have been exposed to different environmental space scenarios using non-invasive and non-destructive techniques such as Spectrophotometry and Spectroscopic Ellipsometry, among others. Space radiation environment has been identified as the main contributor to the optical materials degradation, inducing the formation of colour centres. This damage may produce alterations on the material optical properties, effects that can be continual or transitory.
The interest of this work lies in both improving the knowledge of the evolution of the optical properties of materials commonly used in space instrumentation when they are exposed to the space environment, and developing theoretical models that allow to predict the materials degradation before the optical instrumentation is launched. A data base is developing to provide the optical designer with the needed information about the optical properties behavior, helping them to select the most suitable material to be used in the optical design of a space instrument.
Characterization of diffraction gratings scattering in UV and IR for space applications
Yan Cornil, Quentin Kuperman-Le Bihan
Light Tec, Pôle d’activité hyérois
1128 Route de Toulon Hyères FRANCE
The use of Bidirectional Scatter Distribution Function (BSDF) in space industry and especially when designing telescopes is a key feature. Indeed when speaking about space industry, one can immediately think about stray light issues. Those important phenomena are directly linked to light scattering. Standard BSDF measurement goniophotometers often have a resolution of about 0.1° and are mainly working in or close to the visible spectrum. This resolution is far too loose to characterize ultra-polished surfaces. Besides, wavelength range of BSDF measurements for space projects needs to be done far from visible range.
How can we measure BSDF of ultra-polished surfaces and diffraction gratings in the UV and IR range with high resolution? We worked on developing a new goniophometer bench in order to be able to characterize scattering of ultra-polished surfaces and diffraction gratings used in everyday space applications. This ten meters long bench was developed using a collimated beam approach as opposed to goniophotometer using focused beam. Sources used for IR characterization were CO2 (10.6µm) and Helium Neon (3.39µm) lasers. Regarding UV sources, a collimated and spatially filtered UV LED was used. The detection was ensure by a photomultiplier coupled with synchronous detection as well as a MCT InSb detector. The so-built BSDF measurement instrument allowed us to measure BSDF of ultra-polished surfaces as well as diffraction gratings with an angular resolution of 0.02° and a dynamic of 1013 in the visible range.
In IR as well as in UV we manage to get 109 with same angular resolution of 0.02°. The 1m arm and translation stages allows us to measure samples up to 200mm. Thanks to such a device allowing ultra-polished materials as well as diffraction gratings scattering characterization, it is possible to implement those BSDF measurements into simulation software and predict stray light issues. This is a big help for space industry engineers to apprehend stray light due to surface finishes and to delete those effects before the whole project is done.
We are now thinking of possible improvement on our optical bench to try to get dynamic in IR and UV similar to what we have in visible range (e.g. 1013).
Keywords: scattering, BSDF, stray light, optical design, manufacturing, engineering, ultra-polished, diffraction gratings.
Modelling of diffracted stray light in solar coronagraphs and heliospheric imagers
Kevin Middleton, Jackie Davies, Ian Tosh, James Tappin
Optical Systems Group, RAL Space, STFC Rutherford Appleton Laboratory
Coronagraphs – imagers which view the solar corona – are key to understanding the impact of the solar wind, and in particular coronal mass ejections (CMEs), on human infrastructure – a field commonly known as space weather. To detect CMEs requires blocking light from the solar disc so that the much fainter corona is visible. This is usually accomplished by a series of occulting discs. The instrument’s stray light rejection is ultimately limited by light diffracted at the edges of these occulters, and accurately predicting diffraction effects is key to understanding the limits of achievable performance. We discuss different approaches to modelling diffracted light in coronagraphs and, in particular, the trade between computational time and fidelity of the results.
The challenge of optical filter glass production and the trend to miniaturization
Ralf Biertümpfel
Optical Filter Advanced Optics, Schott Advanced Optics, Germany
Optical glasses and optical filter glasses have a well defined quality. The description "optical quality" means that several properties like refractive index, absorption and transmittance are controlled within their specified tolerances. In order to obtain world class optical quality the glasses are made in noble metal aggregates that ensure a melting process without any contamination. Due to the high viscosity of the glass, there are technical limits of the melting furnace towards miniaturization. That means, that optical glass quality requires a minimum size of the melt. Depending on the glass type this varies from 300 kg up to 10 t.
In a world of decreasing demands for optical glasses, e.g. production of digital still cameras dropped from 121 million in 2009 to 35 million in 2015, the economic production of the glasses becomes more and more challenging. The importance of world wide logistics and forecasting is increasing. Thus, availability of special glasses have to be considered during the design of optical components and a delivery schedule should be negotiated as early in the design phase as possible.
Nodal Aberration Theory: The Legacy of Kevin Thompson and Roland Shack
John R Rogers
Synopsys Inc, USA
Nodal Aberration Theory was developed by Dr. Kevin Thompson and his advisor, Dr. Roland Shack, as Kevin’s doctoral dissertation. In this paper, we outline the fundamental basis of the theory as well as more recent refinements. We discuss several important developments and recognitions that were made possible by Nodal Aberration Theory.
The evolution of infrared countermeasures systems is underpinned by optical engineering and manufacturing innovation
Graham Jeffrey
Finmeccanica Airborne and Space Systems DivisionTransmission is a key parameter in describing an IR-lens, but is also often the subject of controversy. One reason is the misinterpretation of “transmission” in infrared camera practice. If the camera lens is replaced by an alternative one the signal will be affected by two parameters: proportional to the square of the effective aperture based F-number and linearly to the transmission. The measure to collect energy is defined as the Energy Throughput ETP, and the signal level of the IR-camera is proportional to ETP. Most published lens transmission values are based on spectrophotometric measurement of plane-parallel witness pieces obtained from coating processes. Published aperture based F-numbers derive very often from ray tracing values in the on-axis bundle.
Directed Infrared Countermeasures (DIRCM) Systems are designed to protect aircraft from the threat of infrared guided missiles by disrupting the guidance system of the threat and ensuring that they miss the target. Their successful operation relies upon a sophisticated electro-optical system incorporating sensors, laser sources and beam steering optics, which is designed to operate in the harshest of airborne environments. This talk discusses the evolution of the technology since the early 1990s and presents the optical design and manufacturing developments which underpin the latest 4th generation product currently in the market.
Multichannel optics for imaging and nonimaging applications
Pablo Benítez, Juan C. Miñano, Milena Nikolic (presenter)
Universidad Politécnica de Madrid Cedint, Campus de Montegancedo UPM
28223 Madrid, Spain
Compacting devices is an increasingly demanding requirement for many applications in nonimaging and imaging optics. “Compacting” means here decreasing the volume of the space between the entry and the exit aperture without decreasing the optical performance. After reviewing the different techniques to do so, we analyze here the multichannel strategies. These type of designs split the incoming bundle of rays in different sub-bundles that are optically processed (independently) and then recombined in a single outgoing bundle. The optics volume decreases rapidly with the number of sub-bundles. These designs usually need to be combined with freeform optics for optimum performance.
Abuse of Zemax: Monte Carlo modelling of light propagation in biological tissue
and more
Andy Harvey, Guillem Carles, Paul Zammit, Laurence Brewer and Beatriz Grafulla
School of Physics and Astronomy, University of Glasgow, UKRay-tracing programmes such as Zemax embody the fundamental laws of optics governing reflection, refraction and scattering and are immensely powerful for optimisation of complex optical systems. We report here on the incorporation of a rigorous, polarization-tracking, Monte-Carlo model for light propagation in turbid media into Zemax that enables for the first time, the modelling of light propagation from the source, through an optical instrument, through biological tissue to the detector. As an example, a complete model of confocal fluorescence microscopy can now incorporate rigorous models of microscope lenses or mirrors, stops, laser-beam aberrations, stray light, imperfect pinholes as well as complex volumetric representations of biological structures such as neurons and biological tissue of arbitrary shape, represented as conventional CAD (computer-aided design) models and fluorescence characteristics of fluorophors. An important and practical advantage of this approach is that the user-friendly, graphical user interface of Zemax can be used by non specialists to model the optical performance of complex optical-biological systems. Our new holistic Monte-Carlo modelling tool is underpinned by the recognition that the physical principles of optical ray tracing and Monte-Carlo modelling of light transport are very closely related: both propagate optical rays between interactions with media, but whereas in ray tracing the path length and change in direction is deterministic (determined by the application of Snell's law and the Fresnel equations to reflection and refraction at interfaces), the path length and change in direction in Monte Carlo models is determined statistically to reflect empirical or physical models. While the vocabulary of ray tracing involves intensities or rays, the Monte Carlo literature describes weighted amplitudes of photon packets, but the concepts are essentially equivalent. In commercial ray tracing programmes this distinction may be further blurred by the incorporation of statistical scattering models into bulk scattering, although the physical scattering models employed may be uncertain. Additional examples will include the physical-optics modelling of mm-wave imaging systems and coherence phenomena (speckle).
The TRUTHS imaging spectrometer and absolute calibration system
Dan Lobb
Surrey Sattelite Ltd, UKThe TRUTHS mission aims to measure Earth radiance from a satellite in low-Earth orbit, in a spectral band from 300nm to 2400nm, at resolution < 6nm. It will cover a 50km swath at 50m ground sample distance. The key performance target is absolute accuracy better than 0.3%: an order of magnitude better than is currently achieved. Critically, this accuracy is needed to provide evidence of changes in Earth radiance within a relatively brief time scale (e.g. a decade), using repeat launches. The satellite will also improve accuracy of other space-based radiometers by cross-calibration, and of course provide useful hyperspectral data for multiple other purposes. The Earth imager is a dispersive spectrometer, using curved prisms to cover the whole spectral band. The calibration system includes a cryogenic radiometer which measures output power from multiple solid state lasers sampling the 300nm-2400nm spectral range. The laser are in turn used to calibrate a transfer radiometer that measures the absolute radiance of a laser-lit diffuser deployed in front at the Earth imager aperture. The cryogenic radiometer is also used, with the spectrometer, to measure absolute spectral irradiance of the sun.
Poster Abstracts
Anastigmatic ophthalmic lens design
Milena Nikolic
Advanced Optics CeDInt-UPM, Madrid
A new approach for ophthalmic lens design with two aspheric surfaces free of astigmatism is presented. We design a -5D anastigmatic monofocal lens for the gaze directions of up to 30º . The solution is obtained solving a set of implicit differential equations derived from generalized ray tracing.
Two tailored freeform surfaces for specialty imaging applications
Yunfeng Nie, Hugo Thienpont, and Fabian Duerr
Brussels Photonics Team, Department of Applied Physics and Photonics
Vrije Universiteit Brussel
When an optical system is to be designed, the typical method is to start from certain aberration theory to obtain an initial approximate solution. Another typical approach is to use a related already existing optical system design as starting point. However, both approaches are not feasible for solving an inverse problem, where the targets are determined and optical components are assumed to be a “black box” with no priori descriptions. Freeform optics can be seen as an analogy to such a “black box” whilst any shape is possible from the perspective of theoretical solutions. In this work, we propose a “multi-fields” direct design method that is based on Fermat’s principle, where the Optical Path Lengths (OPLs) off different fields to their respective ideal image points are constant.
Some current direct design methods for freeform optics, for example the Simultaneous Multiple Surfaces (SMS) design method and a related analytic design method, allow to perfectly focus two or three ray bundles with two surfaces. However, as for imaging systems they haven’t been fully developed so far: firstly, they do require optical path lengths and corresponding image points that are unknown and need to be optimized. Secondly, their imaging performance is unbalanced due to the perfect imaging of only very few fields, especially unsuitable for wide field of view imaging systems. Last but not the least, these direct design methods do not incorporate a pupil which is of great importance to balance the overall performance for an imaging system.
Keeping these problems in mind, the proposed multi-fields direct design method enables partial coupling of multiple fields with two tailored freeform surfaces to balance the performance over the full field of view. Including an entrance pupil in the design process is crucial to gain control of multiple fields with only two optical surfaces. As a test example, a barcode scanner with only a single freeform lens has been designed with this method, achieving a similar performance in comparison with a patented system consisting of two lenses (one spherical lens and one aspherical lens) in terms of aberration plots.
Given the fact that none of the state-of-the-art optical direct design methods for freeform surfaces is capable of directly calculating more than two surfaces, we have developed a hybrid approach that allows to combine the multi-fields direct design method with classic optical design strategies. For example, adding a negative front lens helps to reduce aberrations from wide angle incident rays, and using different optical materials is important to correct chromatic aberrations. In this way, we were able to design a wide field of view objective with four optical surfaces, showing a better result when compared with its spherical four lenses (eight surfaces) counterpart from the perspective of RMS spot diagram, field curvature, distortion and MTF plots.
We also extended the multi-fields design method to calculate two off-axis mirror profiles, which are added as accessory optical components to a commercial BENQ projector, tailoring the projection distance from 2m to 48cm for a 78.3 inches diagonal screen. This greatly reduced throw ratio is very beneficial for narrow space projection. At the same time, a good image quality as well as a low distortion has been achieved. The image quality can be further improved by re-fitting the mirrors with freeform surfaces, outperforming all the rotationally symmetric accessory optical systems in literature.
Irradiance Tailoring for Extended Sources in 3D by
Implicit Integral Equation Solution
Adam Hirst
OSRAM & CeDInt-UPM, Madrid
A new method for solving the problem of irradiance tailoring in 3D for extended sources is introduced, in which the integral expression for the flux at each of a set of target points for a given Lambertian source is approximated and expressed in terms of parameters defining a freeform surface, and the resulting set of equations solved through said parameters using standard numerical methods to yield the freeform surface satisfying a desired irradiance prescription. The method has previously been shown to work at least in principle for extremely restricted cases, but current work extends the work to apply to more general and more useful set of cases, namely by utilising a B-Spline surface along with the means to counter the virtual edge(s) of a projected source, and some solutions are presented.