Other Stuff Lecture 5 Objectives: Lecture 5 At the end of this lecture you should: 1. Be aware of the Zemax capability to approximate a lens design with catalogue components 2. Be familiar with the use of co-ordinate breaks in Zemax to model off-axis systems 3. Understand the use of non-sequential ray-tracing to model scattered light 4. Appreciate the capabilities of Zemax to model physical optics wave propagation 5. Be able to use Zemax to model the performance of imaging systems using realistic images March 17, 2015 Optical Systems Design 2 COTS Lens Substitution • • • • • • • Zemax can take a custom design and substitute real lenses As an example start from paraxial lens model (DOUBLETELECENTRIC-PARAXIAL-LECT5.ZMX) Select Libraries -> Lens Catalogue Use Vendor(s) drop-down menu to search standard manufacturers catalogues Search on lens type, EFL, pupil size Select best match and Insert (delete paraxial surface) (DOUBLE-TELECENTRIC-EDMUNDOPTICS-LECT5.ZMX) May need to reverse some lens elements to improve performance, since convex surface of doublets always optimised for ∞ conjugate (there is a convenient icon above the lens data streadsheet to do this) 17/03/2015 Optical Systems Design 3 Co-ordinate Breaks • Non-axially symmetric systems where surfaces are tilted or decentered require the use of co-ordinate breaks • Rotate/shift local co-ordinate frame • Positive rotation (in ZEMAX) is clockwise as viewed along +ve axis direction • Subsequent co-ordinate breaks refer to the newly defined axis orientations • If a co-ordinate break is placed immediately before an optical surface, it can be useful to put another one with opposite sign immediately after, thus undoing the tilt etc • There are now simple tools in the Lens Data icon bar to tilt/decentre surfaces and add fold mirrors March 17, 2015 Optical Systems Design 4 Nasmyth Field Derotator FIELDROTATOR-LECT5.ZMX March 17, 2015 Optical Systems Design 5 Non-Sequential Systems • No predefined sequence of surfaces • Objects encountered determined solely by physical positions of surfaces and directions of rays • Co-ordinate system is global • Can deal with Total Internal Reflection (TIR), stray light and illumination systems • Required for prisms, beamsplitters, light pipes, faceted (array) objects etc • In some cases need mixed sequential/nonsequential ray tracing March 17, 2015 Optical Systems Design 6 Non-Sequential Systems Sequential Non-Sequential [PRISM-SEQ-LECT5.ZMX] [PRISM-NONSEQ-LECT5.ZMX] March 17, 2015 Optical Systems Design 7 Non-Sequential Systems PENTAPRISM-NONSEQ-LECT5.ZMX Can convert from sequential design using Tools -> Miscellaneous -> Convert to NSC Group (need to first move STOP to front surface) March 17, 2015 Optical Systems Design 8 Physical Optics Propagation • Geometrical ray tracing is an incomplete description of light propagation • POP uses diffraction calculations to propagate a light modelled as a wavefront through an optical system • Wavefront is modeled by an array of complex amplitudes which is user-definable in terms of its dimension, sampling and aspect ratio • Applications include fibre coupling, diffraction by apertures and beam irradiance calculations March 17, 2015 Optical Systems Design 9 Gibbs Phenomenon March 17, 2015 GIBBS-LECT5.ZMX Optical Systems Design 10 Fibre Coupling March 17, 2015 FIBRE-LECT5.ZMX Optical Systems Design 11 Array Elements • Rectangular array of spherical lenses • Modelled as a userdefined surface (DLL) • LENSLET-LECT5.ZMX March 17, 2015 Optical Systems Design 12 Image Simulation • For an optical designer the lens performance is specified in terms of spot diagrams, ray-fan plots, vignetting, field curvature, astigmatism etc • In some cases its much more effective to demonstrate what images will look like when viewed through the lens • Zemax now has a nice feature called Image Simulation to demonstrate this on an input image March 17, 2015 Optical Systems Design 13 Image Simulation • Object scene is represented by a source bitmap (.BMP or .JPG) • Rays traced using the defined object through the lens to the image plane • At detection surface place a pixellated detector which receives the rays and builds up an image of the source bitmap as seen through the lens March 17, 2015 Optical Systems Design 14 Design for Fabrication • Primary considerations: optical material, component size, shape, and manufacturing tolerances • Minimize cost and delivery time by using COTS items whenever possible • Minimize risk through prototyping and pre-production models March 17, 2015 Optical Systems Design 15 Optical Materials • Over 100 optical glasses available worldwide • Each manufacturer has a list of “preferred” glasses that are most frequently melted and usually available from stock • Generally can substitute similar glasses from different manufacturers (and re-optimise) • Material quality defined by tolerances on spectral transmission, index of refraction, dispersion, striae grades (AA/A/B), homogeneity (H1-H4), and birefringence (NSK/NSSK) • Tighter than standard optical tolerances require additional cost and time • May be more economical to add a lens to the design in order to avoid expensive glasses • Some glasses (e.g. SF-59) made much less frequently than others (e.g. BK-7) March 17, 2015 Optical Systems Design 16 Fabrication • Mechanical properties: hardness & abrasion resistance (manufacture) • Chemical properties: resistance to humidity, acids, alkalis • Thermal properties: expansion coefficients from 4 -16 x 10-6/°K. March 17, 2015 Optical Systems Design 17 Some Other Zemax Examples ZMX/SES files on course website: • Cassegrain Telescope (WHT) • Ritchey Chretien Telescope (AAT) • Off-axis parabola • Melles Griot ball lens • Shack-Hartmann wavefront sensor • Palomar triple spectrographs March 17, 2015 Optical Systems Design 18 Summary: Lecture 5 • Co-ordinate breaks allow Zemax to model arbitrarily complex off-axis systems in a local coordinate system • Need care in use to avoid over-complication • Non-sequential mode allows complex objects to be defined using a global co-ordinate system • Can also be used to model scattered light and illumination systems • Physical optics propagation in Zemax includes the effects of diffraction • Fabrication issues need to be thought about early in the instrument design phase March 17, 2015 Optical Systems Design 19 Exercises: Lecture 5 • Work your way through some of the example Zemax files, evaluating their performance and making sure that you understand the prescription data. March 17, 2015 Optical Systems Design 20 Homework Problem • Design a very simple telephoto lens with the following first-order properties: Effec&ve focal length = 120 mm Back focal distance = 50 mm Image space f/# = 10 Lens separa&on = 40 mm Field angles = 0°, 2°, 3° Wavelength = 0.587 µm • Design goal: maintain all first-order properties and achieve rms spot sizes ≤ 20 µm. Start from two paraxial lenses with focal lengths 75mm and -75mm. • Final solutions should include a layout diagram, spot diagram and system prescription data (also email the Zemax file). • Hand in the solutions to my pigeon hole by Friday 17th April. 17/03/2015 Optical Systems Design 21
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