Sunday, December 11, 2011

Understanding Your Interferometric Test Results by James Mulherin

At Optical Mechanics our mission is to provide professional quality optics to the research and amateur astronomy community. To that end, we produce our OMI Newtonian mirrors adhering to a rigorous quality control process. The result is consistently high optical quality. Each OMI mirror is delivered with an interferometric certification that conforms to industry standards for research-grade astronomical optics. With this technical article we hope you will gain a better understanding of the critical interferometric certification process, what your interferometric test results mean and why interferometry is so important to ensuring that your optic will perform to your expectations. Each OMI mirror must meet or exceed specific wavefront quality specifications before it leaves our facility. These specifications are presented in the form of Peak-to-Valley (P-V), Root-Mean-Square (RMS) wavefront error, and Strehl ratio. The following paragraphs describe how these quantities are measured and what they mean. Below we describe the high-points of the iterative figuring and testing process whereby your mirror surface is polished from a sphere to a paraboloid. During this figuring process we use common phase modulation tests such as the Ronchi and Foucault tests. Then we explain why the final interferometric certification is so important to assuring that the mirror actually meets our pass criterion for wavefront quality. As a point of interest, we describe the string test; a basic technique that is used to make a qualitative assessment of your interferogram, and we present some sample interferograms that demonstrate the appearance of the third order Seidel aberrations; spherical aberration, astigmatism and coma. Finally, we visually inspect our OMI mirrors for scratches and pits during each step of the fabrication process to ensure that the mirror's surface meets well defined cosmetic quality standards. We describe this surface quality standard and what it means. Interferometric Wavefront Quality Specifications In an interferometric test, the shape of the wavefront produced by the optic under test is determined by combining its wavefront with a highly accurate reference wavefront. Constructive and destructive interference between the combined wavefronts produces interference fringes. These interference fringes are analogous to contour lines on an elevation map and they represent deviations of the wavefront under test from the optimal shape. The interference fringes are captured using a CCD camera and image capture board and displayed on a computer monitor. Fringe analysis software then picks hundreds of points along the fringes over the entire wavefront to accurately quantify the deviations. The output of the fringe analysis software describes the quality of the optic under test by reporting its P-V, RMS wavefront error, and Strehl ratio. The Peak-to-Valley wavefront error is a measure of the distance from the highest to the lowest point on the test wavefront relative to the reference wavefront. According to Optical Shop Testing by Daniel Malacara; "The P-V error must be regarded with some skepticism because it is calculated from the worst two interferometric data points out of possible thousands. It might make the system under test appear worse than it really is." Note that a mirror with a true P-V wavefront error of .25 wave, as verified by interferometry, will in any case meet or exceed the RMS wavefront and Strehl ratio criterion described below. Because P-V uses two points without regard to location on the mirrors surface, two mirrors may have the same P-V error but will be very different in overall quality. The difference in quality will be evident in the RMS and Strehl values. As an example, two mirrors may have the same P-V value due to a zone on the mirrors surface. Both of the mirrors have a high zone. The first mirror has its high zone near the center while the second has its high zone near the edge. The first mirror will have better RMS and Strehl values because the surface area covered by the high zone will be smaller in the center than at the edge. To obtain the RMS wavefront error, a large number of interferometric data points are measured over the entire area of the test wavefront. As explained in Optical Shop Testing; "The RMS error is a statistic that is calculated from all of the measured data and gives a better indication of the overall system performance." Due to its statistical nature, professional optical shops consider the RMS wavefront error to be the most useful measure of optical quality. By common convention an optic with RMS wavefront error of 0.0712 wave or less is considered diffraction limited. The Strehl ratio is another statistical measure of optical performance calculated from the interferometric test data. The Strehl ratio is the ratio of intensity of an aberrated wavefront to an unaberrated wavefront. In other words, the use of the Strehl ratio is a fundamental description of the amount of intensity reduction due to wavefront errors. A common convention is to consider an optic with a Strehl ratio of 0.8 or higher to be diffraction limited. The Strehl ratio and RMS wavefront error are mathematically related. It can be shown that a Strehl ratio of 0.8 corresponds to an RMS wavefront error of 0.0714 wave or approximately 1/14 wave. Why Interferometry is Important During the figuring process, the surface of your mirror is polished from a sphere to a paraboloid using an iterative process of testing and polishing to achieve the desired results. During this process we interpret the appearance of fringes in the Ronchi test and shadows in the Foucault test. Once the optician feels that the optic will pass the scrutiny of the interferometer, this test is performed. Due to the qualitative nature of the Ronchi and Foucault tests, and to the subjectivity in their interpretation, it is not infrequent that the interferometer will reject a mirror, sending it back for touch-up polishing. The interferometer is the final impartial go/no-go point in the quality control chain. It is a completely objective and accurate assessment of the quality of the optic under test. Although it is possible to produce diffraction limited mirrors using methods such as the Ronchi and Foucault test, it is impossible to accurately verify and the quality of the entire wavefront to a fraction of a wave without interferometry. The Ronchi and Foucault tests are excellent evaluation tools during mirror fabrication as they show the general shape of the wavefront as well as localized and high frequency errors extremely well. These tests can show localized errors as small as 1/100 wave. This makes them indispensable tools during mirror figuring. However, unlike interferometry, they do not provide a means of accurately quantifying the whole wavefront because they only measure a few points. Interferometry on the other hand, is an extremely strict statistical analysis that assures the customer of a truly diffraction limited optic over its entire wavefront.

Saturday, December 10, 2011

Update for IEC Standards Collection-2010-10(2)

http://www.findstandards.info http://www.int-bizdirectory.com/ http://www.int-bizdirectory.com/standards.htm Quote: Hench2004@sina.com ISO/IEC 19775-2 Ed. 2.0:2010 Information technology — Computer graphics and image processing — Extensible 3D (X3D) — Part 2: Scene access interface (SAI) IEC 62376 Ed. 1.0:2010 Maritime navigation and radiocommunication equipment and systems – Electronic chart system (ECS) – Operational and performance requirements, methods of testing and required test results IEC 60745-2-14 Consol. Ed. 2.2 (incl. am1+am2):2010 Hand-held motor-operated electric tools – Safety – Part 2-14: Particular requirements for planers IEC 60695-6-1 Consol. Ed. 2.1 (incl. am1):2010 Fire hazard testing – Part 6-1: Smoke obscuration – General guidance IEC 60335-2-25 Ed. 6.0:2010 Household and similar electrical appliances – Safety – Part 2-25: Particular requirements for microwave ovens, including combination microwave ovens Project IEC 62660-1 Ed. 1.0:2010 Secondary lithium-ion cells for the propulsion of electric road vehicles – Part 1: Performance testing Project IEC 62150-2 Ed. 2.0:2010 Fibre optic active components and devices – Test and measurement procedures –Part 2: ATM-PON transceivers Project IEC 62026-7 Ed. 1.0:2010 Low-voltage switchgear and controlgear – Controller-device interfaces (CDIs) – Part 7: CompoNet Project IEC 60252-2 Ed. 2.0:2010 AC motor capacitors – Part 2: Motor start capacitors Project IEC 61753-086-6 Ed. 1.0:2010 Fibre optic interconnecting devices and passive components – Performance standard – Part 086-6: Non-connectorised single-mode bidirectional 1 490 / 1 550 nm downstream and 1 310 nm upstream WWDM devices for category O – Uncontrolled environment Project IEC 61300-3-22 Ed. 2.0:2010 Fibre optic interconnecting devices and passive components – Basic test and measurement procedures – Part 3-22: Examinations and measurements – Ferrule compression force Project IEC 61300-2-6 Ed. 2.0:2010 Fibre optic interconnecting devices and passive components – Basic test and measurement procedures – Part 2-6: Tests – Tensile strength of coupling mechanism Project IEC 60684-3-283 Ed. 1.0:2010 Flexible Insulating Sleeving – Part 3: Specifications for individual types of sleeving – Sheet 283: Heat-shrinkable, polyolefin sleeving for bus-bar insulation Project IEC 62660-2 Ed. 1.0:2010 Secondary lithium-ion cells for the propulsion of electric road vehicles – Part 2: Reliability and abuse testing Project IEC 61753-087-2 Ed. 1.0:2010 Fibre optic interconnecting devices and passive components – Performance standard – Part 087-2: Non-connectorized single-mode bidirectional 1 310 nm upstream and 1 490 nm downstream WWDM devices for category C – Controlled environment ISO/IEC/TR 29125 Ed. 1.0:2010 Information technology – Telecommunications cabling requirements for remote powering of terminal equipment IEC 60825-2-am2 Ed. 3.0:2010 Amendment 2 – Safety of laser products – Part 2: Safety of optical fibre communication systems (OFCS) IEC 60601-2-52 Ed. 1.0:2010 Corrigendum 1 – Medical electrical equipment – Part 2-52: Particular requirements for the basic safety and essential performance of medical beds IEC 60312-2 Ed. 1.0:2010 Vacuum cleaners for household use – Part 2: Wet cleaning appliances – Methods of measuring the performance IEC 60312-1 Ed. 1.0:2010 Vacuum cleaners for household use – Part 1: Dry vacuum cleaners – Methods for measuring the performance IEC/TR 60269-5 Ed. 1.0:2010 Low-voltage fuses – Part 5: Guidance for the application of low-voltage fuses Project IEC 61853-1 Ed. 1.0:2010 Photovoltaic (PV) module performance testing and energy rating – Part 1: Irradiance and temperature performance measurements and power rating Project IEC 60747-7 Ed. 3.0:2010 Semiconductor devices – Discrete devices – Part 7: Bipolar transistors Project IEC 60747-15 Ed. 2.0:2010 Semiconductor devices – Discrete devices – Part 15: Isolated power semiconductor Project IEC 60601-2-46 Ed. 2.0:2010 Medical electrical equipment – Part 2-46: Particular requirements for the basic safety and essential performance of operating tables Project IEC 60502-4 Ed. 3.0:2010:2010 Power cables with extruded insulation and their accessories for rated voltages from 1 kV (Um = 1,2 kV) up to 30 kV (Um = 36 kV) – Part 4: Test requirements on accessories for cables with rated voltages from 6 kV (Um = 7,2 kV) up to 30 kV (Um = 36 kV) Project IEC 62148-2 Ed. 2.0:2010 Fibre optic active components and devices – Package and interface standards – Part 2: SFF 10-pin transceivers ISO/IEC 27001-HBK Ed. 1.0:2010 ISO/IEC 27001 for Small Businesses – Practical advice IEC 62475 Ed. 1.0:2010 High-current test techniques – Definitions and requirements for test currents Project IEC 61853-1 Ed. 1.0:2010 Photovoltaic (PV) module performance testing and energy rating – Part 1: Irradiance and temperature performance measurements and power rating Project IEC 60747-7 Ed. 3.0:2010 Semiconductor devices – Discrete devices – Part 7: Bipolar transistors Project IEC 60747-15 Ed. 2.0:2010 Semiconductor devices – Discrete devices – Part 15: Isolated power semiconductor Project IEC 60601-2-46 Ed. 2.0:2010 Medical electrical equipment – Part 2-46: Particular requirements for the basic safety and essential performance of operating tables Project IEC 60502-4 Ed. 3.0:2010 Power cables with extruded insulation and their accessories for rated voltages from 1 kV (Um = 1,2 kV) up to 30 kV (Um = 36 kV) – Part 4: Test requirements on accessories for cables with rated voltages from 6 kV (Um = 7,2 kV) up to 30 kV (Um = 36 kV) Project IEC 62148-2 Ed. 2.0:2010 Fibre optic active components and devices – Package and interface standards – Part 2: SFF 10-pin transceivers ISO/IEC 27001-HBK Ed. 1.0:2010 ISO/IEC 27001 for Small Businesses – Practical advice IEC 62475 Ed. 1.0:2010 High-current test techniques – Definitions and requirements for test currents IEC 60269-6 Ed. 1.0:2010 Low-voltage fuses – Part 6: Supplementary requirements for fuse-links for the protection of solar photovoltaic energy systems IEC 60252-1 Ed. 2.0:2010 AC motor capacitors – Part 1: General – Performance, testing and rating – Safety requirements – Guidance for installation and operation IEC 60060-1 Ed. 3.0:2010 High-voltage test techniques – Part 1: General definitions and test requirements IEC/TS 62257-7-1 Ed. 2.0:2010 Recommendations for small renewable energy and hybrid systems for rural electrification – Part 7-1: Generators – Photovoltaic generators IEC/PAS 62326-14 Ed. 1.0:2010 Printed boards – Part 14: Device embedded substrate – Terminology / reliability IEC 61347-2-12-am1 Ed. 1.0:2010 Amendment 1 – Lamp controlgear – Part 2-12: Particular requirements for d.c. or a.c. supplied electronic ballasts for discharge lamps (excluding fluorescent lamps) IEC 62374-1 Ed. 1.0:2010 Semiconductor devices – Part 1: Time-dependent dielectric breakdown (TDDB) test for inter-metal layers ISO/IEC 29199-2 Ed. 2.0:2010 Information technology — JPEG XR image coding system — Part 2: Image coding Project IEC 62642-2-5 Ed. 1.0:2010 Alarm systems – Intrusion and hold-up systems – Part 2-5: Intrusion detectors – Combined passive infrared / Ultrasonic detectors Project IEC 62642-2-4 Ed. 1.0:2010 Alarm systems – Intrusion and hold-up systems – Part 2-4: Intrusion detectors – Combined passive infrared / Microwave detectors Project IEC 62642-2-3 Ed. 1.0:2010 Alarm systems – Intrusion and hold-up systems – Part 2-3: Intrusion detectors Project IEC 62509 Ed. 1.0:2010 Battery charge controllers for photovoltaic systems – Performance and functioning Project IEC 62271-206 Ed. 1.0:2010 High-voltage switchgear and controlgear – Part 206: Voltage presence indicating http://www.int-bizdirectory.com/ http://www.int-bizdirectory.com/standards.htm http://www.findstandards.info Quote: Hench2004@sina.com

Sunday, December 4, 2011

Most Helpful Customer Reviews

The back cover claims that this is "The definitive guide to draughting to the latest ISO Standards, incorporating BS 8888". I cannot agree. This book seems to be a partial revision of a school or undergraduate drawing textbook. The authors might have achieved their objective if they had started from scratch. As it is, it would be better to call it a Rough Guide. It will be useful to beginners, but it is certainly not "definitive". The description of CAD systems in chapter 3 is heavily biased towards AutoCad, even when describing 3D programmes, in which they have never been dominant. The screenshot examples shown, over five pages, are taken as much from architecture as engineering, and are poorly reproduced. Captions are minimal, and the relevance to engineering of a dragonfly flying over a pond is hard to see. Two potentially informative screenshots of drawings in progress seem to have been printed in soot. The clarity and sharpness of a screen image is entirely lost. The authors appear to have shares in Mechsoft and the inclusion of two pages of AutoCad publicity material do little to advance the subject. The space would have been better used to illustrate the working methods of CAD programmes, particularly showing the difference between 2D and 3D work, and explaining the significance of Surface and Solid modelling, leading on to Hybrid programmes. The further use of 3D models for stress, heat flow, or fluid dynamics could have been illustrated. After pointing out on page 6 that the comma is to be the decimal marker, it is odd that the majority of drawings shown use the full stop, or point. The diameter symbol shown in the text does not agree with that shown in some illustrations, but the use is inconsistent. In both cases the symbol is incorrect. The section on drawing nuts and bolts continues a method which has been a poor approximation for more than fifty years, but makes no mention of using stencils, or CAD libraries, which would give an accurate representation. Chapters 20 to 23 reproduce the symbols for geometrical tolerancing as provided by AutoCad, including the errors. It would have been better to show them proportioned correctly to the standard. Several examples seem to have abandoned the correct use of line thickness. Chapter 26 shows welding symbols to BS 499. The authors should be aware that this was superseded in 1995 by BS EN 22553. Some explanation of the previous ways of working may be needed, but the emphasis should be on the current standard. The engineering diagrams in chapter 27 give a small selection of symbols to current standards, but far more space is given over to poor or non standard examples. The symbols used are inconsistent and no account has been taken of Reference Designations as specified in BS EN 61346. The section on Heating and Ventilation diagrams drifts into design techniques, which would be better covered in a Design textbook. The chapter on bearings similarly becomes a design manual, but the one illustration of the representation of bearings on a drawing is badly printed and incorrect. To add insult to injury, the text states that the drawing is wrong, but it has not been corrected! The final chapter deals with designing with adhesives. No examples of drawings showing assembly with adhesives are given, and we are completely in the world of design, not draughting, techniques. None of the finished drawings shown would be acceptable in my drawing office. The authors need to decide whether they are producing a Draughting or a Design Manual. The illustrations should ALL be up to date with the latest standards they claim to be presenting, and comply in every detail. They should represent the best of the draughtsman's art, not the typical products of those who have not kept up to date with the standards.