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Gérard René Lemaitre - One of the best experts on this subject based on the ideXlab platform.

  • Optical Design with the Schmidt Concept – Telescopes and Spectrographs
    Astronomy and Astrophysics Library, 2009
    Co-Authors: Gérard René Lemaitre
    Abstract:

    The basic principle of the wide-field telescope invented by the Estonian optician and astronomer Bernhard Schmidt in 1928 ([67–70], E. Schmidt [72]), is that a single concave and spherical mirror used with a pupil stop at its Center of Curvature has no unique axis and therefore yields equal size images at all points of its field of view. In the third-order theory, the mounting is free from Coma3 and astigmatism Astm3; all images have the same amount of spherical aberration, Sphe3, coming from the spherical mirror. By using a refractive corrector plate at the mirror Center of Curvature, one therefore yields equally good images in the whole field of view. In the historical context in Europe, three scientists had previously developed the theoretical analysis on aplanatic telescopes in the two-mirror class, but none of them found or realized that the primary mirror could be used off-axis or could be replaced by an on-axis refractive element. Kellner who patented in 1910 [25] several designs using a corrector lens, locates the plate in a wrong position for wide-field compensations. Schmidt placed the aspherical plate at the mirror Center of Curvature and emphasized the importance of this location for the entrance pupil of the telescope. The curved focal surface is a monocentric sphere with the mirror. In 1930–31, he succeeded in constructing the first wide-field telescope, 36 cm clear aperture at f/1.75, with which he demonstrated the wide-field performance on 7.5 arc degrees during the two subsequent years. He obtained with Wachmann, about two hundred exposures onto curved films showing perfect images. Such astronomical object densities were never seen before. In 1932, Schmidt published his famous article “Ein Lichtstarkes Komafreies Spiegelsystem” [70] and photographies [71]. In fact, his coma-free i.e. aplanatic telescope is also free from third-order astigmatism: nowadays, this is called an anastigmatic telescope. Review papers on B. Schmidt’s work were published by Schorr [73], Mayall [47], Wachmann [84, 85], Kross [27] and more recently by E. Schmidt [72], his nephew.

John C. Bancroft - One of the best experts on this subject based on the ideXlab platform.

  • The computation of traveltimes when assuming locally circular or spherical wavefronts
    SEG Technical Program Expanded Abstracts 2006, 2006
    Co-Authors: John C. Bancroft
    Abstract:

    Traveltimes on a grid may be computed using a finite difference solution to the Eikonal equation, however that solution is based on a plane wave assumptions (Bancroft 2005a). However the wavefront may be curved and approximated by a circle. Solving for an unknown traveltime, such as a corner of the square, may involve estimating the Center of the circle (Center of Curvature) and then computing the traveltime to the desired location. The traveltime computations assume the velocity to be locally constant within the square, but this velocity is extended outside the square to the location of the Center of Curvature.

  • Computation of gridded traveltimes using a circular wavefront assumption
    ASEG Extended Abstracts, 2006
    Co-Authors: John C. Bancroft
    Abstract:

    Traveltime computations are an integral part of modelling and imaging seismic data by providing efficient kinematic information on the location of propagated energy. The traveltimes may be computed analytically using simplifying assumptions, or may be estimated on a complex geological structure using raytracing or gridded traveltimes. A basic requirement for the propagation of gridded traveltimes is the estimation of one point on a corner of a square, given the traveltimes on the other three corners. A number of solutions are available to solve for the unknown time and are based on either a plane-wave assumption, a finite difference solution to the Eikonal equation, or an assumption that the wavefront at the square is curved. A solution for a curved wavefront assumption requires estimating the Center of Curvature, and requires solving a quartic equation. An alternate method is presented to estimate the Center of Curvature for a curved wavefront that uses an iterative procedure and does not require solving the quartic equation.

  • The Computation of Gridded Traveltimes When Assuming Circular Wavefronts
    SEG Technical Program Expanded Abstracts 2005, 2005
    Co-Authors: John C. Bancroft
    Abstract:

    Traveltime computations are an integral part of modelling and imaging seismic data by providing efficient kinematic information on the location of propagated energy. The traveltimes may be computed analytically using simplifying assumptions, or may be estimated on a complex geological structure using raytracing or gridded traveltimes. A basic requirement for the propagation of gridded traveltimes is the estimation of one point on a corner of a square given the traveltimes on the other three corners. A number of solutions are available to solve for the unknown time and are based on either a plane-wave assumption, a finite difference solution to the Eikonal equation, or an assumption that the wavefront at the square is curved. A solution for a curved wavefront assumption requires estimating the Center of Curvature, and requires solving a quartic equation. An alternate method is presented to estimate the Center of Curvature for a curved wavefront that uses an iterative procedure and does not require solving the quartic equation.

Pieter J. Erasmus - One of the best experts on this subject based on the ideXlab platform.

  • Anatomical study of the radius and Center of Curvature of the distal femoral condyle.
    Journal of biomechanical engineering, 2010
    Co-Authors: Jurgen Kosel, Ioanna Giouroudi, Cornie Scheffer, Edwin Dillon, Pieter J. Erasmus
    Abstract:

    In this anatomical study, the anteroposterior Curvature of the surface of 16 cadaveric distal femurs was examined in terms of radii and Center point. Those two parameters attract high interest due to their significance for total knee arthroplasty. Basically, two different conclusions have been drawn in foregoing studies: (I ) The Curvature shows a constant radius and (2) the Curvature shows a variable radius. The investigations were based on a new method combining three-dimensional laser-scanning and planar geometrical analyses. This method is aimed at providing high accuracy and high local resolution. The high-precision laser scanning enables the exact reproduction of the distal femurs—including their cartilage tissue—as a three-dimensional computer model. The surface Curvature was investigated on intersection planes that were oriented perpendicularly to the surgical epicondylar line. Three planes were placed at the central part of each condyle. The intersection of either plane with the femur model was approximated with the help of a b-spline, yielding three b-splines on each condyle. The radii and Center points of the circles, approximating the local Curvature of the b-splines, were then evaluated. The results from all three b-splines were averaged in order to increase the reliability of the method. The results show the variation in the surface Curvatures of the investigated samples of condyles. These variations are expressed in the pattern of the Center points and the radii of the Curvatures. The standard deviations of the radii for a 90 deg arc on the posterior condyle range from 0.6 mm up to 5.1 mm, with an average of 2.4 mm laterally and 2.2 mm medially. No correlation was found between the Curvature of the lateral and medial condyles. Within the range of the investigated 16 samples, the conclusion can be drawn that the condyle surface Curvature is not constant and different for all specimens when viewed along the surgical epicondylar axis. For the portion of the condylar surface that articulates with the tibia during knee flexion-extension, the determined Center points approximate the location of the Centers of rotation. The results suggest that the concept of a fixed flexion-extension axis is not applicable for every specimen.

Gene Olczak - One of the best experts on this subject based on the ideXlab platform.

  • Performance of the Center-of-Curvature Optical Assembly During Cryogenic Testing of the James Webb Space Telescope
    Space Telescopes and Instrumentation 2018: Optical Infrared and Millimeter Wave, 2018
    Co-Authors: James B. Hadaway, Conrad Wells, Gene Olczak, Mark Waldman, Tony Whitman, Joseph Cosentino, David Chaney, Michael Zarella, Mark Connelly, Randal Telfer
    Abstract:

    The James Webb Space Telescope (JWST) primary mirror (PM) is 6.6 m in diameter and consists of 18 hexagonal segments, each 1.5 m point-to-point. Each segment has a 6 degree-of-freedom hexapod actuation system and a radius-of-Curvature (ROC) actuation system. The full telescope was tested at its cryogenic operating temperature at Johnson Space Center (JSC) in 2017. This testing included Center-of-Curvature measurements of the PM wavefront error using the Center-of-Curvature Optical Assembly (COCOA), along with the Absolute Distance Meter Assembly (ADMA). The COCOA included an interferometer, a reflective null, an interferometer-null calibration system, coarse and fine alignment systems, and two displacement measuring interferometer systems. A multiple-wavelength interferometer was used to enable alignment and phasing of the PM segments. By combining measurements at two laser wavelengths, synthetic wavelengths up to 15 mm could be achieved, allowing mirror segments with millimeter-level piston errors to be phased to the nanometer level. The ADMA was used to measure and set the spacing between the PM and the focus of the COCOA null (i.e., the PM Center-of-Curvature) for determination of the ROC. This paper describes the COCOA, the PM test setup, the testing performed, the test results, and the performance of the COCOA in aligning and phasing the PM segments and measuring the final PM wavefront error.

  • Characterization of the JWST Pathfinder Mirror Dynamics Using the Center of Curvature Optical Assembly (CoCOA)
    Space Telescopes and Instrumentation 2016: Optical Infrared and Millimeter Wave, 2016
    Co-Authors: Conrad Wells, Gene Olczak, Tony Whitman, James B. Hadaway, Joseph Cosentino, John D. Johnston, Mark Connolly, David Chaney, J. Scott Knight, Randal Telfer
    Abstract:

    The James Webb Space Telescope (JWST) Optical Telescope Element (OTE) consists of a 6.6 m clear aperture, 18 segment primary mirror, all-reflective, three-mirror anastigmat operating at cryogenic temperatures. To verify performance of the primary mirror, a full aperture Center of Curvature optical null test is performed under cryogenic conditions in Chamber A at the National Aeronautics and Space Administration (NASA) Johnson Space Center (JSC) using an instantaneous phase measuring interferometer. After phasing the mirrors during the JWST Pathfinder testing, the interferometer is utilized to characterize the mirror relative piston and tilt dynamics under different facility configurations. The correlation between the motions seen on detectors at the focal plane and the interferometer validates the use of the interferometer for dynamic investigations. The success of planned test hardware improvements will be characterized by the multi-wavelength interferometer (MWIF) at the Center of Curvature Optical Assembly (CoCOA).

  • Performance of the primary mirror Center-of-Curvature optical metrology system during cryogenic testing of the JWST Pathfinder telescope
    Space Telescopes and Instrumentation 2016: Optical Infrared and Millimeter Wave, 2016
    Co-Authors: James B. Hadaway, Conrad Wells, Gene Olczak, Mark Waldman, Tony Whitman, Joseph Cosentino, Mark Connolly, David Chaney, Randal Telfer
    Abstract:

    The James Webb Space Telescope (JWST) primary mirror (PM) is 6.6 m in diameter and consists of 18 hexagonal segments, each 1.5 m point-to-point. Each segment has a six degree-of-freedom hexapod actuation system and a radius of-Curvature (RoC) actuation system. The full telescope will be tested at its cryogenic operating temperature at Johnson Space Center. This testing will include Center-of-Curvature measurements of the PM, using the Center-of-Curvature Optical Assembly (COCOA) and the Absolute Distance Meter Assembly (ADMA). The COCOA includes an interferometer, a reflective null, an interferometer-null calibration system, coarse and fine alignment systems, and two displacement measuring interferometer systems. A multiple-wavelength interferometer (MWIF) is used for alignment and phasing of the PM segments. The ADMA is used to measure, and set, the spacing between the PM and the focus of the COCOA null (i.e. the PM Center-of-Curvature) for determination of the ROC. The performance of these metrology systems was assessed during two cryogenic tests at JSC. This testing was performed using the JWST Pathfinder telescope, consisting mostly of engineering development and spare hardware. The Pathfinder PM consists of two spare segments. These tests provided the opportunity to assess how well the Center-of-Curvature optical metrology hardware, along with the software and procedures, performed using real JWST telescope hardware. This paper will describe the test setup, the testing performed, and the resulting metrology system performance. The knowledge gained and the lessons learned during this testing will be of great benefit to the accurate and efficient cryogenic testing of the JWST flight telescope.

  • Wavefront Calibration Testing of the James Webb Space Telescope Primary Mirror Center of Curvature Optical Assembly
    Modern Technologies in Space- and Ground-based Telescopes and Instrumentation II, 2012
    Co-Authors: Gene Olczak, Conrad Wells, David Fischer, Mark T. Connolly
    Abstract:

    The James Webb Space Telescope (JWST) Optical Telescope Element (OTE) consists of a 6.6 meter clear aperture, all-reflective, three-mirror anastigmat. The 18-segment primary mirror (PM) presents unique and challenging assembly, integration, alignment and testing requirements. A full aperture Center of Curvature optical test is performed in cryogenic vacuum conditions at the integrated observatory level to verify PM performance requirements. Two wavefront calibration tests are utilized to verify the low and mid/high spatial frequency performance of the test system. In this paper the methods and results of the wavefront calibration tests are presented.

  • The Center of Curvature optical assembly for the JWST primary mirror cryogenic optical test: optical verification
    Interferometry XV: Techniques and Analysis, 2010
    Co-Authors: Conrad Wells, Gene Olczak, Cormic K. Merle, Tom Dey, Mark Waldman, Tony Whitman, Eric Wick, Aaron Peer
    Abstract:

    The James Webb Space Telescope (JWST) Optical Telescope Element (OTE) consists of a 6.6 m clear aperture, allreflective, three-mirror anastigmat. The 18-segment primary mirror (PM) presents unique and challenging assembly, integration, alignment and testing requirements. A full aperture Center of Curvature optical test is performed in cryogenic vacuum conditions at the integrated observatory level to verify PM performance requirements. The Center of Curvature Optical Assembly (CoCOA), designed and being built by ITT satisfies the requirements for this test. The CoCOA contains a multi wave interferometer, patented reflective null lens, actuation for alignment, full in situ calibration capability, coarse and fine alignment sensing systems, as well as a system for monitoring changes in the PM to CoCOA distance. Two wave front calibration tests are utilized to verify the low and Mid/High spatial frequencies, overcoming the limitations of the standard null/hologram configuration in its ability to resolve mid and high spatial frequencies. This paper will introduce the systems level architecture and optical test layout for the CoCOA.

Conrad Wells - One of the best experts on this subject based on the ideXlab platform.

  • Performance of the Center-of-Curvature Optical Assembly During Cryogenic Testing of the James Webb Space Telescope
    Space Telescopes and Instrumentation 2018: Optical Infrared and Millimeter Wave, 2018
    Co-Authors: James B. Hadaway, Conrad Wells, Gene Olczak, Mark Waldman, Tony Whitman, Joseph Cosentino, David Chaney, Michael Zarella, Mark Connelly, Randal Telfer
    Abstract:

    The James Webb Space Telescope (JWST) primary mirror (PM) is 6.6 m in diameter and consists of 18 hexagonal segments, each 1.5 m point-to-point. Each segment has a 6 degree-of-freedom hexapod actuation system and a radius-of-Curvature (ROC) actuation system. The full telescope was tested at its cryogenic operating temperature at Johnson Space Center (JSC) in 2017. This testing included Center-of-Curvature measurements of the PM wavefront error using the Center-of-Curvature Optical Assembly (COCOA), along with the Absolute Distance Meter Assembly (ADMA). The COCOA included an interferometer, a reflective null, an interferometer-null calibration system, coarse and fine alignment systems, and two displacement measuring interferometer systems. A multiple-wavelength interferometer was used to enable alignment and phasing of the PM segments. By combining measurements at two laser wavelengths, synthetic wavelengths up to 15 mm could be achieved, allowing mirror segments with millimeter-level piston errors to be phased to the nanometer level. The ADMA was used to measure and set the spacing between the PM and the focus of the COCOA null (i.e., the PM Center-of-Curvature) for determination of the ROC. This paper describes the COCOA, the PM test setup, the testing performed, the test results, and the performance of the COCOA in aligning and phasing the PM segments and measuring the final PM wavefront error.

  • Characterization of the JWST Pathfinder Mirror Dynamics Using the Center of Curvature Optical Assembly (CoCOA)
    Space Telescopes and Instrumentation 2016: Optical Infrared and Millimeter Wave, 2016
    Co-Authors: Conrad Wells, Gene Olczak, Tony Whitman, James B. Hadaway, Joseph Cosentino, John D. Johnston, Mark Connolly, David Chaney, J. Scott Knight, Randal Telfer
    Abstract:

    The James Webb Space Telescope (JWST) Optical Telescope Element (OTE) consists of a 6.6 m clear aperture, 18 segment primary mirror, all-reflective, three-mirror anastigmat operating at cryogenic temperatures. To verify performance of the primary mirror, a full aperture Center of Curvature optical null test is performed under cryogenic conditions in Chamber A at the National Aeronautics and Space Administration (NASA) Johnson Space Center (JSC) using an instantaneous phase measuring interferometer. After phasing the mirrors during the JWST Pathfinder testing, the interferometer is utilized to characterize the mirror relative piston and tilt dynamics under different facility configurations. The correlation between the motions seen on detectors at the focal plane and the interferometer validates the use of the interferometer for dynamic investigations. The success of planned test hardware improvements will be characterized by the multi-wavelength interferometer (MWIF) at the Center of Curvature Optical Assembly (CoCOA).

  • Performance of the primary mirror Center-of-Curvature optical metrology system during cryogenic testing of the JWST Pathfinder telescope
    Space Telescopes and Instrumentation 2016: Optical Infrared and Millimeter Wave, 2016
    Co-Authors: James B. Hadaway, Conrad Wells, Gene Olczak, Mark Waldman, Tony Whitman, Joseph Cosentino, Mark Connolly, David Chaney, Randal Telfer
    Abstract:

    The James Webb Space Telescope (JWST) primary mirror (PM) is 6.6 m in diameter and consists of 18 hexagonal segments, each 1.5 m point-to-point. Each segment has a six degree-of-freedom hexapod actuation system and a radius of-Curvature (RoC) actuation system. The full telescope will be tested at its cryogenic operating temperature at Johnson Space Center. This testing will include Center-of-Curvature measurements of the PM, using the Center-of-Curvature Optical Assembly (COCOA) and the Absolute Distance Meter Assembly (ADMA). The COCOA includes an interferometer, a reflective null, an interferometer-null calibration system, coarse and fine alignment systems, and two displacement measuring interferometer systems. A multiple-wavelength interferometer (MWIF) is used for alignment and phasing of the PM segments. The ADMA is used to measure, and set, the spacing between the PM and the focus of the COCOA null (i.e. the PM Center-of-Curvature) for determination of the ROC. The performance of these metrology systems was assessed during two cryogenic tests at JSC. This testing was performed using the JWST Pathfinder telescope, consisting mostly of engineering development and spare hardware. The Pathfinder PM consists of two spare segments. These tests provided the opportunity to assess how well the Center-of-Curvature optical metrology hardware, along with the software and procedures, performed using real JWST telescope hardware. This paper will describe the test setup, the testing performed, and the resulting metrology system performance. The knowledge gained and the lessons learned during this testing will be of great benefit to the accurate and efficient cryogenic testing of the JWST flight telescope.

  • Wavefront Calibration Testing of the James Webb Space Telescope Primary Mirror Center of Curvature Optical Assembly
    Modern Technologies in Space- and Ground-based Telescopes and Instrumentation II, 2012
    Co-Authors: Gene Olczak, Conrad Wells, David Fischer, Mark T. Connolly
    Abstract:

    The James Webb Space Telescope (JWST) Optical Telescope Element (OTE) consists of a 6.6 meter clear aperture, all-reflective, three-mirror anastigmat. The 18-segment primary mirror (PM) presents unique and challenging assembly, integration, alignment and testing requirements. A full aperture Center of Curvature optical test is performed in cryogenic vacuum conditions at the integrated observatory level to verify PM performance requirements. Two wavefront calibration tests are utilized to verify the low and mid/high spatial frequency performance of the test system. In this paper the methods and results of the wavefront calibration tests are presented.

  • The Center of Curvature optical assembly for the JWST primary mirror cryogenic optical test: optical verification
    Interferometry XV: Techniques and Analysis, 2010
    Co-Authors: Conrad Wells, Gene Olczak, Cormic K. Merle, Tom Dey, Mark Waldman, Tony Whitman, Eric Wick, Aaron Peer
    Abstract:

    The James Webb Space Telescope (JWST) Optical Telescope Element (OTE) consists of a 6.6 m clear aperture, allreflective, three-mirror anastigmat. The 18-segment primary mirror (PM) presents unique and challenging assembly, integration, alignment and testing requirements. A full aperture Center of Curvature optical test is performed in cryogenic vacuum conditions at the integrated observatory level to verify PM performance requirements. The Center of Curvature Optical Assembly (CoCOA), designed and being built by ITT satisfies the requirements for this test. The CoCOA contains a multi wave interferometer, patented reflective null lens, actuation for alignment, full in situ calibration capability, coarse and fine alignment sensing systems, as well as a system for monitoring changes in the PM to CoCOA distance. Two wave front calibration tests are utilized to verify the low and Mid/High spatial frequencies, overcoming the limitations of the standard null/hologram configuration in its ability to resolve mid and high spatial frequencies. This paper will introduce the systems level architecture and optical test layout for the CoCOA.