Dr. Harold E. Bennett Profile

Dr. Harold E. Bennett

Retired

SPIE Involvement:

Author

Publications (21)

Proceedings Article | 29 August 2006 Paper

Large, lightweight, low-scatter, composite active/adaptive mirror development

H. Bennett, H. Blazek, A. Danielson

Proceedings Volume 6306, 63060Q (2006) https://doi.org/10.1117/12.687503

KEYWORDS: Mirrors, Actuators, Composites, Glasses, Adaptive optics, Light scattering, Polishing, Atmospheric corrections, Active optics, Astronomy

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Proceedings Article | 7 July 2004 Paper

Development of lightweight mirror elements for the Euro50 mirrors

Harold Bennett, Robert Romeo, Joseph Shaffer, Peter Chen

Proceedings Volume 5382, (2004) https://doi.org/10.1117/12.566262

KEYWORDS: Mirrors, Actuators, Composites, Zerodur, Adaptive optics, Telescopes, Light scattering, Space telescopes, Surface finishing, Solids

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New, very large telescopes with apertures of 30, 50, and 100 meters are being proposed by the astronomical community. Superpolished or ultrapolished mirrors with low scattered light levels and the use of adaptive optics for near-diffraction-limited performance would make such large telescopes a turning point in astronomy. The secondary mirror for the Euro50 will be a four meter adaptive optic made of a low expansion graphite-filled cyanate ester resin composite produced using a replica transfer technique. We have made three 1/3^rd meter diameter prototype composite adaptive optic mirrors of this cyanate ester composite material. Because of the embedded graphite fibers, the composite material has a measured expansion coefficient in the 10^-8 range, as has Zerodur or ULE glass. It is very much lighter, more rugged and more economical than Zerodur or ULE, and can be fabricated in weeks, not months. The Zerodur mandrels upon which these replica transfer mirrors are made are superpolished using centrifugal elutriation, so the replica surface has an rms roughness of 0.6 to 0.8 nm. It thus scatters about an order of magnitude less light than typical conventionally polished astronomical mirrors. In adaptive optic mirrors with sub-mm thick faceplates the number of plies used is insufficient to produce an isotropic surface. For mirrors 2 mm thick, with more plies, the surfaces are isotropic, and the slight astigmatism sometimes resulting from the mesh in the ply can be corrected by actuators to make them attractive mirrors. They must be supported to maintain a good optical figure over a meter diameter mirror. The support requirement may be met by using a new type of mechanical/piezoelectric actuator adjustable to a fraction of a wavelength. The mechanical actuators have a coarse adjust of over an mm and a fine adjust of less than a wavelength of light. They can be used in series with a novel type of piezoelectric actuator for final static adjustment. The low voltage, up to 2.5 kHz frequency piezoelectric actuators have a displacement of approximately one μm per volt, 82 times greater than conventional piezoelectric actuators, and a throw of ±30 μm or more. Compliant faceplates can be adaptive as well as active. Calculations indicate that for actuator spacings of about 4 cm the effective mirror stiffness equals that of a solid Zerodur mirror with a conventional 6:1 diameter to thickness ratio. The effect of gravitational sag for composite mirrors is calculated to be negligible. They are thus a good choice for the secondary mirror for the Euro50 as well as for the primary or secondary mirrors for other giant telescopes.

Proceedings Article | 30 January 2003 Paper

Development of lightweight mirror elements for a very large astronomical adaptive optic primary mirror

Harold Bennett, Joseph Shaffer, Robert Romeo, Peter Chen

Proceedings Volume 4840, (2003) https://doi.org/10.1117/12.459929

KEYWORDS: Mirrors, Actuators, Adaptive optics, Composites, Glasses, Telescopes, Wavefronts, Prototyping, Space telescopes, Surface finishing

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New very large telescopes with apertures as large as 100 meters are being proposed. They will be made up of mirror segments only a meter or two in diameter and phased together. The diffraction-limited resolution of a mirror is directly proportional to its diameter, and the light-gathering-power is proportional to the square of the diameter. Near-diffraction-limited performance using adaptive optics would make such large mirrors very exciting. We have built two small prototype composite adaptive optic mirrors of graphite fiber impregnated cyanate ester resin driven by actuators spaced 4 cm apart and with a faceplate influence function of 5 cm. The second mirror assembly also makes possible a 2 cm actuator spacing. The overall figure is not yet as good as desired, but we believe that much of this problem can be corrected by mechanical adjustment of the actuator rest positions and use of low expansion mandrels. This mirror concept, when realized in primary mirror segments a meter or more in diameter, should make correction possible for atmospheric turbulence under almost any observatory seeing conditions. The composite optical faceplate in the most recent prototype had a roughness of 0.6 to 0.8 nm. Two centrifugal elutriation super-polishers, each over 1.2 meters in diameter, are in place to produce superpolished mandrels on which to form superpolished faceplates over a meter in diameter. Scattered light from such a mirror surface will be reduced by as much as a factor of ten, as compared to conventional fresh feed polishing. The name "transfer mirrors" rather than the widely recognized but poorer quality "replica mirrors" is given to such faceplates. They have an expansion coefficient comparable to ULE quartz or Zerodur, and are lightweight with 10-20, an aerial density of 17 kg/m² for the mirror with a 4 cm actuator spacing or 34 kg/m² for the mirror with 2 cm actuator spacing. In both cases the effect of gravitational sag is minimized. A 60 volt potential results in actuator displacement of 5 mm as measured inter

Proceedings Article | 4 June 2002 Paper

Space elevator feasibility test using laser power beaming

Bradley Edwards, Harold Bennett

Proceedings Volume 4632, (2002) https://doi.org/10.1117/12.469765

KEYWORDS: Space operations, Composites, Mirrors, Solar cells, Adaptive optics, Free electron lasers, Carbon nanotubes, Laser systems engineering, Satellites, Receivers

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Proceedings Article | 19 July 1999 Paper

FEL powering of satellites: a technically and economically viable program

Harold Bennett

Proceedings Volume 3614, (1999) https://doi.org/10.1117/12.352663

KEYWORDS: Satellites, Solar cells, Adaptive optics, Mirrors, Free electron lasers, Telescopes, Atmospheric optics, Ka band, Space telescopes, Satellite communications

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Proceedings Article | 15 May 1997 Paper

Laser power beaming: an emerging technology for power transmission and propulsion in space

Harold Bennett

Proceedings Volume 2988, (1997) https://doi.org/10.1117/12.274388

KEYWORDS: Satellites, Space telescopes, Sun, Solar cells, Rockets, Adaptive optics, Telescopes, Mirrors, Space operations, Atmospheric optics

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Proceedings Article | 15 May 1997 Paper

Progress in establishing a laser power beaming facility at China Lake California

Harold Bennett

Proceedings Volume 2988, (1997) https://doi.org/10.1117/12.274382

KEYWORDS: Satellites, Free electron lasers, Prototyping, Mirrors, Adaptive optics, Solar cells, Nuclear physics, Integrated optics, Telecommunications, Satellite communications

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Proceedings Article | 15 May 1997 Paper

High-power beam control: results-to-date and relevance to power beaming

John Albertine, Sadegh Siahatgar, Harold Bennett

Proceedings Volume 2988, (1997) https://doi.org/10.1117/12.274389

KEYWORDS: Mirrors, Laser energy, Sensors, Beam expanders, Beam controllers, Laser systems engineering, Telescopes, Chemical lasers, Ocean optics, Relays

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Proceedings Article | 27 May 1996 Paper

Segmented adaptive optic mirrors for laser power beaming and other space applications

Harold Bennett

Proceedings Volume 2714, (1996) https://doi.org/10.1117/12.240355

KEYWORDS: Mirrors, Light scattering, Space telescopes, Satellites, Adaptive optics, Space mirrors, Diffraction, Telescopes, Plasma, Stars

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