Recent experiments conducted under the Optical RF Communications Adjunct program demonstrate and
validate the viability of hybrid free space optical communications links in heavy atmospheric turbulence. Long range
air-to-mountain link closures were established under extreme atmospheric turbulence. The system implemented adaptive
mechanisms such as adaptive optics, an optical automatic gain controller, forward error correction coding, and link-level
retransmission to achieve low packet error rates for long distance links with heavy turbulence. The system, experiments,
and results are presented and comparisons are made to statistical prediction models.
The capacity to integrate RF and free space optical hybrid communications now feasible
given advances in adaptive optics and optical automated gain control. The ORCA program is
developing on operationally capable of highly reliable hybrid communications. This paper
provides an overview of the ORCA systems and discusses some of the key developments in
making the systems a reality.
A free-space optical (FSO) communication demonstration was conducted with JHU/APL and AOptix at the TCOM Test Facility in Elizabeth City, NC in May 2006. The primary test objective was to evaluate the performance of an FSO link from a fiber-tethered aerostat to a ground platform at effective data rates approaching 100 Gigabits/sec using wavelength division multiplexing (WDM) techniques. (Multiple optical channels operating near 1550 nm were modulated at data rates of 1, 10 and 40 Gbps). The test was conducted with a 38 meter aerostat raised to an altitude of 1 km and a ground platform located 1.2 km from the aerostat (limited by property boundary). Error free data transfers of 1.2 Terabits in 30 seconds at 40 Gbps were demonstrated. The total data transferred during the test was greater than 30 Terabits with an average BER of 10-6 without any forward error correction (FEC) coding.
The harsh operating environment of high vacuum and extremely low temperature poses several challenges to cryogenic mechanisms. These challenges include out-gassing, physical property change of metal and nonmetal materials, differential thermal shrinkage of different materials. Many motorized cryogenic mechanisms have been designed and fabricated for various IR instruments at the Institute for Astronomy. These mechanisms include detector focus stages, filter wheels, 2 and 3-position bema selectors, lens switchers, grating tilt stages and gimbal mirror mounts. Cryogenic motors are used for all these mechanisms. The following topics will be discussed in this paper: motor selection, material selection, stress relieve and surface treatment, ball bearing and ball screw selection and treatment, bushing materials, lubrication methods, flex pivots, and Hall effect sensors.
The NIRI for the Gemini North telescope is now undergoing acceptance testing. NIRI is the main near-IR facility camera on the Gemini North telescope and is designed to fully exploit the excellent characteristics of the site and the expected high performance o the telescope. NIRI offers 3 different pixel scales for wide-field, tip-tilt corrected and diffraction-limited imaging. It is equipped with a pupil imaging system to evaluate the telescope emissivity and to optimize the alignment of the instrument with the telescope. NIRI has an IR wavefront sensor so that tip-tilt and focus corrections can be obtained even in dark cloud regions or during daytime observing.
Reviewed is a focus stage designed to accommodate the positioning and stability requirements of the detector arrays in the Gemini North Telescope's Near IR imager (NIRI). Focus axis translation of the two detector arrays is required, while sub-micron deflection stiffness about all other axes is of paramount importance to the successful operation of NIRI. The stiffness requirement coupled with a cryogenic vacuum environment led to a flexure design. Testing of the prototype stage mechanism to date has shown transverse deflections of < 1 micrometers , positioning repeatability of 1 micrometers , and satisfactory cryogenic performance.
The IR camera and spectrograph (IRCS) for SUBARU and Gemini near-IR imager (NIRI) instruments have a common design for all wheels, based on a modified geneva mechanisms with a locking cam actuated detent pin. The geneva design, in combination with the spring loaded detent mechanism, allows the stepper motor/spur gear drive to decouple from the wheel at each aperture position. The detent mechanism positions the wheel precisely. The need for precise motor control and wheel position encoding is reduced because of the detent mechanism. Six of these mechanism are filters wheels requiring repeatable aperture positing. The other seven mechanisms include of a slit wheel, grism wheel, pupil mask wheel, 2 beam steerers, a focal p;lane mask wheel, and a beamsplitter wheel. These mechanisms require repeatable, stable and accurate positioning. The number of aperture positions for the 13 wheels range from 2 to 16. The mechanisms are aligned and tested at room temperature and operated at 60 K, requiring an athermal design, for which the modified geneva mechanism is ideally suited. This paper will discuss the prototype development and final mechanical design of specific wheel mechanisms completed for the IRCS and NIRI instruments at the Institute for Astronomy.
The Gemini Near IR Imager (NIRI) is a cryogenic instrument cooled by two closed-cycle cryo-coolers. The vacuum jacket is a hexagon shaped vacuum vessel made of three sections. Each section is forged out of aluminum 6061. All the internal structural components are made of aluminum 6061T6 except the supporting trusses, which are made of titanium. All the internal structural members are stress relieved to maintain dimensional stability and good optical alignment. The thermal insulation includes floating shields and cold shields. Two closed-cycle coolers are mounted opposite to each other and electronically synchronized in order to cancel the vibration caused by the oscillating expansion valve. Several different fabrication methods and stress relief methods are discussed.
We present a high-resolution gimbal mirror mechanism which will perform the beam steering for the on-instrument wavefront sensor section of the Gemini near-IR imager. In turn, the wavefront sensor will generate correction signals for the tip-tilt and fast-focus secondary mirror. Preliminary testing of the current version of the gimbal assembly has revealed positive result when operated at room temperature, but demonstrated hysterisis problems at cryogenic temperatures. Described in this paper are the specifications, design and performance characteristics, and integration of the gimbal mechanism with the rest of the wavefront sensor system.
We discuss the main design features of the Gemini Near-IR Imager (NIRI) and its scientific capabilities. NIRI is designed to fully exploit the excellent image quality and low telescope emissivity expected from the Gemini telescope on Mauna Kea. It offers a range of pixel scales matched to different scientific objectives and has spectroscopic as well as polarimetric capabilities. One of its main design features is the use of a near-IR 2 X 2 Shack-Hartmann wavefront sensor for tip-tilt and focus control.
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