Telescope Mirror Alignment
Telescope Application Background
Interface Inc has been helping to build telescopes since the early 1980s. Load cells are an integral part of some telescopes' ‘adjustment bed’ for the telescope’s mirror(s). Required, is a good understanding of the physical properties and design criteria for the load cell – properties such as thermal sensitivity, creep factors, and application tolerances. In fact, Tucson Arizona is a much regarded location for many telescopes, and where preeminent scientists, academics, and institutions choose to locate their telescopes on Kitt Peak. Interface Inc., in Scottsdale Arizona, is located just 100 miles from The University of Arizona’s world-renowned Steward Observatory Mirror Lab and Kitt Peak.
The Kitt Peak National Observatory (KPNO) is a United States astronomical observatory located on Kitt Peak’ summit at 2,096 m (6,880 ft) of the Quinlan Mountains in the Arizona-Sonoran Desert on the Tohono O'odham Nation, 88 kilometers (55 miles) southwest of Tucson.
The observatory is considered to be part of the National Optical Astronomy Observatory (NOAO), although some of the telescopes located here, like those at the MDM Observatory, belong to other groups such as the University of Arizona's Steward Observatory. With 23 telescopes, it is the largest, most diverse gathering of astronomical instruments in the world.
Kitt Peak National Observatory, was founded in 1958 under contract with the National Science Foundation and is administered by the Association of Universities for Research in Astronomy. Its principal instrument is the Mayall 158-in. (4-m) reflector. The observatory's equipment also includes 84-in. (2.1 m), 50-in. (1.3-m), 36-in. (0.9-m), and 16-in. (0.4-m) reflecting telescopes as well as a planned 3.5-m telescope. Used for wide angle photographs and electronic images of the sky, the Burrell Schmidt telescope is operated jointly with Case Western Reserve Univ. The 60-in. (1.5-m) Robert McMath Solar Telescope is the largest instrument of its kind in the world.
The University Of Arizona (UA) is a state institution of higher education in Tucson, AZ. UA research units include the Steward Observatory, which operates observatory facilities on Kitt Peak, Mt. Lemmon, Mt. Hopkins, and Mt. Graham for the benefit of faculty and students at the Arizona state universities. The Steward Observatory Mirror Lab has made mirrors for the MMT, Magellan, and Large Binocular Telescope, and Steward Observatory is a partner in these projects.
In 2005, The LSST Corporation has awarded a $2.3 million contract to the University of Arizona Steward Observatory Mirror Lab to purchase the glass and begin engineering work for the 8.4-meter diameter main mirror for the Large Synoptic Survey Telescope (LSST). Acquiring the LSST primary mirror was made possible by a generous, private donation from Arizona businessman Richard Caris. Mr. Caris is the Chairman and Founder of Interface Inc., a manufacturer of load sensors for other mission-critical projects at companies like Boeing, Airbus, NASA, and many others.
The LSST is a proposed world-class, ground-based telescope that can survey the entire visible sky every three nights. It will generate an awesome 30 terabytes of data per night from a three billion-pixel digital camera, producing a vast database of information on the universe.
Telescope Optical Support Structure Designs
While large telescope mirrors are made possible by the expertise of scientists and engineers at the Stewart Observatory Mirror Laboratory, the actual function of the mirror comes down to the fine levels of adjustments using load cells to monitor the forces on the mirrors.
Despite the advantages offered scientists by modern computer techniques and electronics, much of the success of the project has depended on the design of the mechanism supporting the mirrors. The mirrors are made from hexagonal glass blocks, above, fused together and held in shape during use by a system of load cells and hydraulic and pneumatic actuators. At the heart of the structure are hundreds of strain gauge load cells - a technology first used 60 years ago to determine weight and balance data for military aircraft.
In one large telescope design, each support node consists of two load cells, mounted one on top of each other. At the top, a 500N load cell is used for the fine control of the mirror, while immediately below is a 2,000N load cell. Although the two load cells are mounted in series, a force transfer mechanism ensures that the top load cells do not carry the full load of the mirror if the main support fails, the load being transferred to the lower cells.
All the support points are used in conjunction with pneumatic and hydraulic actuators which can pull and push parts of the mirror to maintain optimal shape and to compensate for distortions due to gravity and wind. To keep the mirror circular as the orientation of the telescope changes, 100 special 5,000N load cells are equally spaced around the outside of the rim of the mirror.
The Large Binocular Telescope (LBT) shown above consists of two 8.4-m mirrors on an alt-az mounting, located on Mt. Graham (elevation 3200 m) near Safford, Arizona.
The equivalent circular aperture of LBT is 11.7 m. The f/1.14 primary mirrors were spun cast at the Steward Observatory Mirror Lab.
In another larger telescope design, the Optical Support Structure (OSS) includes a moderate precision (100 micron) repeatable coupling interface to accommodate both RC and PFA top ends. The installation of any particular top end will be accomplished using the observatory dome crane with telescope horizon pointing. In order for the top end coupling to repeatably locate the top end with a known tolerance the load carried by the coupling will need to be controlled during installation. This will be accomplished using a load cell on the dome crane hook. The top end installation procedure will be to load the coupling to some percentage of the total top end load, which will provide the proper deformation to yield the repeatable coupling, and then secure the top end to the truss tubes using captive fasteners. The top end will then be released from the dome crane and the telescope can be placed into service.