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The Kaj Aa. Strand 1.55-meter (61-inch) Astrometric Reflector

This is the largest optical telescope currently operated by the U.S. Naval Observatory. It was designed and built under the direction of the Scientific Director of the Observatory from 1963 - 1977, Dr. Kaj Aa. Strand. It was designed to produce extremely accurate astrometric measurements in small fields, and has been used to measure parallaxes and therefore distances for faint stars. Thousands of the world's most accurate stellar distances and proper motions have been measured with this telescope since 1964. In recent years this telescope has also served as a test-bed for the development of state-of-the-art near-infrared detectors.
  • Construction
    • Optics: Corning Glass Works (blanks); Davidson Optronics (optical figuring)
    • Mounting: L&F Machine and Boller & Chivens
    • Date Completed: 1964
    • Cost: $515,400
  • Optical Design: Folded Newtonian
  • Effective Focal Length: 15.2-meters
  • Overall Focal Ratio: f/9.8
  • Focal Plane Scale: 13.5 arcsec/mm
  • Primary Mirror
    • Diameter: 1.55 meters (61 inches)
    • Material: Laminated Fused Silica
    • Weight: 1350 kg (2970 lbs)
    • Figure: Paraboloid
    • Focal Ratio: f/9.8
  • Secondary Mirror
    • Diameter: 0.89 meters (35 inches)
    • Material: Fused Silica
    • Weight: 190 kg (420 lbs)
    • Figure: Flat
  • Total Weight (Optics, Tube and Mount): 33,000 kg (36 tons)


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