IntroductionPrimary MirrorsFocal Stations
Secondary ComplementOther OpticsSimulated Images


The Large Binocular Telescope will use two 8.408 meter (331-inch), F/1.142 primary mirrors to provide a collecting area equivalent to an 11.8 meter (465-inch) circular aperture. By having both the primary mirrors on the same mounting, the telescope will be able to achieve the diffraction-limited image sharpness of a 22.8 meter aperture. These borosilicate honeycomb primary mirrors are being fabricated at the Steward Observatory Mirror Lab using E6 glass made by Ohara in Japan.

Primary Mirrors

  • Number of Primary Mirrors: 2
  • Primary Spacing: 14.417 meter center-to-center
  • Primary Physical Diameter: 8.417 meter
  • Primary Focal Ratio: F/1.142
  • Central Hole Physical Diameter: 0.889 meter
  • Primary Figure: parabolic
  • Primary Construction
    • cast borosilicate honeycomb
    • 28 mm faceplate thickness
    • edge thickness 894 mm, plano-concave
  • Primary Mirror Mass: approximately 16 metric tons each
Included in the budget for the telescope project are the costs for a pair of F/15 adaptive secondaries, each with 672 voice-coil actuators. Other focal stations and features deemed scientifically important for future expansions will be allowed in the optical and mechanical design of the telescope but not initially implemented.

Focal Stations

A pair of curved secondary mirrors and a pair of tertiary flat mirrors are required to implement those focal sations. These smaller optics are moved in and out of the lightpath by swing arms. The tertiary mirrors rotate to direct the light to several central instrument locations. (Prime focus cameras are not shown on the drawing below.

Images by ADS Italia and Max-Planck-Institut für Astronomie (Heidelberg). The red "T-shaped" instrument in the center of the telescope is an instrument which contains the optics for combining and phasing the beams from the two telescopes.

Secondary Complement

  • infrared Gregorian, F/15
    • interchange: swing arm
    • mirror diameter: 0.911 m (undersized secondary)
    • infrared field-of-view: 4 arcminutes (unvignetted at primary)
    • optical field-of-view: 10 arcminutes (vignetted at secondary)
    • asphere: -0.7328
    • focal ratio of full-aperture parent: F/14.72
    • back focal distance: 3.050 m

The infrared F/15 secondaries are undersized so that the edge of the secondary mirror(s) serves as the aperture stop(s) for the telescope. This exit pupil can be reimaged and baffled inside the infrared instruments to reduce the thermal emission from the structure of the telescope. The infrared instrument will see the telescope pupil surrounded by a cold sky background.

Other Optics

  • tertiary flats (2)
    • interchange: swing arm
    • minor axis diameter: 50 cm (45 cm minimum)
    • major axis diameter: 64 cm
    • location: 2.25 m above primary vertex
    • F/15 field-of-view: 8 x 4 arcminutes (before tertiary vignettes)
  • beam combination optics
    • lateral offset: 0 or 3.5 m
  • wide field correctors
  • small field correctors

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Last modified: Thu Feb 7 19:05:54 2002