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Super Invar 32-5... How Matching Glass and Metal Thermal Expansion Rates Is Critical to the Performance of the World's Largest Telescope

The *Subaru telescope's active primary mirror support system employs 261 actuators and high-precision load sensors with 261 specially designed Super Invar 32-5 Alloy sleeves imbedded in the glass disk at the top of each actuator.

Japan's Subaru telescope, located atop Hawaii's 14,000-ft. Mauna Kea, presented many complex engineering design concerns, including fabricating the enormous glass blank that serves as the primary mirror, and maintaining its accurate shape during observation. The single primary mirror structure:

  • Weighs 33 tons
  • Measures 8.31 meters (27.24 ft.) in diameter
  • Is only 20 cm (8 inches) thick
  • Has a 40.1 ratio of diameter to thickness
  • Requires a highly precise shape
  • Possess the smallest possible coefficient of thermal expansion (CTE)

To meet the primary mirror requirement, a giant disk was fabricated out of an ultra-low-expansion (ULE) glass. To support this primary mirror and maintain an accurate shape during observation, the Subaru telescope has an active mirror support system that employs 261 actuators and high-precision load sensors. At the top of each actuator in the mirror support system, imbedded in the glass disk, are 261 specially designed sleeves made of Super Invar 32-5 Alloy*, a specialty alloy that provides minimum thermal expansion at ambient temperatures and minimizes the difference between the CTEs of the glass and the alloy.

The load sensors monitor any deformation of the mirror due to gravity when the telescope tilts. A control computer analyzes that information, then drives the actuators every tenth of a second to compensate for any distortion of the mirror. The sleeves rest on tapered balls made of chrome-nickel-moly steel with galvanized surfaces to protect the sleeves' thin nickel coating.

Super Invar 32-5

Super Invar 32-5 is a low expansion alloy consisting of 32% Nickel, 5% Cobalt, and balance Iron. Super Invar Alloy exhibits minimum thermal expansion at room temperatures (one half of Invar Alloy), as well as thermal expansion properties less than that of Invar Alloy in the 67/203 °F (-55/95 °C) temperature range. The minimal expansion of Super Invar over this range of temperatures makes this alloy useful in many applications including structural components and substrates in instrument systems requiring precision measurements including optical instruments, telescopes, laser instruments and equipment, ring laser gyroscopes, laser benches, positioning devices, metrology devices, substrates in instrument systems requiring precision measurements, and other applications that require lower expansivity than Invar Alloy provides.

Ed Fagan Inc. Super Invar 32-5 Products

Ed Fagan Inc. stocks Super Invar in a variety of forms and dimensions including:
Rod - 0.50" - 3.50"
Sheet - 0.039" - 0.118"
Plate - 0.50" - 2.50"

For more detailed information on how Super Invar 32-5 is used in the Subaru Telescope visit Cartech Website.

For information on Super Invar 32-5 Linear Coefficient of Thermal Expansion, Physical and Mechanical Properties, Chemistry, Specifications and forms available from stock; download a copy of our Super Invar 32-5 data sheet.

For information on all our Controlled Expansion Alloys Linear Coefficient of Thermal Expansion, Physical and Mechanical Properties, Chemistry, Specifications and forms available from stock; download a copy of our Controlled Expansion Alloys data sheet.

Call us now to speak with an EFI materials expert or to place an order.

* Artist rendering


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Controlled Expansion Alloys

Ed Fagan Inc. stocks a variety of similar alloys for low expansion, controlled expansion, glass-to-metal or ceramic sealing applications that may also satisfy your needs. EFI carries:
Super Invar
42 Alloy
46 Alloy
48 Alloy
52 Alloy

And we also have a large selection of other types of specialty metals and alloys for high technology applications, including:
Electrical/Electronic Nickel
Soft Magnetic Alloys
Refractory Metals and Alloys
Hard to Find Materials