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Use of the material Zerodur in the KECK observatory telescope. The very low CTE makes ZERODUR ideal for use as part of the primary mirror. This means that over the temperature range that is possible that the telescope works in, (0-50C), the materi

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Introduction


Introductionimage00.pngimage01.pngimage02.png

The KECK observatory lies near the summit of Mauna Kea in Hawaii. There are two telescopes close to each other on the summit, KECK I and KECK II. These combine to form one of the largest optical telescopes in the world, second only to the Gran Telescopio Canarias (GTC) in the Canary Islands. For good reason, both the KECK observatory and GTC use the same material for their primary mirrors.

The primary mirrors of the telescopes are the largest mirrors of the operation, and are designed to gather as much light as possible. The bigger the primary mirror, the more light the telescope can gather, and hence the ‘further’ into the solar system the telescope can see. With the need to see more and more of space, larger and larger telescopes are being built. However, making a mirror with a diameter of 10 metres of more out of a single sheet of a reflective substance gives a very large problem; the mirror must be very thick in order to hold its shape. When KECK I was being designed, the engineers came up with an ingenious solution, which involved splitting up the mirror into many hexagonal sections, which, when attached together, would act as a single mirror.

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Middle

m per Kelvin). When by itself, the CTE of a material carries little value; hence it is easier to compare the CTE of ZERODUR® to other known materials in order to get a feel for just how appropriate it is for the usage in telescopes.

Some known values of CTE are shown below. Because the CTE of most metals is not constant across a wide temperature range, the values below are for when the material is at 25˚C. All values use the units 10-6m m-1 K-1;

  • Gold - 14.2.4
  • Porcelain - 4.5.4
  • Glass - 8.5.5
  • Water - 69.5
  • Stainless steel - 17.3.5

Clearly then, with varying conditions within the building in which the telescope is housed; using other materials such as stainless steel would have caused major problems for the mirror. For example, using stainless steel;

CTE: 17.3 μm m-1 K-1

Expansion for a temperature change of 5 degrees across a mirror support 2m in diameter;

2m x 17.3 x 5K = 173μm, or 0.173mm

Obviously, this would produce a notable gap between the mirrors, and more importantly, one which the AO system couldn’t counteract.


Chemical composition and properties

There are several ‘systems’ of glass-ceramics, which in essence all differ in chemical composition. All compositions feature oxides of silicon and aluminium; however the oxides in the material differ depending on the system.

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Conclusion

This combination of a highly reflective coating of aluminium, ZERODUR®, and the adaptive optics system provide scientists with a near flawless system of looking further into the distances of space. This system of splitting the primary mirror into smaller section and using AO systems to keep then in the correct position is cutting edge technology, and is being used in nearly all new telescope designs, including in the James Webb Space Telescope.8


Bibliography

Reference 1

http://www.pa.msu.edu/people/frenchj/digistar/

Reference 2

http://www.jrank.org/space/pages/2429/Keck-Observatory.html

Reference 3

http://www.pgo-online.com/intl/katalog/zerodur.html

Reference 4

http://www.engineeringtoolbox.com/linear-expansion-coefficients-d_95.html

Reference 5

http://en.wikipedia.org/wiki/Coefficient_of_thermal_expansion#Thermal_expansion_coefficients_for_various_materials

Reference 6

http://www.ndt-ed.org/EducationResources/CommunityCollege/Materials/Graphics/QuartzGlass.jpg

Reference 7

http://www.pgo-online.com/intl/katalog/curves/zerodur_dkurve.html

Reference 8

http://www.aviationweek.com/aw/generic/story_channel.jsp?channel=space&id=news/awx/2010/04/28/awx_04_28_2010_p0-222800.xml&headline=James%20Webb%20Space%20Telescope%20Passes%20Milestone

Reference 9

http://www.flickr.com/photos/jclarkhpa/658788908/

Reference 10

http://www.britannica.com/EBchecked/media/152920/Polycrystalline-structure-of-electrical-steel

ZERODUR® in the KECK Observatory

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