This example of a large Zerodur optical flat represents one of a great number of Zerodur products we carry, and a particularly fine representation at that. Measuring six inches in diameter (15.24 cm) and 1/10 wave flatness, this is a tool of great optical precision.
Utility of Optical Flats
Optical flats function as extremely precisely prepared flat surfaces that can be used to determine the relative flatness of a sufficiently reflective test surface via the optical property of interference. In testing, the optical flat is placed on the test surface and a monochromatic light, that is to say, a light of a specified wavelength, is placed above the flat and test object so as to illuminate through the flat. Bands of light known as interference fringes then form which can be observed to determine the flatness of the test surface, at least in relation to the reference flat. If the bands are sufficiently straight and evenly spaced, the test surface can be determined to be at least as flat as the reference surface. If the bands are curved or otherwise malformed, this curvature can be quantified in relation to the wavelength of the light used for testing. Optical flats are accordingly classified by this proportional relationship, such as this optical flat which is rated at 1⁄10 wave flatness. In other terms, the maximum deviation of the surface of the optical flat is λ⁄10 nanometers, λ being the testing wavelength. Industrially, 632 nm is a common standard wavelength used for flatness testing, though shorter wavelengths are sometimes selected for increased accuracy.
As a means of determining the relative imperfectness of a surface, optical flats are an invaluable tool in the production of precision optics.
Zerodur Glass-Ceramics
Zerodur (registered as ZERODUR®) is a type of non-porous glass-ceramic originally created by Schott AG in 1968. It began its initial development in 1966 at the instigation of Dr. Hans Elsässer, director of Heidelberg University's observatory, who sought Schott's manufacturing capability to create a new set of large diameter ceramic castings for telescope optics. Given the relative lagged state of German astronomical infrastructure at the time, especially compared to the large telescopes being created in the United States such as at Mt. Palomar, interest in creating German-manufactured wide diameter astronomic optics had been building for some time. In order to meet Elsässer's requirements, including that the produced optics should have low thermal expansion at room temperature, a new material proved necessary. Schott took up the task, and, under the project direction of Dr. Jürgen Petzoldt and after two years of research, produced a lithium-aluminosilicate glass-ceramic fulfilling all technical requirements, a material consequently named Zerodur.
With the new material in hand, Schott received its formal order from the Max Planck Institute of Astronomy at Heidelberg for eleven mirror substrates to be produced using Zerodur in November 1968. It wouldn't be until late 1973 that Schott successfully cast the largest component of the order, a 4 meter diameter mirror blank that would ultimately be refined into a 3.6 meter telescope mirror post machining. Total production of the large optics would take years, with final machining of the precision glass-ceramics going until 1975. Still, once produced and mounted in the Calar Alto Observatory in Southern Spain, the results of some nine years of effort and millions spent in research and development spoke for themselves. Ultimately, Zerodur would come to be an essential material for the astronomic industry, becoming used, by 2008, in over 50% of large-scale telescopes between 8 - 10 meters. All the same, the impact of Zerodur extends to a number of other key industries, not least of which is the field of microlithography.
What grants Zerodur its immense utility is its unique blend of core characteristics. Perhaps its most defining trait is its extraordinarily low coefficient of thermal expansion which enables its use in a variety of environments and allows it to function to exacting standards of precision. Further, it is a highly homogenous material, often featuring close to no internal inclusions such as bubbles, striae, or miscellaneous material deformities. Finally, Zerodur is a relatively durable material, able to handle a reasonable amount of mechanical stress as well as being quite chemically stable. Combined, these elements make it a popular selection for optical applications requiring immense precision even in extreme environments.
Bibliography
- Mitra, Ina. "Zerodur: A Glass-ceramic Material Enabling Optical Technologies." Optical Materials Express 12, no. 9 (September 2022).
- Döhring, Thorsten et al. "Forty Years of Zerodur® Mirror Substrates for Astronomy - Review and Outlook - Art. No. 70183B." The International Society of Optical Engineering (July 2008).
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