Use of the material Zerodur in the KECK observatory telescope Essay Example

near-negligible coefficient of thermal expansion materials are used for the primary mirrors.

ZERODUR(r) is a glass-ceramic created by Schott, a German company. The mirror segments are based on this material and coated with a thin layer of polished aluminum to add reflection. Prior to being coated with aluminum, the mirror segment is seen in this picture. ZERODUR(r) has numerous qualities that make it ideal for use in telescopes, such as a low coefficient of thermal expansion (CTE), material homogeneity, and easy coating. However, properties like Young's Modulus are not relevant for use in telescopes but still adequate for the job. Despite being brittle, ZERODUR(r) works well since it doesn't require an impacting force acting upon it.

ZERODUR(r) is an excellent material for use in the primary mirror due to its very low CTE. The temperature range that the telescope operates in is 0-50�C, and the minimal expansion and contraction of ZERODUR(r) ensure that any gaps in the mirror are because of the active optics (AO) system. When ZERODUR(r) and a flawless AO system are combined, the primary mirror can work almost perfectly. With an expansion class of 0, which is Schott's highest quality version, ZERODUR(r) has a temperature coefficient of 0 � 0 across the 0-50�C temperature range.

For every 1K increase or decrease, the material will experience a 0 to 0.1 x 10-6m (or 0 to 0.1 um/K) expansion or contraction with a coefficient of thermal expansion of 02 x 10-6 m m-1 K-1.3.

Comparing the CTE of ZERODUR(r) to that of other known materials can help gauge its suitability for telescope use. Generally, the CTE of a material alone is not very meaningful. Below

are some known values for CTE with units of 10-6m m-1 K-1. It should be noted that the CTE of most metals is not constant over a wide temperature range, so the values listed below are specific to when the material is at 25�C. These values are as follows: Gold - 14.2.4, Porcelain - 4.

5.4 units of measurement are for Glass, 8.5.5 units are for Water, 69.5 units are for Stainless steel, and a total of 17 units are unspecified.

3.5 To avoid complications for the telescope mirror due to the diverse conditions in its housing, choosing alternative materials like stainless steel was not a feasible option. The coefficient of thermal expansion (CTE) for stainless steel is 17.3 ?m m-1 K-1. This means that a temperature increase of 5 degrees across a mirror support with a diameter of 2 meters would cause an expansion of 2m x 17.3 x 5K = 173 ?m, or 0.173mm. As a result, an observable gap would emerge between the mirrors that could not be corrected by the AO system.

Chemical composition and properties of glass-ceramics vary depending on the system utilized, featuring oxides of silicon and aluminium with differing materials depending on the system. The MAS System uses oxides of magnesium, aluminium, and silicon, while the LAS System contains oxides of lithium, aluminium, and silicon. ZERODUR(r) is made using the LAS system, which is the most common.

LAS system has diverse applications such as cooktops owing to its capability of managing abrupt temperature changes and low heat conduction coefficient. It is advantageous for the material to not absorb infrared radiation, which is responsible for heating, as the system is almost

transparent to it. Material properties are classified into macrostructure and microstructure. Macrostructure defines how a substance behaves and its associated traits like ductility, brittleness, and hardness; whereas microstructure pertains to the molecular composition of the material including the arrangement of atoms or molecules along with whether it is amorphous or crystalline.

Differentiating between amorphous and crystalline substances is essential because the arrangement of atoms in a molecule has a significant impact on its macroscopic characteristics. A substance lacking a repetitive atomic pattern, with atoms scattered randomly throughout its volume, resembles a natural forest without any tree pattern and is known as an amorphous substance. On the other hand, when atoms are highly ordered in a substance, like trees planted by humans in tidy rows, it's termed as a crystalline substance.

The regular arrangement of atoms in a material creates a crystal, similar to a crystalline substance. Figure 2 illustrates both types of structures.

- The chemical structures of amorphous and crystalline substances are compared in this diagram. The black and white atoms represent two different elements, such as silicon and oxygen. The left diagram illustrates the disordered and random arrangement typically found in amorphous substances, such as wax. Meanwhile, the right diagram displays the ordered and organized arrangement commonly seen in crystalline substances, such as diamond. It should be noted that not all arrangements have the same hexagonal shape as shown in the right diagram. Both atomic structures are significant since ZERODUR(r) is composed of a combination of both amorphous and crystalline phases. The chemicals are first melted and then rapidly cooled to create the mixture.

The amorphous phase is characterized by a disordered arrangement of

atoms, forming amorphous solids which can be classified as super-cooled liquids. In due time, the molecules in the substance will gradually move past each other, behaving like extremely cold liquids. Glass is a perfect example of such a substance, demonstrated by the thicker bottom of windows in old homes and buildings. Nevertheless, it is feasible to create a crystalline substance by slowly cooling down the hot liquid.

Unlike the non-crystalline form, the crystalline counterpart of the material displays a uniform arrangement of its atoms and molecules. This divergence in structure originates from two factors - the incorporation of nucleating agents during the cooling process, which act as anchor points for crystal growth. With several nucleating agents available, numerous crystals develop around them, resulting in a final product that is termed polycrystalline; a single structure comprising several highly ordered atom-containing crystals (refer to figure 3).

- Fewer crystals are formed when the liquid cools slowly, with the possibility of forming one large crystal over an extended period. This is illustrated in Figure 3, which shows a false color electron micrograph of the polycrystalline structure of electrical steel.

Schott achieves the optimal balance between quality and crystallinity by using a combination of nucleating agents and cooling speed. If perfect quality is not necessary, adding more nucleating agents or slowing down the cooling process can result in a cheaper version of ZERODUR(r). It is crucial for the mirror's supporting material to have minimal expansion, making a crystalline substance preferable due to increased homogeneity. A uniform distribution and arrangement of atoms throughout the atomic array can be achieved with crystalline materials.

This paragraph highlights some of the significant properties of ZERODUR(r)

- high homogeneity and zero thermal expansion coefficient within a specific temperature range. This unique material has almost uniform distribution of thermal expansion, ensuring that it expands in all directions at the same rate under extreme heating. Glass-ceramics also exhibit a distinct property of having different coefficients of thermal expansion across diverse temperature ranges due to the varying mix of crystalline and amorphous components. To achieve the desired zero expansion coefficient for application in KECK observatories, finding the right balance of elements, cooling rate, and nucleating agents is crucial. In the case of ZERODUR(r), the perfect combination of these factors results in a 0 thermal expansion coefficient between 0-50�C temperature range, making it an ideal material for use in such high-precision observatories.

Figure 4 depicts a temperature range of 0�C to 50�C where ZERODUR(r) has a 0 expansion coefficient. The graph shows the thermal expansion coefficient of ZERODUR(r) from 0 to 900K. In conclusion, ZERODUR(r) is an ideal material choice for telescopes due to its excellent profile.

ZERODUR(r) has a thermal expansion coefficient close to zero, meaning it minimally expands or contracts during heating or cooling. This property enables the adaptive optics (AO) system to keep individual mirrors operating as a single mirror with high precision. The material's high homogeneity also ensures uniform expansion or contraction, making it easier for the AO system to correct for any movements. Additionally, ZERODUR(r) can be easily coated with reflective aluminium, making it ideal for telescope applications. Combining ZERODUR(r), an aluminium coating, and AO systems creates an advanced technology for observing distant space. This technique involves dividing the primary mirror into smaller sections and keeping them in place using AO

systems, which is incorporated in most new telescopes such as the James Webb Space Telescope.