For as long as humanity can remember, we have turned our gaze to the night sky in order to unlock the secrets of the universe. The naked eye isn't sufficient for this purpose, which is why highly accurate telescopes are used for observation. Astronomical telescopes utilize an array of mirrors, of which the most significant is the primary mirror with a diameter of several meters. This primary mirror captures the light from distant stars and galaxies, focuses it, and directs it onto the smaller, convex secondary mirror. The beam is then further directed to the imaging instruments. Modern telescopes employ an adaptive secondary mirror (ASM), which has the ability to rapidly change its shape and effectively compensate for optical disturbances in the atmosphere.
Typically, these mirrors are made of glass and are subsequently coated with a reflective layer. The manufacturing process involves a lengthy sequence of grinding and polishing procedures. This approach not only leads to significant material consumption but also renders the delicate mirror susceptible to breakage.
In a collaborative study, scientists from the Dutch research institute TNO and the Fraunhofer Institute for Production Technology IPT are developing an alternative process chain for the sustainable and economically viable production of mirror substrates for the ASM of the UH 2.2-Meter Telescope in cooperation with the University of Hawaii. The secondary mirror has a diameter of 620 mm.
The aim of the study is to advance the production of increasingly larger adaptive secondary and deformable mirrors. By enhancing cost, time, and energy efficiency, these advancements are expected to propel astronomical research into new dimensions.
In the new process, the glass is directly shaped into the desired contour using the method of gravity bending. Gravity bending is an isothermal forming process in which the flat glass blank is positioned on the forming tool. Subsequently, the blank and the tool undergo a predefined temperature profile together.
The direct shaping of the mirror into its final geometry significantly reduces the need for elaborate post-processing and results in a considerable reduction in processing time. Compared to established manufacturing methods, the novel approach can produce a mirror shell of this size within a few days, which is about 3 times faster than other methods and reduces production costs by a factor of 3.
As the need for grinding and polishing can largely be eliminated, the process stands out for its exceptional resource efficiency: The innovative technique leads to a more than 70% reduction in material usage. Another major advantage is that even relatively thick glasses with a thickness of up to 3.5 mm can be processed. This not only simplifies the handling of the component but also reduces the risk of glass breakage.
The Fraunhofer IPT developed the process chain for forming the ASM mirror substrate. The forming process was simulated in advance to minimize errors. The researchers used the Finite Element Method (FEM) to determine the optimal forming temperature and predict the glass's shrinkage behavior during cooling. Additionally, simulative analysis is used to determine how the forming tool must be designed to ensure the necessary geometric precision during the forming process.
In the next step, the simulation results were validated through practical experiments. For this purpose, a forming tool was created, and mirror substrates were manufactured. Due to the dimensions of the mirror, a correspondingly large tool was required. Through simulative process design and fine-tuning of process parameters, the desired accuracy was quickly achieved.