In an incredible technical achievement, the primary mirror will be built out of 798 hexagonal segments, each one 1.4 meters in size, as measured from corner-to-corner. ESO/Sergio Dalle Ave & Roberto Ragazzoni (INAF-OAPD) The segmented structure is necessary for a telescope of this size and weight, particularly at the desired optical accuracy. mirror, assembled next to the Asiago Astrophysical Observatory near Asiago, Italy. This aerial image shows a 1:1 scale model of the European Extremely Large Telescope's primary. With material manufactured by SCHOTT, made out of their unique, low-expansion ZERODUR ® material, and then polished by SAFRAN-REOSC, the ELT will boast the largest primary mirror of any optical telescope in humanity's history. You cannot build a single mirror that large to that level of smoothness, so the only option is to do it in segments. (For comparison, Hubble's area is 4.5 square meters.) The surface needs to be smooth down to an incredible 7.5 nanometers: just 1/100th the size of the wavelengths of light it will collect. In order to build a telescope this large, you need to build an effective surface that's properly shaped to focus the incoming light across an area 39 meters in diameter with a large hole in the center: the equivalent of 1000 square meters. I had the opportunity to speak with Marc Cayrel, the project manager of the optics - the eyes of the telescope - for the ELT. Bill Saxton, NRAO/AUI/NSFīut the key to it all is the size and quality of the primary mirrors. With the ELT, new views of a protoplanetary disk like this, including in the optical, will become possible at last. The evolving protoplanetary disk, with large gaps, around the young star HL Tauri.
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