Optical design issues for giant telescopes

A new generation of ground based optical telescopes is now being proposed with diameters up to 100 meters. It is not practical to simply scale up the current successful designs, as this would result in unwieldy structures, extremely high manufacturing costs, and poor performance. This paper summarizes the design for these giant telescopes. ©2002 Optical Society of America OCIS Codes: 350.1260 (Astronomical optics), 220.4830 (Optical systems design) In the 1980’s, many groups around the world were planning, designing, and starting to build a new class of large telescope, with apertures from 6 to 10 meters. In the 1990’s a dozen of these were completed and are now operational. These new telescopes allow measurements with sensitivity that was not possible a few years ago with smaller telescopes. Also very high resolution is obtained when the atmospheric effects are corrected in these telescopes using adaptive optics. Some of these large telescopes are now combined as interferometers, achieving higher resolution yet. What could possibly be better? Enter the era of the really giant telescopes! The groups of astronomers and engineers who recently solved the difficult problems for 6 to 10 meter telescopes are now applying their talents and resources to the design new types of telescopes that increase the aperture (thus the sensitivity) or the field of view (thus the efficiency) by an order of magnitude. It is interesting how different groups have looked at the design problem for giant telescopes. For sizes larger then 8 meters, the primary mirror must be segmented, i.e. the mirror must be constructed from a mosaic of smaller mirrors, all co-pointing and co-phased. The details of the segmentation and the optical design vary widely according to the groups’ experience and engineering choices. Significant design efforts are now underway in the US and in Europe for several of these giant telescopes. There are two groups studying 30-m telescopes in the US. The University of California including Cal. Tech., is developing CELT, the California Extremely Large Telescope. A study is underway by NOAO (National Optical Astronomy Observatory) for a 30 meter Giant Segmented Mirror Telescope GSMT. Both follow conventional optical designs and use 1-m segments. Two efforts in Europe are investigating much larger telescopes. The 100-m OverWhelmingly Large Telescope (OWL) from the European Southern Observatory ESO uses 2-m segments to make a 100-m f/1.4 spherical primary mirror. The baseline optical design uses 5 more mirrors to correct the system, although the group is considering other designs. A consortium led by Lund University in Sweden is studying a 50-m Gregorian type telescope call Euro-50. This telescope uses 2-m segments to create a 50-m f/0.85 primary mirror. The University of Arizona is pursuing a different type of large telescope called 20/20. This telescope, led by Roger Angel, uses twin 21m telescopes on a 100 m circular track to work as a Fizeau imager with adjustable baseline. The primary mirrors are made from 8.4 m segments that can be manufactured at the Steward Observatory Mirror Lab. The DMT (Dark Matter Telescope), also called the LSST (Large Synoptic Survey Telescope) breaks new ground by its combination of large aperture and giant field of view. This three-mirror telescope gives a field of view of 7 square degrees at a final focal ratio of f/1.25. The telescopes currently being developed are shown below: 102 © 2002 OSA/IODC 2002