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J.R.P. Angel (Steward Obs., U. Arizona), C.F. Claver (NOAO), R. Sarlot, H.M. Martin (Steward Obs., U. Arizona), J.H. Burge (Steward Obs. and Optical Sci., U. Arizona), J.A. Tyson, D. Wittman (Lucent Technologies), Kem Cook (LLNL)
The Large Synoptic Survey Telescope will be a facility for digitally surveying the entire visible sky for the purpose of cataloging Earth crossing asteroids, exploring the nature of dark matter and dark energy in the universe and opening the faint optical transient time window on the universe. This concept was strongly endorsed by the National Academy of Sciences in their report "Astronomy and Astrophysics in the New Millenium". In response to this endorsement we present here the design of an 8.4m modified Paul telescope that expands the etendue ("A - Omega") product by a factor of 20-50 beyond any previously realized design.
The telescope presented here will deliver a 3 degree diameter field of view (7 sq. degrees) over the wavelength range 0.3-1\mum. The plate scale of 50 microns/arcsec (f/1.25) is chosen to match the pixel size of a large mosaic CCD detector 0.5 m in diameter. The primary and secondary mirrors are strongly aspheric. The f/1 primary can be made using polishing techniques and metrology methods pioneered at the Mirror Lab for the 8.4 m f/1.1 LBT primaries. The 3.5 m convex secondary is twice the size of the largest secondary yet manufactured, the 1.7 m MMT f/5 secondary. The current proven method for testing during manufacture uses a full size hologram on a transmissive element and would be expensive to scale up. Alternates involving mechanical metrology and sub-aperture optical tests are under consideration.
In operation the primary mirror figure accuracy will be maintained by active optics, as is the practice for all telescope mirrors >= 6.5 m diameter. To maintain the design image quality of >80 field, the optical system must be held in accurate alignment also by an active system. We have explored the tolorances to misalignment, and find that uncompensated errors in decenters and tilts must be kept less than a few tens of microns. A number of techniques are available for continuous or periodic metrology that should allow realization of this goal. They include comercial laser ranging which is now accurate to 7 microns, retro-reflectors, holograms located on the mirrors and wavefront analysis across the field. Correction may be applied to the elements directly, or for some period could be made using the secondary mirror alone as a compensator.