Opening a Window of Discovery on the Dynamic Universe

LSST Optical Design

The optical elements of LSST appear suspended over the coplanar primary/tertiary mirror. The secondary mirror, camera lenses and filters are also visible.

LSST views a 9.62 square-degree patch of sky, more than 40 times the area of the full moon. This huge viewing area is the result of the telescope's unique optical design: a highly optimized, compact, three-mirror telescope augmented by a large refractive corrector, the whole capable of delivering a 3.5 degree field of view covering a 64 cm diameter flat focal plane. The starting point for this design was the Dark Matter Telescope proposed by Roger Angel, director of the Steward Observatory Mirror Lab at the University of Arizona, and his collaborators in 2000.

A cut-away view of the primary/tertiary mirror, showing the honeycomb structure used to reduce the weight of the huge piece of glass.

In the top picture, light from a source in the sky reflects off the three mirror surfaces, through the refractive lenses and is captured by the detector in the camera. The fast optical system coupled with the large collecting area of the mirror and sensitive detectors makes it possible to take very deep images with a very large field of view compared to other 8m class telescopes (right). Over 10 years, more than 800 images will be taken of each point on the sky.

The LSST telescope consists of three aspheric (nonspherical) mirrors: an 8.4-m primary mirror (M1), a 3.4-m convex secondary mirror (M2), and a 5.0-m tertiary mirror (M3). The inner primary and outer tertiary mirrors were fabricated from a single piece of Ohara E6 low expansions glass. The secondary mirror is the largest convex mirror ever made. Details of these mirrors can be found in the mirror design section.

A comparison of the field of view on the sky for a typical 8m class telescope and the LSST field of view enabled by this unique design.

The system length is a very compact 6.4 m from the vertex of the secondary mirror to the vertex of the tertiary mirror. The LSST primary mirror is highly annular (ring-shaped), having an outer clear aperture of 8.36 m and an inner diameter of 5.12 m, giving an effective collecting area of a 6.67 m filled aperture. The 1.8 m diameter hole in the secondary mirror holds the camera body and associated readout electronics. The hole in the tertiary mirror is used to mount equipment for maintaining precise control of LSST's optical alignment.

The camera is located after M3. Its optics consists of three large fused-silica lenses and a filter. The largest lens (L1) is 1.55 m in diameter, half again as large as the 40 inch Yerkes refractor—the world's biggest astronomical refracting telescope. The 0.69 m third lens (L3) also serves as the vacuum barrier for the focal plane array (FPA) cryostat and requires a center thickness of 60 mm. LSST's filters are very large, with a clear aperture of 0.75 m. All of these attributes give LSST superb optical performance. 

The wavelengths of light for which the optical system is optimized range from ultraviolet to near-infrared; these wavelengths are divided into spectral bands labeled u, g, r, i, z, & y to identify wavelengths of specific interest. For example, an infrared, or "r" filter might be used when observing areas obscured by dust, as infrared wavelengths can pass through the dust and the filter will enable imaging of sources behind the dust.

The image quality is better than 0.3 arcseconds (1/3600 of a degree) for all bands as measured by the 80% encircled energy of the image of a point source, which is a standard measure in optics of how well the optical system focuses the gathered light. For the longer wavelengths corresponding to the r, i, z & y spectral bands, the 80% encircled energy is ~0.2 arcseconds or better. Typically imaging will then be limited by the atmosphere, which averages around 0.7 arcseconds.

The throughput of LSST, or the percentage of an object's light the optical system can capture, is 63% at the center of the detector (or image plane) out to a field radius of 0.7 degrees, gradually dropping to 57% at full field (or 1.75 degrees). The resulting etendue (the product of aperture area in square meters and field of view in square degrees)  is 319.5 m2deg2

Image Credit: 
Top to bottom: Todd Mason, Mason Productions Inc. / LSST Corporation; LSST Project Office; LSST Corporation; LSST Corporation

Financial support for LSST comes from the National Science Foundation (NSF) through Cooperative Agreement No. 1258333, the Department of Energy (DOE) Office of Science under Contract No. DE-AC02-76SF00515, and private funding raised by the LSST Corporation. The NSF-funded LSST Project Office for construction was established as an operating center under management of the Association of Universities for Research in Astronomy (AURA).  The DOE-funded effort to build the LSST camera is managed by the SLAC National Accelerator Laboratory (SLAC). 

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