The optical design for the LSST is a highly optimized compact three-mirror telescope system feeding a large refractive corrector producing a 3.5 degree field of view covering a 64 cm diameter flat focal plane. The LSST covers an on-sky area of 9.62 square degrees, nearly 50 times the area of the full moon. The starting point for this design was the Dark Matter Telescope proposed by Angel et al. (2000; see references in Historical Documents: Optical Design for an 8m Telescope with a 3 Degree Field at f/1.25: The Dark Matter Telescope [PDF 620K] ).
The LSST telescope consists of three aspheric mirrors; an 8.4-m primary (M1), a 3.4-m convex secondary (M2), and a 5.0-m tertiary (M3). The inner primary and outer tertiary edges are such that these mirrors can be fabricated from a single piece of Ohara E6 low expansions glass. The secondary mirror will be the largest convex mirror ever made. Details of these mirrors can be found in the mirror design section. The light path through the telescope is shown in the diagram on the right.
The LSST optical performance is superb. The image quality as measured by the 80% encircled energy image is better than 0.3 arcseconds for all spectral bands (ugrizY); for the rizY spectral bands, the 80% encircled energy is ~0.2 arcseconds or better (see figure below). The throughput of the LSST is 62.55% on-axis out to a field radius of 0.7 degrees, and then gradually drops to 57% at full field. The resulting etendue (FOV ◊ collecting area) is 319.5 m2deg2.
Some of the dimension details of the LSST optical design are shown below. The system length is a very compact 6.4 m from secondary vertex to tertiary vertex. The LSST primary mirror is highly annular 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 is used to accommodate the camera body and its associated readout electronics. The hole in the tertiary is used to mount equipment for maintaining precise control over the LSST optical alignment.
The refractive corrector consists of three large fused silica lenses and a filter. The largest lens (L1) is 1.55 m in diameter, half again as a large as the 40-in Yerkes refractor. The 0.69-m third lens (L3) serves as the vacuum barrier for the focal plane array (FPA) cryostat hence requires a center thickness of 60 mm. The filters for LSST are very large with a clear aperture of 0.75 m.