The fore-optics design builds on proven technology by adopting a hybrid Ritchey-Chretien system to achieve excellent image quality over as wide a field as possible. The choice of this simple design also minimizes construction delays for the telescope, which is traditionally a long lead-time item. Our baseline concept assumes an 8.4-m f/1.6 hyperbolic primary and a 2.4-m secondary (~8% central obscuration) which produce a f/4.3 beam (plate scale of 5.3''/mm) at a forward Cassegrain focus. We will be investigating faster focal ratios that may reduce the overall cost of the telescope, but the present combination produces a field which can be corrected by currently buildable (and affordable!) optics. The reflecting surfaces will be coated with a protected silver coating in order to maximize the reflectivity over the 0.36-1.7micron wavelength range.
The curved field delivered at the secondary focus is corrected using a two-element aspheric fused silica corrector, each roughly 1-m in diameter (Figure 15). This results in a corrected 1.5o diameter circular field with excellent image quality (figure 16): the 80% encircled energy diameters for white light (i.e., 0.36-1.7 micron) are less than 0.25'' over the entire 1.5o field! For a typical observation spanning an octave in wavelength, the 80% image diameter is <0.2 arcsec. In order to compensate for atmospheric dispersion, an ADC assembly is inserted between the elements of the corrector assembly (Figure 15). The ADC is constructed from fused silica and LLF6 glasses. The latter introduce a roll-off in the transmission below 3600A; we are investigating alternate designs that do not require LLF6, but the present design works well and meets the science requirements. The elements of the corrector and ADC are roughly 1-m in diameter. Obtaining good quality glass of this size is possible, but may require a special run at Schott. This difficulty is reflected in the cost estimate for the corrector.
