Celestron CGEM II 800 EdgeHD Telescopes Whitepaper EdgeHD Optics - Page 8

14" ƒ/10 Coma-Free SCT -0.8 mm -0.4 mm 0.0 mm +0.4 mm +0.8 mm 14" ƒ/11 Flat-Field EdgeHD -0.8 mm -0.4 mm 0.0 mm +0.4 mm +0.8 mm On-axis 5 mm off-axis 10 mm off-axis 15 mm off-axis 20 mm off-axis Spot diagrams plotted for 0.0, 5, 10, 15, and 20mm off-axis; showing λ = 0.486, 0.546, and 0.656μm. FIGURE 5. In a 14-inch coma-free SCT, the smallest off-axis star images lie on the curved focal surface indicated by the gray line. Since CCD or digital SLR camera sensors are flat, so star images at the edge of the field will be enlarged. In the aplanatic EdgeHD design, the smallest off-axis images lie on a flat surface. Stars are small and sharp to the edge of the field. Telescope tubes must "breathe" not only to enable cooling, but also to prevent the build-up of moisture and possible condensation inside the tube. In the classic SCT, air can enter through the open baffle tube. In the EdgeHD, the sub-aperture lenses effectively close the tube. To promote air exchange, we added ventilation ports with 60µm stainless steel mesh that keeps out dust but allows the free passage of air. In a telescope designed for imaging, users expect to attach heavy filter wheels, digital SLRs, and astronomical CCD cameras. We designed the rear threads of the EdgeHD 925, 1100, and 1400 telescopes with a heavy-duty 3.290×16 tpi thread, and we set the back focus distance to a generous 5.75 inches from the flat rear surface of the baffle tube locking nut. The rear thread on the EdgeHD 800 remains the standard 2.00×24 tpi, and the back-focus distance is 5.25 inches. Many suppliers offer precision focusers, rotators, filter wheels, and camera packages that are fully compatible with the heavy-duty rear thread and back focus distance of the EdgeHD. 6. MANUFACTURING THE EDGEHD OPTICS Each EdgeHD has five optical elements: an aspheric Schmidt corrector plate, a spherical primary mirror, a spherical secondary mirror, and two sub-aperture corrector lenses. Each element is manufactured to meet tight tolerances demanded by a highperformance optical design. Celestron applies more than forty years of experience in shaping, polishing, and testing astronomical telescope optics to every one of the components in each EdgeHD telescope. Our tight specifications and repeated, careful testing guarantee that the telescope will not only perform well for highpower planetary viewing, but will also cover a wide-angle field for superb edge-to-edge imaging. Nevertheless, we don't take this on faith; both before and after assembly, we test and tune each set of optics. FIGURE 6. The mirror clutch mechanism shown in this crosssection prevents the primary mirror from shifting during the long exposures used in imaging. Celestron's founder, Tom Johnson, invented the breakthrough process used to make Celestron's corrector plates. Over the years, his original process has been developed and refined. At present, we manufacture corrector plates with the same level of ease, certainty, and repeatability that opticians expect when they are producing spherical surfaces. Each corrector plate begins life as a sheet of water-white, hightransmission, low-iron, soda-lime float glass. In manufacturing float glass, molten glass is extruded onto a tank of molten tin, where the glass floats on the dense molten metal. The molten tin surface is very nearly flat (its radius of curvature is the radius of planet Earth!), and float glass is equally flat. 8 I The Celestron EdgeHD