Celestron CGEM II 800 EdgeHD Telescopes Whitepaper EdgeHD Optics - Page 7

5. MECHANICAL DESIGN IMPROVEMENTS To ensure that the completed EdgeHD telescope delivers the full potential of the optical design, we also redesigned key mechanical components. With classic SCT designs, for example, an observer could bring the optical system to focus at different back focus distances behind the optical tube assembly, changing effective focal length of the telescope. This caused on-axis spherical aberration and increased the off-axis aberration. In the EdgeHD series, the back focus distance is optimized and set for one specific distance. Every EdgeHD comes equipped with a visual back that places the eyepiece at the correct back focus distance, and our Large T-Adapter accessory automatically places digital SLR cameras at the optimum back focus position. As part of the optical redesign, we placed the primary and secondary mirrors closer than they had been in the classic SCT, and designed new baffle tubes for both mirrors that allow a larger illuminated field of view. To ensure full compatibility with the remarkable Starizona Hyperstar accessory that enables imaging at ƒ/1.9 in the EdgeHD 800 and ƒ/2.0 in the EdgeHD 925, 1100, and 1400, all EdgeHDs have a removable secondary mirror. Because it covers a wide field of view, the optical elements of the EdgeHD must meet centering and alignment tolerances considerably tighter than those of the classic SCT design. For example, because the corrector plate must remain precisely centered, we secure it with alignment screws tipped with soft Nylon plastic. The screws are set on the optical bench during assembly while we center the corrector plate. Once this adjustment is perfect, the screws are tightened and sealed with Loctite® to secure the corrector in position. This seemingly small mechanical change ensures that the corrector plate and the secondary mirror mounted on the corrector plate stay in permanent optical alignment. Centering the primary mirror is even more demanding. In the classic SCT, the primary mirror is attached to a sliding "focus" tube. When you focus the telescope, the focus knob moves the primary mirror longitudinally. When you reverse the direction of focus travel, the focus tube that carries the primary can "rock" slightly on the baffle tube, causing the image to shift. In the classic SCT, the shift does not significantly affect on-axis image quality. However, in the EdgeHD, off-axis images could be affected. Because the baffle tube carries the sub-aperture corrector inside and the primary mirror on the outside, we manufacture it to an extremely tight diametric tolerance. The tube that supports the primary was redesigned with a centering alignment flange, which contacts the optical (front) surface of the primary mirror. When the primary mirror is assembled onto the focus tube and secured with RTV adhesive, this small mechanical change guarantees precise optical centration. Following assembly, the focus tube carrying the primary is placed in a test jig. We rotate the mirror and verify that the primary is precisely squared-on to ensure that the image quality expected from the optics is maintained. In any optical system with a moveable primary mirror, focus shift-movement of the image when the observer changes focusing direction-has been an annoyance. In Celestron's SCT and EdgeHD telescopes, we tightened the tolerances. During assembly and testing, we measure the focus shift; any unit with more than 30 arcseconds focus shift is rejected and returned to an earlier stage of assembly for rework. In the classic SCT, astrophotographers sometimes experience an image shift as the telescope tracks across the meridian. The focus mechanism serves as one support point for the mirror. In the EdgeHD, we added two stainless steel rods to the back of the cell that supports the primary mirror. When the two mirror clutches at the back of the optical tube assembly are engaged, aluminum pins press against the stainless steel rods, creating two additional stabilizing support points (see Figure 6). 8" ƒ/10 Coma-Free SCT -0.8 mm -0.4 mm 0.0 mm +0.4 mm +0.8 mm 8" ƒ/10 Flat-Field EdgeHD -0.8 mm -0.4 mm 0.0 mm +0.4 mm +0.8 mm On-axis 3.5 mm off-axis 7 mm off-axis 10.5 mm off-axis 14 mm off-axis Spot diagrams plotted for 0.0, 3.5, 7, 10.5, and 14 mm off-axis; showing λ = 0.486, 0.546, and 0.656 μm. FIGURE 4. Compare star images formed by a 8-inch coma-free SCT with those formed by an EdgeHD. The sharpest star images in the coma-free SCT follow the gray curve, coming to focus approximately 0.6mm in front of the focal plane. In the EdgeHD, small, tight star images are focused at the focal plane across the field of view, meaning that your images will be crisp and sharp to the very edge. The Celestron EdgeHD 7