Celestron CGX Equatorial 800 HD Telescopes Whitepaper EdgeHD Optics - Page 2

The Celestron EdgeHD A Flexible Imaging Platform at an Affordable Price By the Celestron Engineering Team ABSTRACT: The Celestron EdgeHD is an advanced, flat-field, aplanatic series of telescopes designed for visual observation and imaging with astronomical CCD cameras and full-frame digital SLR cameras. This paper describes the development goals and design decisions behind EdgeHD technology and their practical realization in 8-, 9.25-, 11-, and 14-inch apertures. We include cross-sections of the EdgeHD series, a table with visual and imaging specifications, and comparative spot diagrams for the EdgeHD and competing "coma-free" Schmidt-Cassegrain designs. We also outline the construction and testing process for EdgeHD telescopes and provide instructions for placing sensors at the optimum back-focus distance for astroimaging. 1. INTRODUCTION The classic Schmidt-Cassegrain telescope (SCT) manufactured by Celestron served an entire generation of observers and astrophotographers. With the advent of wide-field and ultra-wide-field eyepieces, large format CCD cameras, and full frame digital SLR cameras, the inherent drawbacks of the classic SCT called for a new design. The EdgeHD is that new design. The EdgeHD offers clean, diffraction-limited images for high power observation of the planets and the Moon. As an aplanatic, flat-field astrograph, the EdgeHD's optics provide tight, round, edge-to-edge star images over a wide, 42mm diameter flat field of view for stunning color, monochrome, and narrow-band imaging of deep sky objects. 2. SETTING GOALS FOR THE EDGEHD TELESCOPE The story of the EdgeHD began with our setting performance goals, quality goals, and price goals. Like the classic SCT, the new Celestron optic would need to be light and compact. Optically, we set twin goals. First, the new telescope had to be capable of extraordinary wide-field viewing with advanced eyepiece designs. Second, the optic had to produce sharp-tothe-edge astrophotography with both digital SLR cameras and astronomical CCD cameras. Finally, we wanted to leverage Celestron's proven ability to manufacture high-performance telescopes at a consumer-friendly price point. In short, we sought to create a flexible imaging platform at a very affordable price. Given an unlimited budget, engineering high-performance optics is not difficult. The challenge Celestron accepted was to control the price, complexity, and cost of manufacture without compromising optical performance. We began with a comprehensive review of the classic SCT and possible alternatives. Our classic SCT has three optical components: a spherical primary mirror, a spherical secondary mirror, and a corrector plate with a polynomial curve. As every amateur telescope maker and professional optician knows, a sphere is the most desirable optical figure. In polishing a lens or mirror, the work piece moves over a lap made of optical pitch that slowly conforms to the glass surface. Geometrically, the only surfaces that can slide freely against one another are spheres. Any spot that is low relative to the common spherical surface receives no wear; any spot that is higher is worn off. Spherical surfaces result almost automatically. A skilled optician in a well-equipped optical shop can reliably produce near-perfect spherical surfaces. Furthermore, by comparing an optical surface against a matchplate-a precision reference surface-departures in both the radius and sphericity can be quickly assessed. In forty years of manufacturing its classic Schmidt-Cassegrain telescope, Celestron had fully mastered the art of making large numbers of essentially perfect spherical primary and secondary mirrors. In addition, Celestron's strengths included the production of Schmidt corrector plates. In the early 1970s, Tom Johnson, Celestron's founder, perfected the necessary techniques. Before Johnson, corrector plates like that on the 48-inch Schmidt camera on Palomar Mountain required many long hours of skilled work by master opticians. Johnson's innovative production methods made possible the volume production of a complex and formerly expensive optical component, triggering the SCT revolution of the 1970s. For more than forty years, the SCT satisfied the needs of visual observers and astrophotographers. Its performance resulted from a blend of smooth spherical surfaces and Johnson's unique method of producing the complex curve on the corrector with the same ease as producing spherical surfaces. As the 21st century began, two emerging technologies -wide-field eyepieces and CCD cameras-demanded highquality images over a much wider field of view than the classic SCT could provide. Why? The classic SCT is well-corrected optically for aberrations on the optical axis, that is, in the exact center of the field of view. Away from the optical axis, however, its images suffer from two aberrations: coma and field curvature. Coma causes off-axis star images to flare outward; field curvature causes images to become progressively out of focus away from the optical axis. As wide-field eyepieces grew in popularity, and as observers equipped themselves with advanced CCD cameras, the classic SCT proved inadequate. To meet the requirements of observers, we wanted the new Celestron optic to be both free of coma and to have virtually zero field curvature. 2 I The Celestron EdgeHD