Celestron CGEM II 800 EdgeHD Telescopes Whitepaper EdgeHD Optics - Page 21

Appendix B: TECHNICAL PROFILE OF THE EDGEHD 0.7× FOCAL REDUCER LENS Perhaps the most useful accessory you can get for an EdgeHD telescope is a focal reducer. Although the long focal length is a great advantage in capturing detailed images of nebulae, galaxies, and especially of planetary nebulae, it also means the field of view is sometimes smaller than desirable. The relatively slow focal ratio also means rather long exposure times. We designed our 0.7× Focal Reducer to provide a field of view 1.4× larger in angular diameter, giving you twice the sky area coverage and halving the exposure time required to reach a given signal-to-noise ratio. If your passion is imaging large deep-sky objects, imaging in Hα, SII, and OIII narrowband, capturing the faint reflection nebulae often found around Barnard's dark objects, or just cutting your exposure (and guiding) times down, the focal reducer is a "must-have" item. Back in the days of film astrophotography, focal reducers came to be poorly regarded. Although they would shorten the focal length, they also produced fuzzy star images, had bad field curvature, and suffered from severe vignetting. But the days of film and ersatz focal reducers are over. The modern EdgeHD focal reducer is the product of optical engineering and precision manufacturing on a par with the design and production of wide- and ultra-wide-field eyepieces. We designed three EdgeHD 0.7x focal reducers, each specifically tailored to the EdgeHD 1400, 1100, and 800, respectively. The 1400 and 1100 reducers contain five precision optical elements, while the 800 contains four elements. To attain a level of performance worthy of the EdgeHD, the designs employ low-dispersion lanthanum rare-earth glass to control both chromatic and geometric aberrations. All optical surfaces are multi-layer anti-reflection coated to maximize light transmission, provide high-contrast images, and minimize image ghosting. The matrix spot diagram shows that star images on-axis are diffraction-limited in green light, while rays at all wavelengths concentrated near the Airy disk. Even at the outer edge of the 42mm image circle, green and blue rays are clustered tightly, while red shows only a weak flare. Both physically and mechanically, the 0.7× Reducer Lens is more than comparable to a top-of-the-line wide-field eyepiece. The CNC-machined housing easily supports the full weight of your CCD camera or digital SLR camera without sag or movement. And for safe storage, each unit is provided with threaded metal covers for both the front and the back. The EdgeHD 0.7× Focal Reducer FIGURE B1. The EdgeHD 0.7x Focal Reducer shortens the focal ratio of the of the EdgeHD 1400, 1100 and 800 while maintaining sharp images across the full field. This enables CCD imagers to reach the same signal-to-noise ratio on extended objects in half the exposure time, and brings even the faintest deep-sky objects within the range of your high-end digital SLR camera. The Celestron EdgeHD 21