Celestron CGEM II 800 EdgeHD Telescopes Whitepaper EdgeHD Optics - Page 13

Imaging with Celestron EdgeHD Telescopes EdgeHD EdgeHD 800 EdgeHD 925 EdgeHD 1100 EdgeHD 1400 Aperture Focal Ratio Focal Length 203.2mm ƒ/10.456 2125mm 235mm ƒ/9.878 2321mm 279.4mm ƒ/9.978 2788mm 355.6mm ƒ/10.846 3857mm Secondary Ø Obstruction1 68.6mm 34 % 85.1mm 36 % 92.3mm 33 % 114.3mm 32 % Back Focus Distance Adapter Thread Size 133.35mm 2.00"-24 tpi 146.05mm 3.29"-16 tpi 146.05mm 3.29"-16 tpi 146.05mm 3.29"-16 tpi 1. The Ø symbol means diameter. Central obscuration is given as a percentage of the aperture. 2. The Rayleigh Limit for resolving doubles with equally bright components. The " symbol means arcseconds. Image Circle Linear Ø Angular Ø 42mm Ø 68.0 arcmin 42mm Ø 62.2 arcmin 42mm Ø 51.8 arcmin 42mm Ø 37.4 arcmin Airy Disk Angular Ø Linear Ø Rayleigh2 1.36" Ø 14.0μm Ø 0.68" 1.18" Ø 13.2μm Ø 0.59" 0.99" Ø 13.3μm Ø 0.50" 0.78" Ø 14.4μm Ø 0.39" Image Scale arcsec/pixel (6.4 μm pixel) 0.62"/pix 0.57"/pix 0.47"/pix 0.34"/pix 9. IMAGING WITH THE EDGEHD The Celestron EdgeHD was designed and optimized for imaging with astronomical CCD cameras, digital SLR cameras, video astronomy sensors, electronic eyepieces, and webcams. We designed the EdgeHD 800 to deliver the best images 5.25 inches (133.35mm) behind the surface of the telescope's rear cell 2.00×24 tpi threaded baffle tube lock nut. The EdgeHD 925, 1100, and 1400 form their best images 5.75 inches (146.05mm) behind the telescope's rear cell 3.290×16 tpi threaded baffle tube lock nut. For best results, the image sensor should be located within ±0.5mm of this back-focus distance. It is easy to place a digital SLR (DSLR) camera at the proper distance using the Small T-Adapter (item #93644) for the EdgeHD 800, or the Large T-Adapter (item #93646) for the EdgeHD 925, 1100, and 1400. The small adapter is 78.35mm long while the large adapter adds 91.05mm, in both cases placing the best focus 55mm behind the T-Adapter. Because 55mm is the industry standard T-mount to sensor distance, add a T-Ring adapter (T-Ring for Canon EOS, item #93419; T-Ring for Nikon, item #93402) and attach your camera to it. That's all there is to placing your digital SLR camera at the correct back-focus location. (By the way, if you've never heard of the T-mount system, you need to know about it. The T-mount is a set of industry standard sizes and distances for camera lenses. A standard T-mount thread (M42×0.75) is available for most astronomical CCD cameras. The standard T-mount flange-tosensor distance is 55mm.) The T-mount system also makes spacing an astronomical CCD camera easy. Consult your CCD camera's documentation to find the flange-to-sensor distance for your CCD camera. Attaching the Celestron T-Adapter to your EdgeHD gives you the standard 55mm spacing. If your CCD's front flange-to-sensor distance is 35mm, you need an additional 20mm distance. A 20mm T-mount Extension Tube, available from many astronomy retailers, will help you achieve the correct back focus distance. If you require a more complex optical train for your CCD camera, consult a trusted astronomy retailer. For imaging, we recommend using T-system components because threaded connections place your CCD camera or digital SLR at the correct back focus distance for optimum performance. Not only are they strong, but they also hold your camera perfectly square to the light path. To mount a high-performance video camera, add the T-Adapter plus a T-to-C adapter. (Like the T-mount system, the C-mount system is an industry standard. It uses 1×32 tpi threads with a back-focus distance of17.5mm.) For consumer video systems such as electronic eyepieces, planetary cameras, and webcams that attach to the telescope using a standard 1.25-inch eyepiece barrel, simply use the same components that you use for visual observing. Just remove the eyepiece from the telescope and replace it with the camera. Many imaging programs allow you to shoot short exposures through the telescope. On a solid, polar-aligned equatorial mounting, you may be able to expose for 30 seconds or more. With such exposure times, you can capture wonderful images of the Moon, planets, eclipses, bright star clusters, and objects like the Orion Nebula. However, for long exposures on deep-sky objects, you will need to guide the telescope. The days of guiding by eye are over; electronic auto-guiders are the new standard. A functional and relatively inexpensive autoguiding setup consists of a small refractor mounted piggyback on your EdgeHD telescope. You will need a dovetail bar attached to the EdgeHD tube. Celestron offers an 80mm guide telescope package (item #52309) to be used with the NexGuide Autoguider (item #93713). For sub-exposures exceeding 10 minutes or so, piggybacked guide telescopes potentially suffer from differential flexure; for such imaging, consider the Off-Axis Guider (item #93648). The Celestron EdgeHD 13