A FLEXIBLE IMAGING PLATFORM AT AN AFFORDABLE PRICE Superior flat-field, coma-free imaging by the Celestron Engineering Team Ver. 04-2013, For release in April 2013.
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.
EdgeHD Series Edge HD 800 Edge HD 925 Edge HD 1100 Edge HD 1400 FIGURE 1. Celestron’s EdgeHD series consists of four aplanatic telescopes with 8-, 9.25-, 11-, 14-inch apertures. The optical design of each instrument has been individually optimized to provide a flat, coma-free focal plane. Each EdgeHD optic produces sharp images to the edge of the view with minimal vignetting.
OPTICAL ABERRATIONS For those not familiar with the art of optical design, this brief primer explains what aberrations are and how they appear in a telescopic image. OFF-AXIS COMA Coma is an off-axis aberration that occurs when the rays from successive zones are displaced outward relative to the principal (central) ray. A star image with coma appears to have wispy “hair” or little “wings” extending from the image.
THE OPTICAL PERFORMANCE OF THE EDGEHD COMPARED TO OTHER SCTS CLASSIC SCT “COMA-FREE” SCT EDGEHD 100 μm On-Axis 5.00 mm 10.00 mm 15.00 mm 20.00 mm Off-axis distance (millimeters) FIGURE 2. Matrix spot diagrams compare the center-to-edge optical performance of the classic SCT, “coma-free” SCT, and EdgeHD. The EdgeHD clearly outperforms the other optical systems. The classic SCT shows prominent coma.
4. OPTICAL PERFORMANCE OF THE EDGEHD Optical design involves complex trade-offs between optical performance, mechanical tolerances, cost, manufacturability, and customer needs. In designing the EdgeHD, we prioritized optical performance first: the instrument would be diffraction-limited on axis, it would be entirely coma-free, and the field would be flat to the very edge. (Indeed, the name EdgeHD derives from our edge-of-field requirements.
5. MECHANICAL DESIGN IMPROVEMENTS 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.
14” ƒ/10 Coma-Free SCT -0.8 mm -0.4 mm 0.0 mm +0.4 mm 14” ƒ/11 Flat-Field EdgeHD +0.8 mm -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.
We cut corrector blanks from large sheets of the glass, then run them through a double-sided surfacing machine to grind and polish both surfaces to an optical finish. The blanks are inspected and any with defects are discarded. The Johnson/Celestron method for producing the polynomial aspheric curve is based on precision “master blocks” with the exact inverse of the desired curve.
EDGEHD’S CLOSE-UP ON THE PELICAN NEBULA FIGURE 8. After all the testing is done, the ultimate test is the night sky. This close-up image of the Pelican Nebula testifies to the EdgeHD’s ability to focus clean, neat, round star images from center to edge. The telescope was a 14-inch EdgeHD on a CGE Pro Mount; the CCD camera was an Apogee U16m. The mage above shows a 21.5×29.8mm section cropped from the original 36.8mm square image.
Autocollimation Testing Telescope being checked Precision optical flat Beamslitter Eyepiece and Ronchi grating Green Laser (532 nm) FIGURE 9. In autocollimation testing, light goes through an optical system, reflects from a plane mirror, and passes through again. This super-sensitive test method doubles the apparent size of all errors. After we remove each set of optics from the autocollimator, we send the components to our in-house coating chamber.
Throughout the telescope-building process, we maintain a quality-assurance paper trail for each instrument. All test images are numbered and cross referenced. Should a telescope be returned to Celestron for service, we can consult our records to see how well it performed before it left our facility. Once a telescope has passed the FAT, we apply Loctite® to the set screws to permanently hold the alignment of the corrector plate. The instrument is then inspected carefully for cosmetic defects.
Imaging with Celestron EdgeHD Telescopes 1. 2. EdgeHD Aperture Focal Ratio Focal Length Secondary Ø Obstruction1 Back Focus Distance Adapter Thread Size Image Circle Linear Ø Angular Ø Airy Disk Angular Ø Linear Ø Rayleigh2 Image Scale arcsec/pixel (6.4 μm pixel) EdgeHD 800 203.2mm ƒ/10.456 2125mm 68.6mm 34 % 133.35mm 2.00”-24 tpi 42mm Ø 68.0 arcmin 1.36” Ø 14.0μm Ø 0.68” 0.62”/pix EdgeHD 925 235mm ƒ/9.878 2321mm 85.1mm 36 % 146.05mm 3.29”-16 tpi 42mm Ø 62.2 arcmin 1.18” Ø 13.2μm Ø 0.
EdgeHD: The Versatile Imaging Platform Celestron's EdgeHD: The Versatile Celestron’s Imaging Platform EdgeHD 800 5.25 inches 133.35±0.5 mm Small T-Adapter Camera Adapter Digital SLR Reducing Ring Small T-Adapter T-to-1.25” Adapter Webcam Large T-Adapter T-Ring Adapter Digital SLR EdgeHD 925, 1100, and 1400 5.75 inches 146.05±0.5 mm Large T-Adapter T-to-C Adapter Astro Video Camera Large T-Adapter T-system Spacer CCD Camera FIGURE 14.
10. CONCLUSION The classic Schmidt-Cassegrain telescope introduced tens of thousands of observers and imagers to astronomy and nurtured their appreciation for the wonder of the night sky. Today, observers and imagers want a more capable telescope, a telescope that provides sharp close-ups as well as high-quality images all the way across a wide, flat field of view. But, consumers want this advanced optical technology at an affordable price. Celestron has designed the EdgeHD to meet customers’ needs.
Appendix A: Technical Profiles of EdgeHD Telescopes 8-inch When evaluating astronomical telescopes, astroimagers must bear in mind the many factors that influence image quality. The major factors at play are: Airy Disk and Seeing Blurs • The “seeing” quality during exposure • The guiding accuracy during exposure To aid astroimagers, this Appendix presents a spot matrix plot for each of the telescopes in the EdgeHD series.
Celestron EdgeHD 800 Celestron EdgeHD 800 On-axis, the spots show that the 8-inch EdgeHD is diffractionlimited in both green (for visual observing) and red (for imaging). And because blue rays are strongly concentrated inside the Airy disk, the 8-inch EdgeHD is diffraction-limited in blue light. Off-axis, its images remain diffraction-limited over a field larger than the full Moon. For an imager using an APS-C digital SLR camera, relative illumination falls to 84% at the extreme corners of the image.
.Celestron EdgeHD 925 Celestron EdgeHD 925 The spot matrix shows that on-axis images are diffraction limited at all three wavelengths, and remain diffraction-limited over the central 15mm. While blue and red are slightly enlarged, in green light images are fully diffraction-limited over a 38mm image circle. The size of the off-axis blue and red spots remain nicely balanced.
Celestron EdgeHD 1100 Celestron EdgeHD 1100 The 11-inch EdgeHD is optimized to produce its sharpest images in green and red; at these wavelengths it is diffractionlimited over roughly two-thirds of the full 42mm image circle. The relative illumination remains 100% across the central 16mm, then falls slowly to 83% at the very edge of a 42mm image circle. For pictorial images with an APS-C digital SLR camera, flats are unnecessary.
Celestron EdgeHD 1400 Celestron EdgeHD 1400 In the matrix spot diagrams, note the tight cluster of rays in green light, and the well balanced spherochromatism in the blue and red. These spots are far better than spots from a fine apochromatic refractor of the same aperture. In green light, the EdgeHD 1400 is diffraction-limited over a 28mm image circle, although atmospheric seeing enables it to display its full resolution only on the finest nights.
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.
Celestron EdgeHD 0.7× Reducer Celestron EdgeHD 0.7× Reducer Large T-Adapter T-Ring Adapter Digital SLR EdgeHD 1100 and 1400 5.75 inches 146.05±0.5 mm 0.7× Reducer Large T-Adapter T-system Spacer CCD Camera The matrix spot diagrams show that the bulk of rays cluster tightly in or near the Airy disk, with a diffuse scatter most strongly seen in the red light.
Image by André Paquette Imagine the thrill of seeing the first images from your Celestron EdgeHD! A quick glance at the whole image shows that you have captured your target’s faint outer extensions. Across the field, from one side to the other, star images are sharp, crisp, and round. As you process your image, fine details in the target object reveal themselves. Star clouds, delicate dust lanes, subtle HII regions—it’s all there, credit to your skill and the design of your EdgeHD telescope.
The Celestron EdgeHD • ©2013 by Celestron • All rights reserved. Torrance, CA 90503 U.S.A. • www.celestron.
