EdgeHD is an aplanatic flat-field Schmidt telescope designed to produce aberration-free images across a wide visual and photographic field of view. The optical system was designed to reduce more than just off-axis star coma, but also to give an astrograph-quality flat focal plane all the way to the edge of the field of view.
True Astrograph Quality
Many optical designs advertise themselves as “astrographs”; in reality, these only produce pinpoint stars across a curved focal plane. This may be acceptable for visual observing, but when used with the flat sensor of a digital camera, stars appear out of focus at the edges. EdgeHD optics produce a focal plane more than three times flatter than a standard Schmidt-Cassegrain telescope and dramatically flatter than other competing coma-free designs. This guarantees you visibly sharp stars across some of the largest CCD chips available today.
A curved focal plane does not perfectly coincide with a flat CCD chip. Therefore, stars will appear out of focus at the edges.
Even when focused sharply in the center of the field of view, a curved focal plane will produce out-of-focus stars at the edges. The wider the field of view, the more pronounced the defect becomes. Compare the two images of M3 below; the first was taken with a competing coma-free optical design and the second with EdgeHD. Both images were taken with the same 35 mm chip (42 mm diameter focal plane) camera under the same conditions.
When focused on-axis, both images appear to be sharp in the center. But in the first image, as you examine the stars closer and closer to the edge, the stars become larger and less focused until finally, at the very corners, the stars are so defocused that they appear as rings instead of pinpoints.
Now look at the second image. As you move from the center outward, the stars retain their crisp focus from edge to edge, appearing as crisp, solid points of light.
Fig. 1: The competing coma-free design, when focused on the axis, shows sharp stars in the center of the image but dramatically out-of-focus stars (rings or donuts) at the edges.
Fig. 2: EdgeHD, when focused on axis, produces a sharp focus across the entire FOV.
Superior edge performance not only creates rounder, more pleasing stars, but actually improves the resolution and limiting magnitude when compared to telescopes of equal aperture.
Celestron’s High Definition optics let you resolve smaller spot sizes all the way to the outer edge, revealing finer detail and greater contrast. Compare the two cropped images below, both taken with a 37 mm chip (52 mm diameter focal plane) camera:
Fig. 1 (left): The off-axis coma inherent in many optical designs (such as the SCT shown above) distorts star images and compromises resolution.
Fig. 2 (right): EdgeHD focuses sharply across the entire FOV, revealing fine detail lost with other systems.
EdgeHD optics give you smaller, more concentrated stars that create brighter images and allow you to see a fainter magnitude than other equally-sized telescopes. Poor edge quality can spread out starlight so much that the brightness of a star appears the same as the sky background, making it undetectable to your eye or camera. With EdgeHD optics, you can capture the faintest of galaxies all the way to the corner of your full-frame camera chip. Compare the two cropped images below, both taken with a 37 mm chip (52 mm diameter focal plane) camera:
Fig. 1 (left): In telescopes that exhibit off-axis coma, faint starlight can go unnoticed at the edge of the field of view.
Fig. 2 (right): EdgeHD optics resolve faint detail missed in telescopes of equal aperture.
See For Yourself
The image below is one 60-second guided image taken with the EdgeHD 1100 and a Canon 5D (full-frame) camera. The field of view represents almost one full degree from corner to corner. Enlarge this image to 100% and inspect the star images both on-axis and all the way out to the edge.
StarBright XLT Coating
All EdgeHD optical tubes feature Celestron’s StarBright XLT coating system on all mirror and lens surfaces. With this dynamic combination, you get maximum light throughput across the widest visual and photographic spectrum. For more information about StarBright XLT, visit its technology page.
The EdgeHD tube features an innovative mechanical design to ensure you get the most from your optics each and every night:
- Mirror support: Flexible tension clutches hold the primary mirror in place and reduce image shift when rotating the tube around the mount. The flexible rods allow the mirror to be held in place without putting any force or pressure on the mirror assembly. This keeps the image centered in the eyepiece or chip.
- Tube vents: Cooling vents located on the rear cell allow hot air to be released from behind the primary mirror. Each vent has an integrated micromesh filter guaranteed to let warm air out without letting dust in.
- Fastar versatility: EdgeHD is the most versatile imaging telescope available today. At its native f/10, you can achieve the image scale necessary to capture the smallest of deep-sky objects. With the optional EdgeHD reducer lens, you can increase your field of view without sacrificing optical performance. A Barlow lens gives you added power for high-resolution planetary, lunar, and solar imaging. And, of course, all EdgeHD optical tubes are Fastar-compatible for ultra-fast f/2 wide field imaging. Each EdgeHD tube is fitted with a removable secondary mirror and is opto-mechanically aligned on a laser bench to ensure they are axially symmetric in any configuration.
- Easy collimation: Precision-machined mechanics and a spherical secondary mirror mean that EdgeHD optics can be collimated accurately and easily to guarantee ideal performance every time.
The Celestron Product Development team has released a whitepaper outlining the technology behind EdgeHD. This paper describes the development goals, design decisions, optical performance, and their practical realization in EdgeHD’s 8-, 9.25-, 11-, and 14-inch apertures. We include cross-sections of the EdgeHD series, comparative spot diagrams for the EdgeHD and competing “coma-free” Schmidt-Cassegrain designs, a table with specifications for visual and imaging, graphics showing how to place sensors at the optimum back-focus distance, and details on the construction and testing of the EdgeHD telescope series. Download a PDF of the EdgeHD Whitepaper here.