EdgeHD is an aplanatic 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.
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 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.
|Learn about EdgeHD Optics straight from Celestron's engineers:|
Many optical designs that advertise themselves as "astrograph" quality actually only produce pinpoint stars across a curved focal plane. While this may be acceptable for visual observing, stars will appear out of focus at the edge when used with a flat chip sensor of a digital camera. 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.
Above: A curved focal plane does not perfectly coincide with a flat CCD chip and therefore 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 below. Both images were taken with the same camera under the same conditions. When focused on-axis both appear to be sharp in the center of the image. But as you view the stars closer and closer to the edge, the stars in the first image (taken with a competing coma-free design) become larger and less focused until finally, at the very corners, the star images are de-focused so much that they appear as rings instead of pinpoints.
Competing Coma Free Optical Design
Celestron EdgeHD Optics
Fig 1 - Competing Coma Free design focused on axis, shows sharp stars in the center of the image, but dramatically out of focus stars (donuts) at the edges.
Fig 2 - EdgeHD focused on axis, focuses sharply across the entire FOV.
Both M3 images taken with a 35 mm chip (42 mm diameter focal plane) camera
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 outer edge, revealing finer detail and greater contrast.
EdgeHD optics give you smaller (more concentrated) stars that create brighter images and allow you to see down to 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 will be able to capture the faintest of galaxies all the way to the corner of your full frame camera chip.
The image below is one 60 second guided image taken with the EdgeHD 11" and 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 on-axis and all the way out to the Edge.
For more information about Celestron's XLT coating, go to the XLT coating web page.
In addition to Celestron's new optimized optical design, the EdgeHD tube has been redesigned to make sure you get the most from your optics each and every night.
Mirror Support - Flexible tension clutches hold the 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, keeping 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 60 micron micro-mesh filter guaranteed to let warm air out without letting dust in.
Fastar Versatility -The EdgeHD are the most versatile imaging scopes available today. At its native f10 you can achieve the image scale neccesary to capture the smallest of deep sky objects. With the optional reducer lens (coming soon) you can increase your field of view without sacrificing optical performance. A barlow gives you the 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 tube has been fitted with a removable secondary mirror and are opto-mechanically aligned on a laser bench to ensure they are axially symmetric in any configuration.
*requires the use of the Hyperstar lens assembly
Easy Collimation - With precision machined mechanics and a spherical secondary mirror, EdgeHD optics can be collimated accurately and easily to guarantee that they perform to their potential every time you use them.