The EdgeHD design is an “aplanatic Schmidt-Cassegrain”. What that essentially boils down to is a Schmidt-Cassegrain with corrector lenses inside the telescope’s baffle tube to compensate for edge-of-field aberrations inherent in the regular SCT optical design. The exact specifications beyond this are largely proprietary. The EdgeHD can be compared to Meade’s “ACF” (formerly RCX) line but features mechanical improvements the ACF telescopes lack, along with a greater range of focal reducer options—namely in the form of Starizona’s HyperStar system.
Apart from the corrector lenses, the EdgeHD 8 works like any other Schmidt-Cassegrain. There’s no significant loss of light gathering ability to the lenses; the optics are all coated with Celestron’s Starbright XLT coatings; and visually, you’d probably be hard-pressed to tell the difference between the EdgeHD and a regular 8” Schmidt-Cassegrain unless you are extremely particular about the way stars at the very edge of the field of view look in a low-power, wide-angle 2” eyepiece. The EdgeHD’s correction of slight coma and a less-than-flat field are unnoticeable with the vast majority of eyepieces, which either have aberrations of their own towards the edge of the field or have an insufficiently wide field to reveal any coma. Most eyepieces, especially wide-angle, do not have flat fields anyway, which tends to cancel out or obviate the field curvature inherent in a normal SCT. The real advantages of the EdgeHD optics only really show up with a camera.
Besides the EdgeHD optics, the only other significant change from the EdgeHD 8 compared to a normal 8” SCT such as those offered by Celestron is that the baffle tube is actually 2” in diameter, so there is no vignetting with even the widest possible 2” eyepieces or large format camera sensors.
Mechanics of EdgeHD 8
The EdgeHD 8 focuses the same way almost all SCTs do, by sliding the primary mirror along a threaded rod inside the telescope. To combat potential “mirror flop” during long exposures, Celestron has added a pair of locking knobs to secure the primary mirror in a fixed position once focus has been reached.
To collimate the EdgeHD 8, you adjust the tip and tilt of the secondary mirror with a set of 3 Philips head screws. Some people opt to replace these with thumb screws. However, we don’t recommend this as thumbscrews have a tendency to not remain stiff and tight enough to hold collimation during a car trip or even just a long imaging session.
One of the improvements Celestron has made to the EdgeHD optical tubes is a set of rear vents to help accelerate cooldown time; warm air trapped inside the tube can hinder the ability of the scope to achieve sharp images. These vents also have HEPA-grade filters so that they don’t let in pollen, dust, and other debris, which could contaminate the optics of the telescope and is difficult to remove. Another improvement comes in the form of an attached carry handle on the back end of the telescope tube.
As with all of Celestron’s SCTs except for the C5, the EdgeHD 8 is compatible with Starizona’s HyperStar system, a series of corrector lenses that convert the telescope into an f/2 Schmidt camera by replacing the secondary mirror. However, if your main goal is to use this scope at f/2, we’d recommend just getting Celestron’s 8” RASA, as it’s cheaper than outfitting the EdgeHD 8 with a Hyperstar, has a better focusing system designed for the tolerances of an f/2 telescope, and is more accommodating of different sensors.
Attaching accessories to the back of the EdgeHD 8 is the same as with any SCT with standard rear port threads, while the scope is offered with your choice of either a Vixen-style dovetail or a “CGE”-style dovetail, which is for all intents and purposes identical to a Losmandy D-style dovetail plate. You can attach a second dovetail to the top of the tube to piggyback accessories such as additional carrying handles or a guide scope.
The EdgeHD 8 comes with a 1.25” visual back, a 1.25” prism star diagonal, and a 40mm, 1.25” barrel Plossl eyepiece which provides 51x magnification. We would recommend you replace the diagonal with a 2” model if you plan on doing much viewing, but the included 1.25” prism does the job. The 40mm Plossl, while quite sharp and comfortable to use, has a narrow field of view of only about 40 degrees – compared to a typical Plossl at 50-55 degrees. This limitation is a concession of it being stuffed into a 1.25” diameter.
For a finderscope, the EdgeHD 8 is provided with a 9x50mm straight-through finderscope mounted on a rather non-standard bracket on the side of the telescope. This finder will show you stars a few magnitudes fainter than the naked eye and has a field of view around 5 degrees, or 10 full Moons, across with an upside-down view and standard crosshairs. Since the EdgeHD 8 is almost certainly going to be something you use on a GoTo mount, this finder is arguably overkill. and if you’re concerned about weight, replacing it with a lightweight, zero-power red dot or reflex sight would be a very wise decision.
The EdgeHD 8 with Celestron’s 0.7x reducer is still more than 1400mm in focal length and thus needs a beefy, accurate, and stable GoTo German equatorial mount to hold it for any kind of imaging – and you shouldn’t make any concessions even if you’re using it with a Hyperstar at f/2. Your rig is probably going to weigh over 20-25 pounds when all is said and done. Thus, a Sky-Watcher EQ6R, Celestron CGX, or similarly heavy-duty mount is the minimum payload class we would recommend for supporting this telescope.
For visual astronomy, an HEQ5-class mount will do, as will Celestron’s NexStar Evolution mount, but for best results, we would still stick with something like an EQ6R.
Should I buy a Used Celestron EdgeHD 8?
A used EdgeHD 8 is a fine scope; there’s a little more to worry about with a poorly-maintained unit but Celestron will help you if anything is amiss. Obviously, if there is significant fungus, corrosion, or a cracked front Schmidt corrector plate, the scope should be avoided.
Aftermarket Accessory Recommendations
The EdgeHD 8 definitely needs a dew shield. A basic flexible dew shield such as those offered by Celestron or AstroZap is really all you need (or you can make your own), but heated plastic and metal dew shields are also an option for those in very humid climates. A dew shield keeps condensation from forming on your front corrector on cold and damp nights, which can fog up images and eat away at the StarBright XLT coatings over time. It also makes it harder to accidentally touch the corrector plate. Additionally, however, a dew shield keeps stray light from entering the telescope, be it from direct nearby sources like streetlights and screens, moonlight, bright stars and planets, or even some light pollution. Similarly to a lens shade, a dew shield slightly improves contrast visually in the telescope compared to using it without one.
The EdgeHD 8 can fully illuminate the field of a 2” eyepiece, so a focal reducer isn’t really necessary or recommended for visual use. For astrophotography, however, deep-sky imaging at f/10 and 2032mm focal length is not the best idea and as such we’d almost certainly recommend you pick up Celestron’s 0.7x reducer made specifically for the EdgeHD 8, which brings the scope down to f/7 and about 1425mm in focal length. It screws onto the back of the telescope ahead of your camera or off-axis guider. You can again also purchase the Starizona HyperStar attachment for the EdgeHD 8, which turns the telescope into an f/2 system with a mere 400mm of focal length, but again, given the cost and complexity of doing so we’d recommend just getting the 8” RASA if you want to image mainly at f/2.
If you plan on doing any deep-sky imaging with a piggybacked guide scope, purchasing an additional dovetail rail for the EdgeHD 8 – either CGE-style or a smaller Vixen-style rail – is a good idea. Large guide scope brackets can directly attach to either size rail, and will keep the center of gravity of the scope in line with whatever mount you use it on, easing the load on gears and providing more accurate tracking as a result.
For planetary imaging with the EdgeHD 8 and a high frame rate astronomical CMOS video camera like the ZWO ASI224MC (which doubles as a great guide camera for deep-sky imaging) you really need a Barlow lens or focal extender to bring the EdgeHD 8 to between f/20 and f/30 for optimum image scale. The Apertura 2.5x Barlow lens is a good low-cost solution and will bring the scope up to f/25 and 5080mm, while at the higher end, a Tele-Vue 2.5x Powermate or Explore Scientific 2x or 3x Focal Extender is a bit sharper and easier to use. If you tend to have really bad seeing, sticking with 2x is a good idea; for those with very stable skies, 3x is a good idea, while 2.5x is right in the middle. These devices can all be used visually with 1.25” eyepieces to amplify magnification as well; there’s really no point in using a 2” Barlow or focal extender with cameras, and buying one for one or two 2” eyepieces is a bit of a waste.
Choosing eyepieces to use with the EdgeHD 8 for visual observing is a matter of personal preference, but we’d definitely recommend getting a 2” star diagonal such as the dielectric, carbon fiber screw-on unit offered by Apertura. For low power, the Apertura 38mm SuperView provides 53x and a true field of 1.3 degrees, while for medium and high magnifications, we’d recommend some nice wide-angle oculars such as the Agena Starguiders, Baader Hyperions, or Explore Scientific 82-degree line. You’ll probably want at least 3 or 4 high-quality eyepieces to get the most out of your EdgeHD 8. A good UHC filter like the Orion UltraBlock UHC 2” is also ideal for viewing nebulae, even if you already live under dark skies.
For focusing for astrophotography use, a Bahtinov mask which you can either purchase or make yourself is a must-have for the EdgeHD 8, and if you’re concerned about fine focus a ZWO EAF motor focuser, Celestron motor focuser, Pegasus motor focuser, or FeatherTouch fine focus upgrade isn’t a bad idea. You can also just lock the mirror in place and install a conventional GSO dual-speed Crayford focuser on the back of the scope if you wish – and add a motor focuser on top of that if you want – though this may not achieve the optimal back focus or spacing, or work whatsoever, with all reducer/Barlow/camera setups and adds a lot of weight to the back of the scope.
When mounted atop a good mount like the Sky-Watcher EQ6R or Celestron CGX, combined with proper autoguiding, a 0.7x focal reducer or HyperStar, a good camera like the ZWO ASI294MC (though a DSLR will do), the EdgeHD 8 is a powerhouse for deep-sky imaging. At f/7, you can go after galaxies, smaller nebulae, and star clusters, while at f/2 the scope is well suited for wide-field images of star clusters, nebulae, and galaxy groups.
With a Barlow lens or focal extender and high-speed, high-framerate astronomical CMOS video camera as previously noted, the EdgeHD 8 is also a spectacular planetary imaging in any environment (though good seeing does help).
What can you see with Celestron EdgeHD 8?
The EdgeHD 8 is a great scope for viewing the Moon, planets, and deep-sky objects. For deep-sky objects, the maximum field of view you can achieve with this scope and a 2” wide-angle eyepiece is around 1.3 degrees, or about 2.5 times the angular diameter of the full Moon. A typical 8” Dobsonian can do up to 2.25 degrees, so the EdgeHD 8 is a little cramped by comparison, but not too bad.
As with any instrument with around 8” of aperture, the EdgeHD will have no trouble resolving globular clusters into individual stars under most viewing conditions, pretty much regardless of your light pollution levels. You’ll also be able to see thousands of open star clusters, though the larger ones may not fit in the scope’s field of view entirely. Brighter, compact planetary nebulae like the Cat’s Eye or the Blinking Planetary appear as blue, green, or aquamarine fuzzes, with small details visible at high magnifications during moments of exceptional seeing conditions. Bright emission nebulae like Orion (M42), the Lagoon (M8), and the Swan (M17) are visible even under suburban skies, but are greatly improved with dark skies and/or a good UHC nebula filter like the Orion UltraBlock.
Galaxies are extremely plentiful throughout the night sky, but you will need dark skies free of significant light pollution to see much of them besides dim fuzzy smudges with the EdgeHD 8, or indeed any telescope. Under dark skies, however, you can expect to see the spiral arms in M51, M33, M101, and M81 at low magnifications, at least somewhat. Galaxies with prominent dust lanes such as M82, M64, and M104 (the Sombrero) are striking and show off their details at first glance most of the time. And there are of course many galaxy pairs, groups, and clusters visible too, mainly in the spring and autumn sky looking away from the plane of our own Milky Way in constellations like Draco, Ursa Major, Leo, Coma Berenices, Fornax, and Sculptor.
The EdgeHD 8 is, of course, also a great scope for viewing the Moon and planets. You can, of course, see the phases of Venus and tiny Mercury, as well as a few dark markings and the polar ice caps on Mars when it’s close to Earth. You can also see a ton of detail on the Moon, with craters and other features mere miles across during moments of good seeing and with constantly changing shadows depending on where the Moon is in its orbit. Jupiter’s cloud belts look sharp and colorful, with the Great Red Spot also present. When Jupiter’s four large Galilean moons (Io, Europa, Ganymede, and Callisto) transit in front of it, the moons can be resolved as tiny disks with perfectly round black shadows following them across the planet’s cloud belts. The EdgeHD 8 will also have no trouble showing you the rings of Saturn, the Cassini division in the rings that splits them in half, and some cloud belts and details on Saturn itself. You can also see a few of Saturn’s moons, the brightest being Titan, which is about as big as Ganymede and larger than the planet Mercury. Uranus is a greenish dot with one or two moons faintly visible, while Neptune is barely distinguishable from a star but with its moon Triton, it is somewhat easier to see than those of Uranus. Viewing Pluto is possible but extremely difficult, even under dark skies with the EdgeHD 8, and it’ll look like a dim star-like point with any telescope anyway.