The EdgeHD 8 Optical Tube
The Celestron NexStar Evolution 8″ EdgeHD is a type of telescope known as an “aplanatic Schmidt-Cassegrain.” This design includes corrector lenses inside the telescope’s baffle tube to compensate for edge-of-field aberrations, which are a characteristic of regular Schmidt-Cassegrain telescopes. The EdgeHD optical tubes can be compared to Meade’s “ACF” telescopes in terms of optical performance, but it has various mechanical improvements that the ACF telescopes lack, such as mirror locks for astrophotography and HEPA-grade filtered vents to accelerate the cooldown time of the telescope. The Evolution 8” EdgeHD has, as with the normal C8 XLT/NexStar Evolution 8, an 8” (203mm) primary mirror and 2032mm focal length, with a resulting f/10 focal ratio. It is compatible with Celestron’s dedicated 8” EdgeHD f/7 focal reducer as well as the Starizona HyperStar f/2 conversion kit.
Aside from the corrector lenses, the Evolution 8” EdgeHD functions like any other Schmidt-Cassegrain telescope, and views are basically the same as through the regular 8” NexStar Evolution. The only difference is that wide-angle 2” eyepieces won’t suffer from the slight vignetting at the outer edges of the field of view like with the normal C8 XLT optical tube; the reduction in field curvature and coma is negated by other more noticeable aberrations produced by most low-power wide-angle eyepieces. The added vents are probably the biggest performance improvement over the regular 8” Evolution.
Like all other Schmidt-Cassegrain and derivative telescope designs, the Evolution 8” EdgeHD uses a focusing mechanism inside the optical tube, with a knob at the back adjusting the primary mirror on a carriage to change the spacing and thus the positioning of the telescope’s focal point. The back has standard Schmidt-Cassegrain threads for attaching a focal reducer, camera adapters, a screw-on 2” star diagonal, or the included 1.25” visual back. You attach the EdgeHD 8” optical tube to a mount with the provided Vixen-style dovetail bar, which can allow you to easily attach the scope to a heavy-duty equatorial mount for deep-sky astrophotography if you desire.
The NexStar Evolution 8” EdgeHD includes a 1.25” visual back and a 1.25” prism star diagonal to insert your eyepieces. The visual back for the EdgeHD telescopes is a little longer than that supplied with regular Celestron Schmidt-Cassegrains for more optimal spacing and sharper views, and the included diagonal is the same high-quality unit provided with most other Celestron telescopes.
As with the other Celestron NexStar Evolution telescopes, you get two 1.25” eyepieces with the Evolution 8” EdgeHD: an “E-Lux” 40mm Plossl (51x), and a 12.5mm Plossl (163x). The 40mm E-Lux Plossl, as with any 40mm 1.25” ocular, has a narrow 43-degree apparent field of view as it is limited by the barrel diameter of the 1.25” format; this results in a true field of only about 0.85 degrees. A 2” star diagonal or f/7 reducer is needed to achieve a wider true field. The 12.5mm Plossl is fine but a little short on eye relief.
For a finder, Celestron provides their “StarPointer Pro” red dot sight instead of the standard “StarPointer” red dot for the Evolution 8” EdgeHD. Hypothetically, you shouldn’t even need a finder at all since the StarSense AutoAlign should align the Evolution mount for you, so this really is just a pointless aesthetic upgrade. In any case, the only difference is that the Pro uses a “bullseye” reticle rather than a dot, which if anything makes aiming slightly more difficult given its sole task of aligning the telescope on bright stars.
The NexStar Evolution Mount + StarSense AutoAlign
The NexStar Evolution mount is an alt-azimuth fork arm with left-right and up-down pointing capability and computer-driven motors, similar to other Celestron NexStar telescopes. Unusually, the mount actually has clutches on both the altitude and azimuth axes, allowing the telescope to be pointed manually. However, the clutches must be locked when using the telescope’s motors. If the mount is powered on while the telescope is being moved manually, the alignment of the mount to the night sky will be disrupted, leading to inaccurate slewing and tracking, and thus there’s really no use in trying to aim manually. The only purpose of the clutches is to make sure you have balanced the optical tube correctly on the altitude axis by sliding it forward-backward in its Vixen-style dovetail saddle.
The Evolution 8” EdgeHD includes Celestron’s StarSense AutoAlign by default. The StarSense AutoAlign is essentially just a packaged plate-solving device for Celestron’s GoTo mounts, with a small camera that guesses where the telescope is pointed in lieu of the user manually aiming the telescope at a few bright stars for alignment. It actually takes so long to do this that you don’t really save any time compared to a manual alignment, and unlike a human operator, the StarSense is easily confused by trees, buildings, or thin wispy clouds, and light pollution getting in the way of even a small portion of the sky. By the way, you are paying hundreds of dollars for this feature.
Unlike the other NexStar Evolution telescopes, the 8” EdgeHD version doesn’t include a hand controller at all. The only way to operate the telescope is with your smartphone or tablet, using the SkyPortal app or another app such as SkySafari. On paper, this makes operating the Evolution 8” EdgeHD (assuming you know how to collimate and focus the telescope itself) as simple as setting up and leveling the tripod, putting together the telescope, and hitting “connect and align” on your phone/tablet screen to start the StarSense up. However, should the StarSense have trouble, you will have to perform a manual 2- or 3-star alignment, much in the same manner as with any other GoTo mount. Using a phone app has advantages and disadvantages compared to a hand controller, both of which heavily depend on the user. For the most part, it’s nice to have WiFi operability and a much simpler user experience compared to a hand controller for beginners, but it’s odd that Celestron still includes hand controllers with all of the other NexStar Evolution models and not their flagship.
The NexStar Evolution mount has a built-in lithium battery, specifically a LiFePO4 unit. This avoids the cost and inconvenience of having to manage a separate power supply and deal with the possibility of it disconnecting, shutting off the scope, and causing you to have to start all over with alignment. The Evolution mount’s battery charge typically lasts around 12 hours at most when it is new. This could be an issue if you plan on going somewhere and expect to have long observing sessions without access to reliable electricity to recharge the mount. The StarSense auto-align will also use up quite a bit of battery as it swings around the sky attempting to plate solve.
The NexStar Evolution mount is marketed as being more suitable for astrophotography than Celestron’s cheaper alt-azimuth mounted instruments, which is why the Evolution 8” EdgeHD in particular is advertised as an “all-in-one setup” capable of producing fabulous deep-sky images. In reality, it really doesn’t compare to an equatorial mount, even when placed on a wedge to track accurately for long exposures and used with an autoguider. The Evolution mount’s high-quality worm gears, if perhaps overkill, do provide a quieter experience when using the telescope, and its gears will also last longer than those in cheaper alt-azimuth GoTo mounts, though this advantage is somewhat offset by the limited battery life of the internal LiFePO4 battery, which will probably fail and need replacing within around 10 years.
Should I buy a Used Celestron NexStar Evolution 8” EdgeHD?
If you can find a used NexStar Evolution 8” EdgeHD in good condition with a fresh battery and fully functional electronics, there is nothing inherently wrong with buying it. However, there are a ton of things that can go wrong between the StarSense, the delicate EdgeHD optics, the mount itself, and its internal battery, so you should make sure everything is in great condition. Problems with the EdgeHD optical tube or mount will require sending them back to Celestron for repairs at your own expense if you buy a used unit, as DIY fixes are simply not possible.
The NexStar Evolution 8” EdgeHD is a nice telescope, but there are better options for your money. Here are a few we’ve selected for your consideration:
- The Apertura AD10/Zhumell Z10/Orion SkyLine 10 offers more resolving and light-gathering power than the Evolution 8” EdgeHD, as well as a wide range of accessories and a wider achievable field of view, at a very low price.
- The Celestron StarSense Explorer 10” Dobsonian doesn’t include as many features bundled with it as the AD10/Z10 but has a significantly lighter and more portable Dobsonian base as well as Celestron’s StarSense Explorer technology (no relation to the StarSense AutoAlign) to help you navigate the night sky with your smartphone.
- The Apertura AD12/Zhumell Z12/Orion SkyLine 12 is far more capable, particularly for deep-sky observing, than an 8” or 10” telescope and comes with a variety of high-quality features and accessories, as well as a sturdy and easy-to-use Dobsonian mount.
- The Sky-Watcher 12” Collapsible Dobsonian is available in both manual and GoTo configurations. The GoTo version includes FreedomFind encoders, which allow it to be aimed by hand even with the mount powered on, a feature not shared by the NexStar Evolution scopes. The telescope’s collapsible tube is easy to set up and helps with storage and portability.
- The Sky-Watcher 10” Collapsible Dobsonian can be purchased in either manual or GoTo configurations, and shares the features of the larger 12” model but with greater portability albeit less light gathering or resolving power.
- The regular Celestron NexStar Evolution 8” offers exactly the same views, accessories, and general performance as the 8” EdgeHD StarSense, apart from the obvious lack of StarSense AutoAlign or EdgeHD optics.
- The Celestron Advanced VX RASA 800 essentially offers the same performance as the EdgeHD 8 with a Hyperstar f/2 kit already added in, with a super-fast f/2 focal ratio and short 400mm focal length allowing for incredible detail with short exposures. The Advanced VX is an acceptable mount for astrophotography with the RASA 800.
- The Celestron Advanced VX 9.25” SCT has more light-gathering and resolving power than the 8” EdgeHD, making it superior for visual observation or planetary imaging.
Aftermarket Accessory Recommendations
A 2” star diagonal and 2” eyepieces, or a 0.7x focal reducer, are necessary to achieve a wider field of view at low powers with the NexStar Evolution 8” EdgeHD. If you choose the former, the Apertura 38mm SWA (53x) and Apertura 2” dielectric screw-on diagonal are a great match. It is also a good idea to have a higher power eyepiece than the stock 12.5mm Plossl, such as the Explore Scientific 8.5mm 82-degree (239x) or an inexpensive 9mm redline/goldline (226x). A good Barlow lens will work in conjunction with the stock or additional eyepieces to achieve higher powers and can be used for planetary imaging as well. To improve your views of nebulae, consider getting an Orion UltraBlock or similar UHC nebula filter.
Any Schmidt-Cassegrain telescope, including the NexStar Evolution 8” EdgeHD, benefits from a dew shield at the front to keep dew, fingerprints, and pollen away from the corrector, as any of these contain chemicals that will damage the 8” EdgeHD corrector plate’s StarBright XLT coatings or the actual glass itself. The dew shield also acts like a lens hood, reducing sources of glare and stray light as well as background sky glow to improve contrast slightly at the eyepiece.
What can you see?
With a 2” wide-angle eyepiece, or by using a 0.7x reducer with the provided 40mm Plossl, the NexStar Evolution 8” EdgeHD can provide a maximum field of view of no greater than 1.3 degrees. This is relatively narrow compared to the true field of around 2 degrees provided by a comparably sized Dobsonian and low-power wide-angle eyepiece. This limited field unsurprisingly restricts the telescope’s capabilities in viewing large open star clusters and nebulae. However, the NexStar Evolution EdgeHD 8 is still capable of providing excellent views of smaller open clusters such as M11 or M35 and can resolve many bright globular star clusters from the Messier catalog into individual components. Bright nebulae like the Orion Nebula (M42) look fantastic, especially with a UHC filter or under dark skies, and you can also observe small planetary nebulae such as the Ring (M57), some of which, such as the Cat’s Eye Nebula, show bluish or greenish coloration. You can also observe details in galaxies under dark skies, such as dust lanes or fuzzy spiral arms in the brightest of them, along with plenty of galaxy groups and clusters with the NexStar Evolution 8” EdgeHD.
Planetary and lunar views are also a delight with the NexStar Evolution 8” EdgeHD. Our own Moon shows details miles across, such as craters, ridges, mountain ranges, and various unusual details, which can vary drastically in appearance depending on the Moon’s positioning and the angle of the Sun shining on the surface. You can see the disks of all 4 Galilean moons of Jupiter as tiny dots when they transit, along with their shadows. Jupiter’s Great Red Spot can be seen amidst its vivid, pastel-shaded cloud belts and storms, Mercury and Venus’ phases can be resolved clearly, and Mars shows several dark markings and its polar ice caps when it is close to Earth. Saturn’s rings are a sight to behold, along with the Cassini Division within; the Encke gap in the rings can be picked up under very steady skies, and you can also see plenty of its moons. Uranus is a turquoise ball with possibly one or two moons accompanying it if you can pick them out of its glare, while Neptune’s moon Triton shows up easily despite the planet appearing hardly distinguishable from a star. Pluto is technically bright enough to be visible with the NexStar Evolution 8” EdgeHD telescope but will probably require a larger aperture telescope to be seen, even under dark skies.
Deep-sky astrophotography is out of the question with the NexStar Evolution 8” EdgeHD. Even with the Evolution mount on a suitable equatorial wedge, you just can’t get accurate enough tracking or guiding for long exposures of consistent quality. A mount like the Sky-Watcher EQ6Ri Pro or Celestron CGX is necessary for this task.
The NexStar Evolution 8″ EdgeHD’s tracking is more than adequate for planetary imaging, however, and is ideal for the task thanks to its sharp optics. Taking images of the Moon and planets can be done with a high-speed planetary video camera like the ZWO ASI224MC and the use of a Barlow lens with a magnification of 2-3x to bring the scope up to f/20-f/30, which is needed for them to occupy enough of the camera’s frame to resolve small details.