The 8″ EdgeHD Optical Tube
The Celestron CGX 800 EdgeHD package features the Celestron 8″ EdgeHD optical tube, an 8″ (203mm) f/10 Schmidt-Cassegrain telescope with a focal length of 2032mm, which can be reduced to 1422mm and f/7 with the Celestron f/7 focal reducer, to 560mm and f/2.8 with the Celestron f/2.8 focal reducer, or to an f/2 391mm system with a Starizona HyperStar kit. The 8″ EdgeHD, an upgraded version of the classic C8 design, has a redesigned optical system that I’ve noticed eliminates coma, field curvature, and off-axis astigmatism, offering a flat and sharp field of view across the entire 2″ diagonal or large camera sensor. The 8″ EdgeHD also has a larger baffle tube than the C8 XLT, which allows it to fully illuminate a 2″ eyepiece or camera, eliminating vignetting. Furthermore, the 8″ EdgeHD has mirror locks that prevent image shift and mirror flop during focusing and imaging, as well as vents that help the telescope reach thermal equilibrium faster and maintain a stable focus.
Collimating the 8″ EdgeHD is the same process the C8 XLT or any other Schmidt-Cassegrain telescope relies upon, and involves adjusting three screws to tip and tilt the secondary mirror. This is a fairly infrequently-required process, and our collimation guide explains more.
Focusing the 8″ EdgeHD, as with all of Celestron’s Schmidt-Cassegrain telescopes, is achieved with a knob at the back of the telescope, which moves the primary mirror and shifts the focal plane dramatically. However, unlike the C8 XLT, the 8″ EdgeHD has two mirror locks that can be tightened to prevent the mirror from moving once focus is achieved. This eliminates the problems of image shift and mirror flop that can cause blurry stars or loss of focus during long exposures. The mirror locks can be hard to access or adjust with some accessories or cameras attached, so it is advisable to use a low-profile visual back or camera adapter, or a motorized focuser that can be controlled remotely.
Accessories such as visual backs, focal reducers, and camera adapters can be attached directly to the SCT threaded port at the back of the 8″ EdgeHD’s tube. The 8″ EdgeHD bundled with the CGX 800 package comes with a CGE-style dovetail rail to fit the CGX or any other mount compatible with a CGE or Losmandy D-style plate, along with holes at the top drilled for a second dovetail and a carry handle on the back of the tube for easy transportation.
Accessories
The CGX 800 package includes a 9×50 finderscope, which has a field of view of approximately 7°, displayed upside-down relative to the naked-eye view. It has crosshairs, which are not illuminated but work just fine for the task of centering stars to align the GoTo system of the CGX mount. It attaches to the 8” EdgeHD optical tube with a non-standard bracket, so replacing it is a bit of a pain. We would recommend you just keep it.
The CGX 800 EdgeHD also includes a standard 1.25” visual back, 1.25” prism star diagonal, and a 25mm focal length, 1.25” diameter E-Lux Plossl eyepiece with a 52° apparent field of view, yielding 81x and a true field of 0.65° – slightly larger than the angular diameter of the full Moon – with the 8″ EdgeHD. You will almost certainly want additional eyepieces for the 8″ EdgeHD offering higher magnifications and lower magnifications for a wider field of view, which may also require the purchase of a 2” star diagonal and eyepieces.
The CGX Equatorial Mount
The CGX is Celestron’s flagship equatorial mount and a significant step up from the CGEM II in terms of performance, features, and reliability. The CGX has a stated weight capacity of 55 lbs, which means that it can easily handle the 8″ EdgeHD and any imaging accessories with plenty of margin for stability and accuracy. The CGX has many improvements over the CGEM II, such as better motors, belt drives, worm gears, spring-loaded worm blocks, adjustable altitude and azimuth knobs, internal cabling, home and limit sensors, a USB port, and a much more ergonomic and user-friendly design. The CGX is one of the best mounts in its class for deep-sky astrophotography, and can rival or surpass more expensive mounts from other brands. Our review of the CGX goes into further detail about its advantages and capabilities.
The CGX has a dual dovetail saddle capable of accepting Vixen, Losmandy, and Celestron CGE style dovetail plates, the latter being used with the provided 8″ EdgeHD optical tube. The CGX uses a standard ¾” counterweight shaft and comes with two 11 lb counterweights to balance the 8″ EdgeHD, with plenty of margin for additional weight from accessories. You can connect the mount to your PC with a USB 2.0 cable and control it with software such as NINA via ASCOM drivers, and a standard ST4 port is built in for an autoguider, which you’ll need for deep-sky astrophotography with the CGX and any telescope. Additionally, the CGX mount allows you to connect add-ons such as Celestron’s StarSense AutoAlign, SkySync GPS, and SkyPortal WiFi adapter. You can power the CGX with either AC or DC connectors, and a cigarette lighter DC cord is provided with the mount.
Setting up the CGX for visual use or imaging has been straightforward for me, even though it’s fairly heavy and bulky compared to smaller mounts. After balancing the optical tube, adjusting the counterweights, and leveling the tripod, you can polar align with a polar scope, Celestron’s All-Star Polar Alignment method, or a software-assisted method such as Sharpcap or PoleMaster. With the NexStar+ hand controller or a suitable WiFi adapter/smart device, you align the mount with the sky. Simply center several alignment stars in the eyepiece or camera and confirm them to complete the alignment process. You can then automatically scroll to and track anything you select in the mount’s 40,000 object catalog.
Should I buy a Used Celestron CGX 800 EdgeHD?
A used CGX 800 EdgeHD package can be a great deal, as the mount and telescope are fairly new and durable products that should not have any major issues unless abused or damaged. Check to make sure that the mount powers on and operates smoothly, and that the 8″ EdgeHD telescope is free of corrosion or damage to the mirrors and corrector plate, which requires replacing the whole telescope or optical set to fix. Missing accessories, a missing hand controller, or a missing counterweight/bar are easily replaced but should accordingly result in a price reduction on a used item.
Alternative Recommendations
The Celestron CGX 800 EdgeHD is a hard-to-beat telescope for the money when it comes to a combo visual/astrophotography setup, but there may be some other options that suit your needs or budget better. Deep-sky astrophotographers may want to consider a different mount and optical tube package, such as what’s outlined in our rankings, while for visual use or planetary imaging, we’ve picked a few options that might be a better fit:
Under $1800
- The Apertura AD10/Zhumell Z10/Orion SkyLine 10 offer an impressive array of features for an incredible price. It has a larger aperture, more accessories, and a much simpler mount than the CGX 800 EdgeHD.
- The Apertura AD12/Zhumell Z12/Orion SkyLine 12 offers over double the light-gathering power and 50% more resolution than the CGX 800 EdgeHD, along with the same accessories and features of its 10” and 8” counterparts, but is a bit less convenient to set up and transport due to its massive solid tube.
- The Sky-Watcher 10” GoTo Collapsible Dobsonian is perfect for those looking for something more portable, as the collapsible tube makes it easier to transport. Plus, it can be controlled via your smartphone/tablet and also features Sky-Watcher’s FreedomFind technology to allow for manual aiming if the telescope’s electronics are in use, without any interference with its GoTo functionality.
$1800+
- The Sky-Watcher 12” Collapsible Dobsonian is fairly compact when disassembled thanks to its collapsible tube and is available in both manual and GoTo versions, with the latter offering the same smartphone operability and FreedomFind encoders as the 10” Collapsible model.
- The Celestron NexStar Evolution 8” SCT is just as capable as the CGX 800 EdgeHD for visual use or planetary imaging but is slightly cheaper, easier to set up and use, and more compact. You can also get an EdgeHD version with StarSense auto-align technology for an even more convenient setup if you wish. However, the EdgeHD 8” or any telescope requires an equatorial mount like the CGX for deep-sky astrophotography.
- The Celestron CGX 925 EdgeHD is a step up from the CGX 800 EdgeHD in terms of aperture and resolution but also in terms of weight and cost. It is ideal for those who want the ultimate in performance and versatility from a Schmidt-Cassegrain telescope and mount package, with the ability to be used for all types of viewing and astrophotography without exceeding the limits of the CGX mount or becoming too unwieldy to set up every night.
Aftermarket Accessory Recommendations
The Celestron 0.7x focal reducer is highly recommended for any deep-sky imaging with the 8″ EdgeHD. They screw onto the back of the tube ahead of your camera or adapters and bring the scope down to f/7 or f/2.8 respectively, increasing the field of view and reducing the exposure time. You can also use a focal reducer for visual astronomy with the 8″ EdgeHD, but a 2” star diagonal and eyepieces are usually the preferred choice for achieving a wider field of view. If you choose the latter, Explore Scientific’s 40mm 68° (51x) and 2″ dielectric mirror screw-on diagonal are perfect for your needs. The 40mm 68° will provide the widest possible field of view with the 8″ EdgeHD at about 1.3°. A GSO 42mm SuperView or Apertura 38mm SWA (48x or 53x, respectively) will also do nicely. Additionally, a good UHC nebula filter, such as the Orion UltraBlock, will improve your views of nebulae with the 8″ EdgeHD under almost any conditions.
The 8″ EdgeHD can handle up to 400x magnification on a steady night, and you’ll want to be able to achieve a range of magnification with additional 1.25″ or 2″ oculars, starting with a medium power eyepiece such as Explore Scientific’s 14mm 82° (145x) or 24mm 82° (85x). Explore Scientific’s 8.5mm 82° (239x) or, alternatively, a 9mm redline/goldline (226x) are both excellent choices for higher power eyepieces. You could also use a medium-power ocular with a Barlow lens in lieu of a high-power eyepiece if you are planning on getting a Barlow for planetary imaging purposes anyways.
The 8″ EdgeHD can benefit from the use of a dew shield to prevent condensation from forming on the front Schmidt corrector plate. Dew can not only obstruct views/images, but its contents – namely trace acids and other chemicals from industrial processes or pollen – can damage the StarBright XLT coatings or even the glass itself composing the corrector. Additionally, adding a dew shield can help reduce stray light entering the telescope, improve contrast, and protect the corrector plate from accidental touches and fingerprints. A flexible dew shield is inexpensive, can be rolled up for storage/transport, and is all that most people will need. However, a heated plastic or metal dew shield may be necessary for more humid climates.
A polar scope or PoleMaster is a must for the CGX, regardless of what you are using it for, as accurate polar alignment cannot be otherwise obtained except with All-Star Polar Align or software-assisted methods. Additionally, either an AC adapter or a rechargeable power supply such as the Celestron PowerTank Lithium Pro is necessary to use the CGX.
For imaging, a motor focuser and Bahtinov mask are highly recommended for use with the 8″ EdgeHD; Starizona’s HyperStar will enable you to shoot with the 8″ EdgeHD at 391mm focal length, which increases tolerance for tracking errors and provides a very wide field. However, this option is expensive, and you are limited to using certain one-shot-color CMOS cameras and small sensor sizes with this configuration. If you’re looking to use a larger aperture and focal length telescope like the 8″ EdgeHD for deep-sky astrophotography, a larger sensor monochrome CCD or CMOS camera with a filter wheel and narrowband filters would be a better choice. You will also need an autoguider setup in the form of a suitable guide camera and either a guide scope/off-axis guider for deep-sky astrophotography.
What can you see?
The 8″ EdgeHD telescope has a maximum field of view of 1.3° when using a 2” wide-angle ocular or 1.25” eyepiece and f/7 reducer, which is smaller but still somewhat comparable to the 2° field of view that 8-10” Dobsonian telescopes can achieve and slightly larger than the 1.1° field of view that the C8 XLT can achieve with similar accessories before it runs into vignetting issues. This means that the 8″ EdgeHD can show many large open star clusters and nebulae in their entirety, such as the Pleiades (M45), the Double Cluster, the Beehive Cluster (M44), the Orion Nebula (M42), and the Andromeda Galaxy (M31). Smaller open star clusters are also spectacular in the eyepiece, as are many of the brighter globular star clusters from the Messier catalog, which display individual stars at high magnifications. Smaller planetary nebulae like the Ghost of Jupiter Nebula and the Cat’s Eye Nebula show many details and colors, while larger emission nebulae such as the Lagoon (M8), the Swan (M17), the Eagle (M16), and the Trifid (M20) can be seen in stunning detail, especially under dark skies, and are further enhanced with a UHC filter.
Under light-polluted skies, galaxies viewed through the 8″ EdgeHD or really any telescope will appear as little besides washed-out smudges and are often hard to see at all. However, if you transport the telescope to a darker location, you can begin to resolve details such as hints of spiral arms, dust lanes, and H-II regions in brighter galaxies from the Messier and NGC catalogs. Additionally, you can observe many galaxy groups and the various huge galaxy clusters in Virgo, Leo, Fornax, and Coma Berenices.
With the 8″ EdgeHD, you can get stunning views of the Moon and planets. The phases of Mercury and Venus, as well as dark markings and polar ice caps on Mars, are clearly visible. Jupiter’s cloud belts and the Great Red Spot show up sharp and colorful, with its four Galilean moons visible even at the 9×50 finder. At high power during frequently-occurring transits, the 8″ EdgeHD reveals their small disks along with their shadows moving across the planet’s cloud belts. Saturn’s rings, cloud belts, and a handful of moons are visible. The Cassini Division in Saturn’s rings can also be observed on a typical night of good seeing, as can the Encke gap on an exceptional night. Uranus and Neptune show little besides their tiny round disks, with a couple of Uranus’ moons barely visible and Triton showing up fairly conspicuously next to Neptune. It is technically possible to observe Pluto with an 8” telescope like the 8″ EdgeHD under dark skies, and the CGX will easily locate it for you, but Pluto is already on the verge of being too dim for the most seasoned observers to spot and will continue to lose brightness as it grows more distant from the Sun.
Astrophotography Capabilities
If you’re looking to use the 8″ EdgeHD and CGX for deep-sky astrophotography, they can produce excellent results with almost any camera if a suitable focal reducer and a suitable autoguider are used. The flat field and fully illuminated field of view of the 8″ EdgeHD make it ideal for imaging with large sensors and wide fields, and the mirror locks prevent any image shift or mirror flop during exposure. The CGX mount is a smooth and accurate mount that can track and guide with high precision and stability, and has many features and accessories to make imaging easier and more convenient. You will need a guide camera and a guide scope or off-axis guider for this setup to obtain good results, as the CGX is still not able to track perfectly on its own and user error with balance, polar alignment, and leveling is inevitable to some degree.
Celestron’s 8” EdgeHD focal reducer will bring the 8″ EdgeHD to f/7, increasing the field of view with your camera and reducing the exposure time drastically compared to trying to shoot at f/10. A Starizona HyperStar will enable you to shoot with the 8″ EdgeHD at a mere 391mm focal length and f/2, which increases tolerance for tracking errors and provides a very wide field. However, this option is expensive, and you are limited to using certain one-shot-color CMOS cameras and small sensor sizes with this configuration.
If you’re looking to do planetary imaging, the 8″ EdgeHD telescope is a great choice – though, of course, a larger scope can resolve more under good seeing conditions. To make the most out of the 8″ EdgeHD for planetary imaging, you’ll need to pair it with a 2x to 3x Barlow lens, which boosts its focal length to between 4000mm and 6000mm for optimal image scale. You’ll also need a high-speed planetary video camera, such as the ZWO ASI224MC (which also doubles as a guide camera), and a laptop. With this setup and good seeing conditions, you can take beautiful photos of the Moon, Venus, Mars, Jupiter, and Saturn.
