The Optical Tube
The 8” FlexTube GoTo Dobsonian is an 8” (200mm) f/6 Newtonian reflector with a focal length of 1200mm. At f/6, you won’t notice coma at all except with maybe the widest-field 2” eyepieces and it’s still negligible, while cheap wide-angle Erfle-type oculars still provide sharp views across the whole field and focusing with a single-speed Crayford like the simple 2” one provided is not much of a problem. The optics in these scopes tend to be pretty good, as with the majority of mass-manufactured Dobsonians nowadays.
The Sky-Watcher FlexTube design reduces the 8” model from a 45” to 33” length when it is collapsed by simply locking/unlocking 3 knobs and pushing or pulling on the scope to extend or retract the struts. This sounds beneficial, but in reality unless you are dealing with an extremely tight space in a car or closet there isn’t much benefit to this system, and in a vehicle the hefty Dobsonian base of the scope is going to be a more formidable challenge to fit than the tube would be anyways in most situations, collapsible or not. The weight is also not any different from a solid tube, and you need multiple covers for the lower tube assembly as well as the secondary mirror instead of one big cap for the end of the tube.
Collimating the 8” FlexTube requires tools for both the primary and secondary mirror adjustments. The secondary is adjusted with a hex key, which is fairly standard and seldom required anyway. The primary mirror, however, uses an annoying push-pull system of 3 Phillips head screws and 3 tiny, recessed hex screws. The cheap alloy used in the stock screws tends to strip very easily and making fine adjustments in the field is rather annoying.
The majority of reflectors use a spring-loaded knob to adjust the primary mirror collimation without tools, including Sky-Watcher’s larger scopes, but the 8” models sold by Sky-Watcher and Celestron (both merely brand names under the larger Synta corporation) still use the substandard and antiquated screw design, probably a cost-saving carry over from older telescope manufacturing lines. Thankfully once adjusted the scope rarely if ever goes out of collimation and the tolerances at f/6 are fairly lax. Our collimation guide goes into more detail on how this process is done; you’ll also want either a Cheshire or laser tool to help out since Sky-Watcher does not provide any collimation tools with the telescope by default.
To attach the 8” FlexTube GoTo Dobsonian to its base, you rotate the slotted left side bearing to line up with the attachment on the tube and lower it into place, then lock the knob holding it on. The other altitude bearing just rests on the base like a manual Dobsonian.
The 8” FlexTube GoTo Dobsonian comes with 1.25″ 25mm (48x) and 10mm (120x) “Super Plossl” oculars, which have a narrow 50-degree apparent field of view typical of the Plossl design. While they are sharp and work quite well with this telescope given its f/6 focal ratio, the eye relief of the 10mm unit is rather short, as with any short focal length Plossl, requiring you to press it close to your eye to get the full field of view.
The 8” FlexTube’s provided extension tubes are required for it to reach focus with eyepieces, and as with the other Sky-Watcher Dobsonians you get separate 2” and 1.25” ones, which won’t allow you to thread on filters and of course grip your eyepieces with simple set screws. Replacing them with a dedicated 2” extension tube and 1.25” adapter, with filter threads and
For aiming, the 8” FlexTube GoTo Dobsonian comes with a 9×50 straight-through finder scope with crosshairs. This finder has an approximately 6-degree true field, and an upside-down view. Although it is helpful for aligning the GoTo system of the scope, it is rather uncomfortable to use in comparison to an easier-to-use right-angle finder or reflex sight. Fortunately, switching out for one of these alternatives is simple due to its standard Synta/Vixen-style finder shoe.
The 8” FlexTube GoTo Dobsonian uses an alt-azimuth Dobsonian mount made mostly out of particle board, with enclosed servo motors and slip clutches attached to the base. It features a motorized bearing on one side, while the other side holds the weight of the scope with a traditional Dobsonian bearing consisting of a cylinder on two plastic cylinders to minimize the motor torque and electrical power needed to move it around. The slip clutches on both the altitude and azimuth axis can be unlocked for manual pointing, while Sky-Watcher’s FreedomFind dual encoders keep track of where you’ve aimed the telescope without interruption to GoTo or tracking accuracy. Switching between electronic commands and manually pushing the scope is a completely seamless process.
Two options are available for controlling the 8” FlexTube GoTo Dobsonian’s electronics; the two versions come with either a plug-in SynScan hand controller or with a WiFi dongle for easy control via the SynScan app or SkySafari Pro on your device. You can convert one to the other by buying a WiFi adapter or SynScan controller as well, respectively. We would recommend the WiFi version of the 8” FlexTube GoTo Dobsonian due to its simpler interface and lower price compared to the SynScan-equipped unit.
The 8” FlexTube GoTo has a base weighing 7 lbs more than its 26 lb manual counterpart, making the total scope balloon to a total weight of 57 lbs. This is comparable in weight to the manual 10” FlexTube, which weighs 59.5 lbs.
Should I buy a Used Sky-Watcher 8” FlexTube GoTo Dobsonian?
A used GoTo telescope should always be approached with a little more caution than a manual one, and the 8” FlexTube is no different. Problems which can be easily combated or are simply innocuous with a manual scope, such as any warping or damage to the base, will completely ruin the GoTo functionality and of course you should always make sure the electronics work too. As with any used reflector, you should also check that the coatings on the mirrors are not corroded as recoating is usually not cost-effective unless you get the scope for a very low price. Additionally, the FlexTube system should smoothly extend and collapse.
The Sky-Watcher 8” FlexTube is a nice scope, but unless you absolutely need the collapsible tube, there are better and cheaper options from a variety of different manufacturers. Given that you could either get a 10” manual scope for a lot less and spend the saved money on accessories or get a 10” GoTo Dob for just a bit more, we really don’t think Sky-Watcher 8″ FlexTube to be the best option. A 10” will show you far more than whatever benefits the GoTo provides on an 8” scope and is essentially the same physical size and weight, while a 12” Dobsonian blows away either smaller aperture with vastly brighter and more detailed views of deep-sky objects.
- Computerized Scope (Best): The Sky-Watcher Virtuoso GTi 150P has a similar collapsible tube design to the 8” Flextube but with full motorized GoTo operation controlled by your smartphone, a huge field of view, and an extremely compact form factor, all with only slightly less aperture and a much lower price.
- Manual Scope: The Sky-Watcher Heritage 150P features a similar collapsible tube to the 8” FlexTube but is a lot cheaper and more compact; it’s identical to the Virtuoso GTi 150P apart from lacking electronics.
- Manual Scope (Best Value ): The Apertura AD8/Zhumell Z8/Orion SkyLine 8 (all are the same model, made by GSO as with the AD10) are slightly superior optically to the Sky-Watcher and Orion scopes, and also include a plethora of high-quality accessories like a 2” wide-angle eyepiece and cooling fan, as well as sporting dual-speed Crayford focusers. You can’t go wrong with this one, and AD8 is our most recommended 8″ Dobsonian.
- Partially Computerized Scope: The Celestron StarSense Explorer 8” Dobsonian offers similar views to the 8” Flextube and a lightweight base, along with Celestron’s fabulous StarSense Explorer technology to aid in locating deep-sky objects. However, collimation adjustments are similarly annoying to the 8” FlexTube and you don’t get much in the way of accessories.
- Manual Scope: The Explore Scientific 10″ Hybrid Dobsonian is even more compact than the 8” FlexTube and offers better movements on the altitude axis thanks to its huge glassboard-covered altitude bearings, along with larger aperture. However, it needs various additional accessories and upgrades to work well.
- Manual Scope: The Apertura AD10/Zhumell Z10/Orion SkyLine 10 offers more aperture than the 8” FlexTube and a fairly portable form factor, along with bonuses like a dual-speed focuser and a 2” wide-angle eyepiece, but at a higher price tag.
- Partially Computerized Scope: The Celestron StarSense Explorer 10” Dobsonian offers the same great StarSense Explorer technology as the 8” model but in basically the same form factor with more aperture and easier collimation adjustments. It’s significantly lighter and easier to set up/transport than many other 10” Dobsonians and even some 8” units thanks to its weight-optimized base and plenty of handles and grab points on both the tube and mount.
- Computerized Scope: The Celestron NexStar 6SE is less capable than an 8” or 10” Dobsonian and its field of view is restricted by its long focal length and 1.25”-only eyepiece compatibility. However, it is a sturdy and well-designed option for those who want a compact scope with GoTo and motorized tracking.
Aftermarket Accessory Recommendations
A shroud is a must-have for the 8” FlexTube, as with any open-tubed scope, to keep stray light out and avoid dust, dirt, and fingerprints on the optics and mitigate the formation of dew or frost. Making one yourself can be a challenge as many fabrics will just droop into the light path due to the scope’s 3-pole arrangement; AstroZap sells a ready-made one for your convenience.
As previously mentioned, the stock extension tubes/adapters provided with the FlexTube Dobsonians are very low quality and won’t let you screw filters on. If you plan on using 1.25” and 2” eyepieces, this is especially problematic as you’ll want to be able to use 2” filters with your 1.25” eyepieces. A 2” extension tube and 1.25” adapter with 48mm filter threads and brass compression rings will solve this issue and create a more rigid and parallel connection, avoiding issues with laser collimators becoming tilted and not leaving any marks on your eyepiece barrels. You might also want to replace or complement the stock 9×50 finder with something else. A Rigel QuikFinder takes up fairly little space on the upper tube assembly while the Explore Scientific ReflexSight slides into the 9×50’s standard finder shoe. A Telrad may not fit on the scope.
Additional eyepieces are a must-have with almost any scope and the 8” FlexTube GoTo Dobsonian is no exception. The Apertura 38mm SWA (32x) is an ideal low-power sweeping eyepiece, providing a true field of 2.2 degrees with the 8” FlexTube, or 4.5 times the angular size of the full Moon and ideal for the largest deep-sky objects. For medium power, a 15mm SWA or redline eyepiece (80x) bridges the gap in magnification of the stock 25mm and 10mm oculars while a 6mm “goldline” or “redline” (200x) is ideal for high-power planetary viewing. A Barlow lens or higher-power eyepiece for up to 400x can also work if your atmospheric conditions support it, and you’ll need a collimation tool (preferably a Cheshire) for the scope as well. And being a GoTo scope, a power supply is required to run the 8” FlexTube GoTo Dobsonian; you’ll need something compact that can ride attached to the base since any cord leading to a wall outlet or nearby power source is going to quickly wrap around the telescope and unplug itself.
Lastly, a good UHC nebula filter such as the Orion UltraBlock is a must-have for deep-sky viewing with almost any telescope, improving contrast in nebulae under light-polluted skies and allowing you to see previously unnoticed objects under good conditions. The 2” model combined with a threaded extension tube/adapter like the ones listed above is a good idea even if you don’t plan on purchasing any 2” oculars just yet as it will future-proof you and work with either 1.25” or 2” eyepieces.
What can you see?
The 8” FlexTube GoTo Dobsonian is fairly easy to transport to dark skies far from light pollution, as are most telescopes of this size on Dobsonian mounts. Under dark skies, you can resolve globular star clusters such as M13 into individual stars, view colorful planetary nebulae like the emerald Cat’s Eye or the eponymous Blue Snowball at high magnifications, and see detail in galaxies such as dust lanes in M82, M31’s orbiting companions, or the huge Virgo Cluster. You can also see fantastic detail in emission nebulae like Orion (M42) or the Lagoon (M8), while a UHC filter reveals the huge Veil Nebula supernova remnant with a suitable low-power eyepiece.
Light pollution will severely limit your views of galaxies and will make it harder to resolve globular clusters and planetary nebulae, as well as emission nebulae like Orion (M42), but you’ll still be able to see plenty of beautiful open clusters such as the Double Cluster or the Pleiades
(M45), and of course resolve thousands of double stars with the 8” FlexTube GoTo Dobsonian.
The 8” FlexTube GoTo Dobsonian is of course a great scope for viewing and imaging the Moon and planets too. Expect to have no trouble resolving the phases of Mercury and Venus, the polar ice caps on Mars, and even some dark markings on the Red Planet during its more favorable apparitions. The Moon delights with countless small details, while Jupiter’s moons are clearly visible in the scope’s 9×50 finder and are resolved as disks with the 8” FlexTube GoTo Dobsonian at high magnifications during transits, along with their shadows. Jupiter shows many vivid cloud details including the Great Red Spot which change as weeks and months go by. You’ll also be able to see the rings of Saturn and the Cassini Division within them alongside a handful of moons and Saturn’s own cloud bands. Uranus and Neptune are resolved as turquoise and blue disks with the 8” FlexTube GoTo Dobsonian, but neither shows much detail, while Neptune’s moon Triton can be seen. Uranus’ moons are technically within range of an 8” telescope but are easier to see with a larger telescope, as is distant and faint Pluto.
The Sky-Watcher 8” FlexTube GoTo Dobsonian is a decent ideal choice for planetary astrophotography due to its large aperture and motorized tracking capabilities. 8” is considered the minimum size required for good high-resolution planetary imaging, and the 8” FlexTube performs no differently than an 8” Schmidt-Cassegrain for the task, though you’ll need a 4x or 5x Barlow lens to get the scope to a suitable focal ratio between f/24 and f/30, which can be a bit harder to find than the 2-3x units used with SCTs. A good planetary CMOS video camera like the ZWO ASI224MC and a laptop is the only other kit you need. Deep-sky astrophotography is of course not viable with this alt-azimuth-mounted, unguided telescope and you are likely to be disappointed in any attempt; long-exposure astrophotography is more about having a fast f-stop and a good mount than large aperture.