The Optical Tube
Optically, the 8” FlexTube is an 8” (203mm) f/6 (thus 1200mm focal length) Newtonian reflector. At f/6, you can use fairly inexpensive eyepieces without resorting to well-corrected fancy wide-angle stuff. You won’t see much coma, and collimation tolerances are not extreme.
The FlexTube’s tube is made up of two thin-walled steel segments with aluminum castings on each end, with a set of three struts that you can easily extend with no tools. When collapsed, the tube length is reduced from 45” to 33”.
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It’s nice, but a 33” long tube is still pretty long and, in most circumstances, this difference probably isn’t going to make or break being able to fit the tube in a car or carry it. There’s no weight savings here compared to a solid-tubed instrument either.
Ironically, the “flex” in the FlexTube could arguably be used to refer to one of the biggest drawbacks of the design. A 3-strut telescope is simply not the stiffest configuration, and the struts can bend, not all extend to exactly the same point, or slide a bit under their own weight when the scope is pointed nearly vertical. As a result, the scope may shift in focus and/or collimation throughout the night and is unlikely to maintain collimation when repeatedly collapsed, extended, and collapsed again. This is in contrast to a solid-tube instrument where you might only need to collimate the scope occasionally to maintain sharp views.
Speaking of collimation, the Sky-Watcher 8” models all require a screwdriver to adjust their primary mirror collimation (most good scopes have spring-loaded hand knobs). The screws are prone to stripping, especially if you are fiddling around with them in the dark with a screwdriver, so beware. Replacing them with thumbscrews is possible, but annoying. As with most scopes, adjusting the secondary mirror collimation (thankfully seldom ever needed) requires a small hex key which is included with the scope.
For a focuser, the Sky-Watcher 8” FlexTube has a single-speed 2” Crayford unit. This focuser works quite well and is really all you need for such a scope. As with the other Sky-Watcher Dobsonians, the scope uses an unusual extension tube system; to use 1.25” eyepieces, you have to swap in the 1.25” extender for the 2” extender unless you buy an aftermarket standard 2” to 1.25” adapter.
The 8” FlexTube includes two 1.25” “Super” eyepieces: a 25mm unit providing 48x and a 10mm providing 120x. These “Super” oculars have a little more eye relief than a standard Plossl eyepiece and are quite lightweight on account of the body being made of plastic. These two eyepieces are good enough to get you started, but you’ll probably want a lower-power 2” eyepiece for a wider field of view and something with higher magnification than 120x at the minimum.
The 8” FlexTube’s finder is a 9×50 right-angle, correct image unit. It can take some practice to get used to this finder; you need to sight along the tube to roughly aim the telescope and finder in the right direction and then use what’s visible in the finder to zero in on your target. This requires a good star atlas or an app like SkySafari and can be especially challenging under light-polluted skies, though the 9×50 will show stars much fainter than what you can see with your naked eye regardless of conditions, by several magnitudes.
A variety of different covers for each part of the optical tube, along with an eyepiece rack, are also included with the 8” FlexTube. Collimation tools are not included with the FlexTube, however—you’ll need at least a homemade collimation cap or to collimate on stars. Our collimation guide details how to collimate the scope with tools or on stars, which isn’t too difficult of a process apart from the difficulty of using a screwdriver to adjust the primary mirror in the dark.
The Collapsible Dobsonian Mount
The 8” FlexTube uses a Dobsonian mount with the same design as Sky-Watcher’s other full-sized Dobsonians. The scope pivots up and down on a pair of plastic altitude bearings resting on plastic cylinders and is held against the sides of the rocker by a pair of glorified bicycle handlebars, which Sky-Watcher calls an advanced tension adjusting system and even has a patent on. In actuality, these bike handle knobs are one of the biggest weaknesses in the design of the mount. They stick out and easily bump into things or get caught on loose fabric, and if the scope is using heavy accessories or a light shroud, it becomes top-heavy and needs the tension handles tightened down in order to stay put, resulting in jerky and awkward motions in altitude. Telescopes with oversized or adjustable bearings or spring-tensioned bearings, which are offered by the majority of other Dobsonian manufacturers, don’t suffer from this problem.
For moving side-to-side (in azimuth), the 8” FlexTube uses a fairly standard set of Teflon pads sliding against the melamine-coated base. The ground board for the FlexTube is circular instead of triangular, which significantly adds to the weight of the telescope.
An eyepiece rack and a carry handle are attached to the front of the base, should you need to use them. The whole base assembles with the included screws and hex key in just a few minutes, though it’s not designed to be dismantled and reassembled repeatedly.
Should I buy a Used Sky-Watcher 8” FlexTube?
A used 8” FlexTube is a good choice. The open tube means that it’s easier for the scope’s optics to gather dust and dirt, especially with a neglectful owner, but cleaning the optics is, in most cases, a trivial matter. However, corrosion of the coatings isn’t, and you should be careful to avoid a scope with damaged mirror coatings as recoating them will likely end up costing you as much as, if not more than, the savings compared to purchasing a brand new unit. Small scratches, light dust, etc. are not a big deal. Dents in the tube are more of a problem with the FlexTube scopes as they may inhibit the collapsible struts from moving, locking, or staying square with the mirrors, but they are still often easy to ignore or fix. A damaged Dobsonian base can be replaced or repaired relatively easily and cheaply, either by a third-party manufacturer or with a sheet of plywood and a handful of tools.
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.
- 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.
- 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. A 10” model is available too.
- The Apertura DT8 has better altitude bearings than the Sky-Watcher scopes and is essentially a stripped-down AD8 with minor mechanical changes and a rock-bottom price tag compared to most other 8” Dobsonians.
- The Sky-Watcher 8” Classic Dobsonian has the exact same mount, optics, and focuser as the 8” Collapsible, but with a straight-through instead of a right-angle finder and a solid tube. The views and general operation are the same, but the price is lower.
- The Orion SkyQuest XT8 is basically the same as the DT8, but with a red dot finder.
- The Apertura DT10 offers more aperture and a better-designed bearing system and focuser than the 8” FlexTube, and is still fairly portable, even in a fairly small car. However, you don’t get a right-angle finder and only one eyepiece is provided.
- The Apertura AD10/Zhumell Z10/Orion SkyLine 10 (identical copies of the GSO Deluxe 10” model) similarly offers more aperture 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.
- If you’re looking for something smaller, 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.
Aftermarket Accessory Recommendations
The most essential accessory we’d recommend for the Sky-Watcher 8” FlexTube is undoubtedly a shroud. This will keep moisture, curious hands, bugs, dirt, and stray light out of the optical tube. It needn’t be expensive—pretty much any fabric draped over the scope that can be secured and won’t droop into the optical tube will do. Sewing one yourself out of Lycra or Spandex, or having someone do it for you, is easy and inexpensive—or you can always purchase one.
The Sky-Watcher 8” FlexTube includes two decent eyepieces, but the 125x provided by the included 10mm ocular isn’t nearly as much as what the scope is capable of on a clear and steady night. A 2x 1.25” Barlow lens will work with the 10mm to double its magnification to 250x, which is an optimal magnification for planetary viewing and splitting double stars under most conditions. Alternatively, a 6mm goldline will provide about 200x with the 8” FlexTube, and in conjunction with a Barlow will provide 400x – at the limit of what the 8” FlexTube’s optics will handle and higher than typical atmospheric conditions will allow without producing a blurry image. Additionally, a 15mm SVBONY wide-angle eyepiece (80x) fits nicely in between the included 25mm and 10mm, and will provide 160x with a Barlow, which arguably makes a 6mm ocular largely redundant.
A wide-angle 2” eyepiece would be something we’d strongly recommend for the 8” FlexTube were it not for the balance issues it is likely to cause. A 38mm Apertura SWA provides 32x and a huge 2.2-degree field of view with the Sky-Watcher 8” FlexTube, nearly double the true field of the stock 25mm, making it a bit easier to locate deep-sky objects and fit larger things such as open star clusters in the field of view with room to spare. Fancier wide-angle eyepieces exist, but their weight exacerbates the problem of having to tighten the FlexTube’s adjustable altitude bearing brakes.
A nebula filter isn’t a magical “light pollution filter” as some suggest, and the typical broadband filters many advertise are basically useless placebos for visual astronomy. However, a good UHC filter with a relatively narrow bandpass increases contrast on emission nebulae like the Lagoon, Swan, Orion, or the Rosette and may make a significant difference under light-polluted skies, arguably providing a contrast benefit under even the most pristine night skies by dimming the naturally gray sky background and reducing the contrast-robbing effects of city light pollution. The Orion UltraBlock is our favorite; the 2” model will need a regular threaded 2” to 1.25” compression ring adapter to be used with any 1.25” eyepieces thanks to the convoluted Sky-Watcher focuser adapter system. If you never plan on purchasing any 2” oculars, the 1.25” version is just fine too.
What can you see with the Sky-Watcher 8” FlexTube?
An 8” Dobsonian like the Sky-Watcher 8” FlexTube is often cited as the ideal beginner telescope, as it’s quite portable and can still show you a lot. You can set the scope up by just carrying it outside fully assembled, or fit it in the back of almost any vehicle to transport it to dark skies. The 8” FlexTube can resolve globular star clusters into individual stars with ease, even under fairly poor viewing conditions, and you can start to see the differences between the brighter globular clusters, as no two are alike. Some are loosely-bound like M4, and some have very dense or even unresolvable cores like M15 or M14. Others are out-of-round like M92, or have dust lanes throughout them like M13’s Propeller. Open star clusters can contain hundreds or even thousands of stars, which are usually spectacularly colorful too, even under light-polluted skies.
Emission nebulae such as the Orion Nebula (M42/43), or the lesser-visited Lagoon (M8) and Swan (M17) in the summer sky, look great with the 8” FlexTube, especially with a UHC filter screwed into your eyepiece and a shroud attached. Planetary like the Ring (M57) and Dumbbell (M27) are easy to spot, while smaller planetaries like the Cat’s Eye and Blinking Planetary are colorful greenish dots with intricate features visible during moments of good seeing at high magnification.
With dark or at least somewhat dark skies when the Milky Way is detectable overhead, an 8” Dob like the FlexTube can show you a lot of galaxies. You can easily see thousands of them, including the entirety of the Messier catalog and the Herschel 400, though most are barely-detectable smudges. Brighter galaxies like M51 and M33, however, show spiral arms with careful observation, while others, like M104, M64, M31, and M82, have prominent dust lanes against the backdrop of their spiral disks. The Virgo Cluster is littered with dozens of members, and the companions of bright galaxies like M31 or groupings like the Leo Triplet (M65/66) are easy to spot with the 8” Flextube.
The Moon and planets are just as exciting and are easy to see under pretty much any viewing conditions, though good atmospheric seeing will allow you to use higher magnifications and resolve smaller details with the 8” FlexTube. You’ll be able to see tens of thousands of lunar features such as craters, ridges, mountains, and valleys. The phases of Mercury and Venus are easy to see, though neither planet will show any other details, even with a larger instrument. Mars’ ice caps are visible, and when the red planet is close to opposition biannually, the 8” FlexTube has sufficient resolving power to reveal a few dark markings on the planet, some of which correspond to geological features.
The 8” FlexTube will show the moons of Jupiter with ease, and furthermore, it can resolve them into tiny disks along with their shadows, which follow them when they transit across Jupiter-a frequent and visually exciting occurrence. Jupiter itself has a number of colorful cloud belts and storms, along with the Great Red Spot, which may take high power to distinguish clearly from the surrounding, often similarly colored, southern equatorial cloud belt on the planet.
Saturn’s rings are easy to see with the 8” FlexTube, as with any good telescope. You can also resolve the Cassini Division, a gap in the rings, at high magnification, along with some linear cloud bands on Saturn itself, which aren’t the highest in contrast. Saturn has more than half a dozen moons visible with the 8” FlexTube, the easiest being Titan and Rhea.
Uranus and Neptune are difficult to locate with the 8” FlexTube, though both are easy enough to spot in the 9×50 finder even under light-polluted skies, and don’t look like much more than fuzzy, bluish stars. Neptune’s moon Triton is bright enough to be seen with an 8” telescope, but Uranus’ moons are out of reach. Pluto used to be bright enough to see with an 8” instrument like the FlexTube, but is slowly receding from the Sun and getting dimmer to the point that a 10” or even 12” telescope is now required to view it at all.