Sky-Watcher 10″ FlexTube Dobsonian Review: Recommended Scope

The Optical Tube’s Performance

The Sky-Watcher 10” FlexTube Dobsonian, also known as FlexTube 250P, is optically a 10” (254mm) aperture Newtonian reflector with a focal ratio of f/4.7 and focal length of 1200mm.

With an f/4.7 reflector, as expected, I could notice some coma at the edges of the field of view with wide-angle, low-power eyepieces, but it wasn’t enough to be bothersome or really dictate the need for a coma corrector. However, I was more concerned when using cheap wide-angle eyepieces such as “SWA” and other Erfle-like designs that I own, as they introduced aberrations like astigmatism and field curvature that can be obnoxious. Unfortunately, the stock 25mm eyepiece provided with the telescope also does this somewhat.

The optics in the 10” FlexTube are quite good, as is to be expected with most quality telescopes, particularly Dobsonians. I can achieve sharp images up to 500x with a good 10” Dobsonian, though atmospheric conditions are likely to limit me to 200x or below on most nights. High power is not always what I want to use on all targets anyway.

The Collimation Process

Collimation is fairly important at such a fast focal ratio.

Thankfully, unlike the smaller 8” Classic and 8″ FlexTube Dobsonians, I can adjust the 10” FlexTube’s primary mirror without using a screwdriver or wrench. But for the secondary mirror, I do require a hex key for adjustment unless I swap in aftermarket thumb screws in the place of the secondary mirror’s screws.

We don’t get any collimation tools with the 10” FlexTube, and you’ll probably need to purchase one.

The Collapsible Tube Feature

Of course, the collapsible tube of the FlexTube Dobsonians is their main feature.

I simply loosen/tighten a set of knobs on the lower half of the scope to extend/collapse the tube, which takes seconds. Provided I fully extend the tubes and lock them into place, extending/collapsing the FlexTube doesn’t result in much, if any, shift in the collimation of the mirrors.

However, it’s important to note that the 10” FlexTube’s optical tube is exactly 5 lbs (2 kg) heavier than its 10″ solid-tubed counterpart, and collapsing it fully saves me a whopping 13” of length. I also need a shroud to keep stray light out of the open tube (and a rigid one that won’t droop into the light path!), which is one more thing to purchase or make as well as put on during assembly.

Theoretically, I could fit the collapsed 10” FlexTube upright in a car seat or otherwise take advantage of the decreased length to fit it in an otherwise impossible configuration. But I still have to deal with the massive particle board Dobsonian base supplied with the scope. The only saving grace I see is that the collapsed 10” FlexTube tube (together with the built-in handles) is a little easier to carry than an equivalently sized solid tube, and it’s still easy to add handles or lifting straps to the tube if it’s too awkward. It’s unlikely the difference in tube length will otherwise help much.

The Single-Speed Crayford Focuser

The Sky-Watcher FlexTube 250P, like many of Sky-Watcher’s other Dobsonians, is powered by a 2″ single-speed Crayford focuser, with provided 2″ and 1.25″ focus extension tubes for focusing eyepieces (theoretically, this means the scope has enough in-focus travel for using a DSLR camera with it, but since this is a manual Dobsonian, you can’t take good long-exposure photos with it!).

Instead of a single 2” extension tube that I can leave installed on the focuser along with a 1.25” adapter for my 1.25” eyepieces, Sky-Watcher provides separate 1.25” and 2” extension tubes that I must swap out depending on which size eyepiece/accessory I’m using. You can avoid doing this if you bite the bullet and buy a regular 1.25” adapter (which is also required for you to use 2” filters with your 1.25” adapters).

I’m also not a fan of the provided extension tubes since they are of low quality and use thumb screws that dig into my eyepieces rather than brass compression rings, as does the focuser itself.

I can state that the provided focuser works well enough. But adjusting the focuser’s tension requires a hex key, and I believe a dual-speed function would’ve been nicer at higher magnifications for finer focus adjustment.

The Decent Eyepieces and Finderscope

The 10” FlexTube 250P Dobsonian comes with two “Super” eyepieces, which are 1.25”-barreled and variants on the Konig wide-angle, 3-element design: a 25mm yielding 48x and a 10mm for 120x. I found these to be fairly decent eyepieces with more eye relief than standard Plossl or Kellner oculars; a 10mm Plossl or Kellner practically requires me to jam my eyeball in to see anything, which is uncomfortable.

You’ll probably want additional eyepieces as time goes on, but the stock eyepieces are enough to get started with.

For a finder, the 10” FlexTube comes with a 9×50 right-angle, correct-image finder scope, which attaches via a standard bracket that is interchangeable with many other finderscope bases. I’ve noticed some users reporting that the provided finder scope is shipping with a mirror diagonal that gives flipped left-right images; this no longer seems to be the case to me.

A 9×50 finder shows me more stars and other objects than I can see with my eyes alone, but its limited field of view and the non-intuitive nature of looking perpendicular to the sky while trying to roughly aim is an issue. I think augmenting or replacing the provided 9×50 finder scope with a zero-power red dot or reflex sight might be a good idea.

The Particle Board Dobsonian Mount

The 10” FlexTube Dobsonian uses a Dobsonian mount, which pivots in an alt-azimuth configuration (up-down, left/right).

Like many mass-manufactured Dobsonians, this base is made out of ¾” particle board, the same stuff a lot of knock-down furniture is comprised of. I assemble it with a hex key and a bunch of included wood screws in the same manner that I would put together a typical piece of furniture from a big box store. This is pretty much a one-time procedure. I’ve learned that the particle board cannot withstand the screws being threaded and unthreaded more than once or twice before the screws simply no longer dig into the material anymore. 

I’m not a fan of particle boards, as it is also notoriously heavy and infamous for its lack of durability. An impact will make it crumple like cardboard, and lots of moisture will cause it to warp and disintegrate. I need to take good care of the base and I know that it will not last forever unless I simply do not take the telescope outside on a regular basis. But most people observe the night sky in a damp, dew-ridden environment.

For azimuth motion, the melamine-coated surface of the 10” FlexTube’s particle board base glides against three small plastic pads. The pads are not made of true PTFE (Teflon) and are somewhat undersized. As such, motions can be a little jerky at high magnifications. I realized that the altitude axis isn’t also much better.

As with the other manual Dobsonians Sky-Watcher sells, rather than using large altitude bearings resting on Teflon pads to pivot smoothly up and down while staying in place as needed, the 10” FlexTube uses a pair of small, circular bearings riding on plastic cylinders. The close-together cylinders, combined with the tiny bearings, simply don’t have enough friction to keep the scope in place on its own.

What’s worse is that due to the minuscule size of the bearings, using even a moderately heavy eyepiece, adding an additional finder, or installing a shroud is enough to shift the center of gravity of the telescope completely outside the altitude bearings. As a result, to compensate, Sky-Watcher has come up with the “innovative” solution of installing two bicycle handles as clutches (which they have, amazingly, patented).

The handles pull the rocker sides together tighter against the bearings, increasing the friction, which also means altitude movements are inevitably jerky or simply frozen altogether. The only solution I know is to either add counterweights to the back of the tube and swap them whenever you swap accessories, or build/retrofit a new mount with larger altitude bearings to compensate. Neither is exactly convenient.

Should I buy a Used Sky-Watcher 10” FlexTube Dobsonian?

A used 10” FlexTube Dobsonian would be an excellent telescope.

But make sure that the struts extend smoothly, the base is in decent shape (or be prepared to pay less to account for the cost of making a new one yourself), and that the optics are in good condition. That means that the mirrors are still clearly reflective and don’t show signs of corrosion; a little dust is fine. Small dents to the optical tube are not an issue as long as they don’t impact the struts’ operation; big ones can usually be pulled out with an automotive dent puller or hammered.

Alternative Recommendations

The 10” FlexTube Dobsonian isn’t bad, but there are arguably better choices for the money and telescopes that offer various different features and accessories. Here are a few of my favorites, sorted by price range:

Under $700

• The Explore Scientific 10” Hybrid Dobsonian uses a true truss tube design to be more compact than the 10” FlexTube Dobsonian, and its all-metal structure isn’t as fragile as the FlexTube’s massive particle board base. The altitude bearings are vastly better designed to maintain balance with any accessory without the need for clutches, and collimation is easier.
• The Apertura AD8 is of course smaller than the 10” FlexTube Dobsonian, but in addition to a 9×50 finder, it includes a high-quality 2” SuperView 30mm eyepiece, a built-in cooling fan, and altitude bearings that can slide along the optical tube for optimal balance.
• The Sky-Watcher Virtuoso GTi 150P features a miniaturized version of the FlexTube’s collapsible tube, with the same Super 25mm and 10mm eyepieces included. The Virtuoso GTi 150P easily fits in a backpack and can be carried outside and set up in seconds thanks to its tabletop Dobsonian design. It also features full motorized GoTo and tracking, controlled by your smartphone or tablet, though you can aim it manually freely with or without powering the scope on. A manual version is also available as the Heritage 150P, alongside 130mm Virtuoso GTi and Heritage tabletop Dobsonian models.

$700-$1000

• The Apertura AD10/Zhumell Z10/Orion SkyLine 10 offers the same performance as the 10” FlexTube Dobsonian, but with the same accessories and features provided as the AD8/Z8. The simplicity of the solid tube design is a nice bonus, as are the adjustable altitude bearings.
• The Celestron StarSense Explorer 8” Dobsonian has less aperture than the 10” FlexTube and features the same flawed altitude bearing design (though without the obnoxious handles), but with Celestron’s StarSense Explorer technology, carry handles attached to the optical tube, and cutouts to reduce the weight of the particle board base.
• The Sky-Watcher 8” FlexTube Dobsonian has basically the same features and drawbacks of the 10” model, but scaled down and with the inability to adjust the primary mirror collimation without a screwdriver. The collapsible tube is even less useful with an 8” telescope, however.

$1000-$1500

• The Apertura AD12/Zhumell Z12/Orion SkyLine 12 is much like a scaled up AD10/AD8, though the longer tube and monstrous base may cause some difficulty in setup or transport, particularly if you have a smaller vehicle or lack sufficient storage space.
• The Celestron StarSense Explorer 10” Dobsonian features the same optics and focuser as the 10” FlexTube Dobsonian, but with Celestron’s StarSense Explorer technology, a solid tube, and a dramatically lighter weight design.
• The Explore Scientific 10″ Truss Tube Dobsonian is similar to the 10” Hybrid but features a dual-speed focuser and a more minimalist upper tube assembly, along with built-in cooling fans in the mirror box.

Aftermarket Accessory Recommendations

A shroud is de facto mandatory for the 10” FlexTube Dobsonian. Without one, stray light will get into the tube, your optics are more likely to dew up, and if you are setting up for other people, it’s more likely someone will end up putting their hand or dropping something inside the tube and smudging/damaging the optics as a result. You can purchase a shroud or sew your own out of a suitable stretchy fabric like Ripstop Nylon, Lycra, or Spandex.

While you can get by with the 10” FlexTube Dobsonian’s included 9×50 right-angle correct-image finder alone, a zero-power reflex sight is a good complement to its abilities or a replacement altogether. The Rigel Quikfinder is lightweight and sticks to the tube, while the bulkier Telrad or QuInsight are limited in where you can place them without getting in the way of the FlexTube’s strut hardware and knobs. All provide a zero-power view with a “bull’s eye” reticle; the QuInsight features rotatable crosshairs and a dew shield, while the QuikFinder has a built-in dew shield and a blink/pulse mode.

The 10” FlexTube Dobsonian’s included eyepieces are just fine, but you might want some stuff for lower and higher magnifications. A 15mm wide-angle or redline eyepiece (80x) bridges the gap in the magnifications provided by the stock 25mm and 10mm eyepieces, while a high-power planetary eyepiece like the Astromania 4.5mm (267x) provides suitable magnification for close-up views of the Moon, planets, and double stars on a steady night. The Apertura 32mm SWA 2” eyepiece (38x) is a great pick if you are able to compensate for the balance issues it will probably cause; it delivers lower magnification and a vastly wider field than the stock 25mm Super eyepiece.

A 2” to 1.25” adapter will allow you to leave the 2” extension tube permanently seated in the FlexTube’s focuser, as well as use 2” filters with your 1.25” eyepieces and grip your eyepieces with a non-marring compression ring rather than gouging their barrels with a metal screw. There are a lot of options to choose from, but the Astromania twist-lock unit is a good deal and takes thumb screws out of the equation altogether.

A UHC (ultra high contrast) nebula filter increases the contrast and brings out faint detail in nebulae, making previously invisible targets possible to see under good conditions. Our top pick in this category is the 2” Orion Orion UltraBlock filter. You really don’t need any other filters; moon/planetary color or dimming filters just blur your view and don’t help at all in most cases, while specialized H-Beta and Oxygen-III nebula filters are seldom even used by experienced observers.

What can you see?

Our views through any telescope of deep-sky objects—faint objects outside the Solar System such as nebulae, galaxies, and star clusters—are heavily affected by light pollution.

Under light-polluted skies, we’ll be able to see open star clusters, which are usually composed of bright and colorful stars that look vivid and stunning with a large-aperture telescope like the 10” FlexTube. The Double Cluster in Cassiopeia, M11 in Scutum, and M35 in Gemini are all examples of these. There are many more open star clusters throughout the Milky Way, often adjoining bright and dark nebulae or larger “star cloud” regions.

Globular star clusters are more crowded, with hundreds of thousands of stars crammed into areas that are only about a hundred light-years across. Globular star clusters are a little harder to see in places with a lot of light pollution, but the 10″ FlexTube Dobsonian should be able to see the brightest ones, like M13 and M15, in almost any sky at high magnification. Some clusters, like M92, show an ellipsoidal shape; others, like M13, have dust lanes, and smaller and dimmer globulars are difficult to resolve or even see with the 10” FlexTube Dobsonian due to their smaller size and greater distance from us.

The brightest nebulae can be seen with the 10” FlexTube Dobsonian under light-polluted skies, but the best views are from dark skies far from city lights. I was able to see the jaw-dropping details and embedded star clusters of the Orion Nebula (M42) or the Lagoon (M8), and a UHC nebula filter brings out fainter wispy details as well as increasing contrast as a whole. The Swan Nebula (M17) looks magnificent, and a filter brings out its dark lanes and wispy regions far from the main nebula. The 10” FlexTube can also show me the magnificent Veil Nebula supernova remnant or the gigantic Helix planetary nebula, provided I use a good UHC nebula filter and have access to at least semi-dark skies. Small planetary nebulae like the Ring or Cat’s Eye are bluish or greenish in color, are less affected by light pollution due to their brightness and small size, and show all sorts of fine detail on a steady night.

Galaxies are severely affected by light pollution, and even the brightest ones appear as mere dim fuzzy ovals under city skies. But if I can get my 10” FlexTube Dobsonian out to a dark location, I’ll be able to see tens of thousands of them. I’ll be able to start resolving the spiral arms of galaxies like M51 or M33, and look at the dust lanes in dozens of galaxies like M31, M64, M82, and M104. Small galaxy groups like the Leo Triplet are abundant, and many of their members display hints of detail, and there’s always the huge Virgo Cluster, containing hundreds of galaxies with dozens displayed at a time in one low-power field of view.

Light pollution doesn’t really change the way Solar System objects look (except for making some faint moons visible), and the 10″ aperture of the FlexTube gives better resolution than a smaller telescope.

Mercury and Venus show their phases—though little else—at any magnification.

• The Moon provides jaw-dropping vistas of detail, with countless craters, ridges, mountains, and ridges mere miles across visible regardless of phase or viewing conditions.
• The 10” FlexTube reveals the polar ice caps of Mars and any ongoing dust storms almost any time the planet is visible, and when Mars is at or near its closest approach to Earth, you can make out a few dark markings on its surface as well.
• The 10” FlexTube Dobsonian easily shows me Jupiter’s four large Galilean moons (even the 9×50 finder should reveal all 4) and on a steady night, their disks and shadows can be seen when they transit in front of Jupiter. Jupiter itself shows many colorful cloud belts and storms, which constantly change as the days and weeks go by. The Great Red Spot can also be resolved amidst Jupiter’s turbulent cloud bands.
• I could also see the rings and several moons of Saturn, with the Cassini Division in the rings and cloud belts on Saturn itself visible on a steady night with the 10” FlexTube.
• Uranus appeals as a turquoise-teal dot with the 10” FlexTube Dobsonian, with some cloud details and a few faint moons visible on the best nights at very high magnifications.
• Neptune’s disk is hard to resolve clearly even under good conditions, but its moon Triton is not difficult to spot beside it with the 10” FlexTube Dobsonian.
• Pluto can also be seen as a faint, star-like point if you know where to look, provided you have dark enough skies.