The Optical Tube
The Skywatcher Skyliner 250P FlexTube Dobsonian is optically a 10” (254mm) f/4.7 Newtonian reflector with a focal length of 1200mm. An f/4.7 reflector has some coma at the edges of the field of view with wide-angle, low-power eyepieces, but it’s not enough to be bothersome or really dictate the need for a coma corrector. However, cheap wide-angle eyepieces such as “SWA” and other Erfle-like designs will show aberrations like astigmatism and field curvature that can be obnoxious; the stock 25mm eyepiece provided with the telescope does this somewhat. The optics in the 10” FlexTube are quite good, as is to be expected with most quality telescopes, particularly Dobsonians. You can achieve sharp images up to 500x with a good 10” Dobsonian, though atmospheric conditions are likely to limit you to 200x or below on most nights, and high power is not always what you want to use on all targets anyway.
Collimation is fairly important at such a fast focal ratio; thankfully, unlike the smaller 8” Classic and FlexTube Dobsonians, the 10” FlexTube’s primary mirror can be adjusted without using a screwdriver or wrench, though the secondary mirror requires a hex key for adjustment unless you swap in aftermarket thumb screws. You don’t get any collimation tools with the 10” FlexTube, and you’ll probably need to purchase one.
Of course, the collapsible tube of the FlexTube Dobsonians is their main feature. You simply loosen/tighten a set of knobs on the lower half of the scope to extend/collapse the tube, which takes seconds. Provided you fully extend the tubes and lock them into place, extending/collapsing the FlexTube shouldn’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 solid-tubed counterpart, and collapsing it fully will save you a whopping 13” of length. You also need a shroud to keep stray light out of the 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, you 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 you still have to deal with the massive particle board Dobsonian base supplied with the scope, and it’s unlikely the difference in tube length will otherwise help much. The only saving grace 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, though most able-bodied adults should have no trouble carrying a solid-tubed 10” optical tube, and it’s easy to add handles or lifting straps to one if it’s too awkward.
The 10″ FlexTube, like many of Sky-Watcher’s other Dobsonians, is powered by a 2″ single-speed Crayford focuser, with provided 2″ and 1.25″ extension tubes for reaching focus with an eyepiece (theoretically, this means the scope has enough in-focus travel for a DSLR camera, but this is a manual Dobsonian – you’re not going to be taking long-exposure photos with it!). Rather than give you a single 2” extension tube that you can leave installed and a 1.25” adapter for your 1.25” eyepieces, Sky-Watcher provides 1.25” and 2” extension tubes that must be swapped out depending on which size eyepiece/accessory you’re using, unless 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). The provided extension tubes are low-quality and use thumb screws that dig into your eyepieces rather than brass compression rings, as does the focuser itself. The provided focuser works well enough, but adjusting the tension requires a hex key, and a dual-speed function would be nice at higher magnifications for finer focus adjustment.
The Skywatcher Skyliner 250P FlexTube 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. These are fairly decent eyepieces with more eye relief than standard Plossl or Kellner oculars; a 10mm Plossl or Kellner practically requires you to jam your 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. Some users have reported the provided finder scope as shipping with a mirror diagonal that gives flipped left-right images; this no longer seems to be the case. A 9×50 finder will show you more stars and other objects than you can see with your eyes alone, but the limited field of view and the non-intuitive nature of looking perpendicular to the sky while trying to roughly aim can be an issue. Augmenting or replacing the provided 9×50 finder scope with a zero-power red dot or reflex sight might be a good idea.
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—and you assemble it in the same manner that you would put together a typical piece of furniture from a big box store with a hex key and a bunch of included wood screws. This is pretty much a one-time procedure; 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.
Particle board 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. You should take good care of the base and keep in mind that it will not last forever unless you simply do not take the telescope outside on a regular basis, as 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. The altitude axis isn’t 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 is to either add counterweights to the back of the tube and swap them whenever you swap accessories, or built/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.
The 10” FlexTube Dobsonian isn’t bad, but there are arguably better choices for the money and telescopes that offer various different features and provided accessories. Here are a few of our favorites, sorted by price range:
- The StellaLyra 8”/Zhumell Z8/Orion SkyLine 8 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.
- The StellaLyra10”/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.
- The StellaLyra 12”/Zhumell Z12/Orion SkyLine 12 is much like a scaled up StellaLyra 10”/StellaLyra 8”, 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
Before considering any other costly accessories, it’s absolutely essential to purchase a shroud for the Skyliner 250P FlexTube’s open tube, as is the case with almost any open-tubed strut or truss telescope. Without a shroud, glare from the Moon, nearby sources of light such as street lamps or passing cars, and the general glow of light-polluted skies will significantly impact contrast at the eyepiece, severely affecting the views through this telescope, especially those of faint deep-sky objects like nebulae and galaxies where maximum contrast and darkness are required to see anything at all. A shroud also mitigates the formation of dew and frost on your secondary mirror and will tend to keep your optics cleaner too.
You’ll also want a wider selection of higher-quality eyepiece than the simple oculars provided with the Skyliner 250P FlexTube Dobsonian. A wide range of magnifications is crucial for observing different types of objects under different conditions. For low-power viewing with the Skyliner 250P FlexTube, a 28mm UWA (43x) or 32mm SWA or PanaView (38x) eyepiece will offer approximately the lowest useful magnification and the broadest feasible field of view for this relatively fast telescope. Additionally, a coma corrector such as the Explore Scientific HRCC or Baader MPCC can help eliminate coma present at the edges of your low-power field of view with this scope, as it is otherwise fairly obvious at f/4.7.
For intermediate magnification with the Skyliner 250P FlexTube, we suggest a 16mm Ultra Wide Angle (UWA) eyepiece (75x magnification) or, as a more budget-friendly alternative, a 15mm redline/goldline ocular (80x magnification) compatible with the Skyliner 250P FlexTube Dobsonian. You might also consider replacing the stock 10mm ocular with a 10mm UWA (120x) or 9mm redline (133x magnification) eyepiece for sharper and more immersive views. For higher magnification, we recommend 7mm (171x) and 4mm (300x) UWA or planetary eyepieces to obtain sharp and detailed views of the Moon, planets, and double stars with the Skyliner 250P FlexTube Dobsonian when atmospheric conditions permit.
Users of the Skyliner 250P FlexTube and other Dobsonian telescopes may find aiming with just a right-angle finder, such as the supplied 9×50, to be rather challenging. This is due to the fact that coarsely aiming without looking down the barrel of the telescope is not the most intuitive method. A widely adopted and strongly suggested solution is to replace the finder scope entirely or to supplement it with a Telrad.
It is also worth considering the replacement of the Skyliner 250P FlexTube’s stock 1.25″ extension tube/adapter system and 2″ extension tubes with higher-quality alternatives. A compression ring 1.25″ adapter with filter threads will facilitate the use of 2″ filters with your 1.25″ eyepieces, providing a secure, non-marring grip without the risk of your eyepieces being scratched or falling out of the adapter. Similarly, a 35mm threaded extension tube with a 2″ compression ring adapter will offer the same benefits for heavy and expensive 2″ eyepieces. These upgraded adapters will also enable more precise collimation by properly aligning a collimation tool with the telescope and focuser body. At f/4.7, accurate collimation is crucial – investing in a Cheshire collimation tool is a wise and relatively minor investment that will considerably simplify the process of accurately collimating the Skyliner 250P FlexTube.
Lastly, a narrowband Ultra High Contrast (UHC)/OIII nebula filter can substantially enhance your views of nebulae, such as the Orion Nebula, when using virtually any telescope, including the Skyliner 250P FlexTube. This filter also augments the visibility of planetary nebulae by diminishing the brightness of surrounding stars, making it more straightforward for you to locate them at low power. Furthermore, it offers sufficient contrast improvement to reveal previously invisible nebulae, such as the Veil Nebula or the Flame Nebula, utilising the Skyliner 250P FlexTube under dark skies. A 2″ filter will attach to an aftermarket threaded 1.25″ adapter, like the compression ring adapter mentioned previously, thus ensuring compatibility with either size eyepiece.
What can you see?
Your 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, you’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 will be 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 may be 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 will be from dark skies far from city lights. You’ll be 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 you the magnificent Veil Nebula supernova remnant or the gigantic Helix planetary nebula, provided you 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 you can get your 10” FlexTube Dobsonian out to a dark location, you’ll be able to see tens of thousands of them. You’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, and 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 will easily show you 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. You can 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.