The Optical Tube
The SkyWatcher Skyliner 250P FlexTube GoTo is a 10” (254mm) f/4.7 Newtonian reflector with a focal length of 1200mm. Though the included 25mm Plossl eyepiece works well enough for low power, cheap wide-angle eyepieces such as “SuperView” or “SWA” eyepieces have edge-of-field astigmatism and field curvature at f/4.7. Low-power eyepieces with a field stop wider than the included 25mm Plossl will also have noticeable coma around the edges of the field of view – though other aberrations such as astigmatism may mask it. It’s important to take this into account when shopping for eyepieces as cheaper wide-angle models may not offer the best performance and could be a waste of money when used in conjunction with this telescope.
The Skyliner 250P FlexTube GoTo of course features Sky-Watcher’s FlexTube collapsible tube system, which allows the tube to retract from its normal length of 44” to 32”. What do you gain from this? In actuality, not much. The 44” tube is already short enough to fit in most vehicles, and the FlexTube design makes no difference in the overall weight of the telescope’s tube compared to a conventional solid-tubed design. You also will need a shroud to cover the exposed section of the tube and keep out stray light, and collimation adjustment might be required a bit more frequently.
Collimation of the Skyliner 250P FlexTube’s primary mirror can be adjusted with standard hand knobs at the back, while secondary adjustment requires a hex key; collimation is a fairly straightforward process, and our collimation guide provides more information. Swapping the secondary mirror’s hex screws for thumb screws is unnecessary and will likely lead to more collimation shift. The trusses have a second setting providing enough in-focus travel for binoviewer or camera use, and both 1.25” and 2” extension tubes are provided as is typical with Sky-Watcher’s Dobsonians.
Attaching the Skyliner 250P FlexTube GoTo to its Dobsonian Base is fairly straightforward – simply rotate the left side bearing holder into place, lower it onto the base horizontally, and tighten one knob for secure attachment.
The SkyWatcher Skyliner 250P FlexTube GoTo Dobsonian comes with 1.25” 25mm (48x) and 10mm (120x) “Super Plossl” oculars, which are decently well-corrected but have a narrow 50-degree apparent field of view. This means that there won’t be any issues with coma or astigmatism, though they are not the best eyepieces to use given the size and cost of this scope. The 10mm, being a Plossl, is also fairly short on eye relief, requiring you to jam your eye into the lens to take in the full field of view.
As with the other Sky-Watcher Dobsonians, the typical separate 1.25” and 2” extension tubes supplied, which are required to reach focus, are of very low quality, will mark up your eyepieces and don’t allow for the attachment of filters. They can also interfere with the accuracy of a laser collimator; replacing them would be a good idea.
For aiming and alignment purposes, this telescope includes a 9×50 straight-through finder scope with crosshairs offering an approximately 6-degree true field and an upside-down view just like you see through the telescope itself. Although overkill for aligning the scope’s GoTo system, it is not as user-friendly or comfortable to look through as a right-angle finder or reflex sight. Fortunately, replacing it with either alternative is simple, thanks to its standard Synta/Vixen-style finder shoe.
As with most GoTo Dobsonians, the Skyliner 250P FlexTube GoTo Dobsonian uses an alt-azimuth Dobsonian mount made mostly out of particle board with servo motors and slip clutches attached. The altitude axis uses a motorized bearing on one side which you clamp the scope to, with a traditional Dobsonian bearing on the other side to hold the remaining weight and minimize the power needed to move the scope around. The slip clutches on both the altitude and azimuth axis can be unlocked and allow you to freely aim the scope around the sky, while Sky-Watcher’s FreedomFind dual encoders keep track of where the scope is pointed allowing for no interruption of tracking and pointing capabilities. The motorized kit does make the Skyliner 250P FlexTube GoTo’s Dobsonian base a bit heavier than its manual counterpart, which can be something to consider – at 39 lbs vs. the manual scope’s 27 lbs, lifting it can be a bit of a chore.
Depending on which version you buy, the Skyliner 250P FlexTube GoTo Dobsonian comes with either a SynScan hand controller or a WiFi dongle, allowing you to control it via the SynScan app which can be coupled to SkySafari Pro for easy pointing on your smartphone or tablet. Given the simpler interface of the app and the lower price of the WiFi version we would probably recommend it over the SynScan hand controller, which can be a little more difficult to navigate and set up.
Should I buy a Used Skywatcher Skyliner 250P FlexTube GoTo Dobsonian?
A used Skywatcher Skyliner 250P FlexTube GoTo has a fair amount of things that can go wrong – any damage to the base can render the GoTo system inoperable and you should of course make sure that the electronics work. The poles should collapse and extend smoothly, and of course any corrosion to the primary and secondary mirrors can necessitate a recoat which may or may not be worth the cost compared to simply buying a new scope instead of a damaged used unit. However, a used Skyliner 250P FlexTube GoTo in good shape can last you a lifetime.
The 10’ FlexTube GoTo Dobsonian is certainly one of the better Dobsonians in its size range available and certainly the best 10” GoTo. However, you may want to consider a few alternative options.
- The StellaLyra 12”/Apertura AD12/Orion Skyline 12’s large 12” aperture gives it significantly more light-gathering and resolving power than a 10” scope and the provided accessory package is quite good – however, it’s rather heavy/bulky on account on its massive solid tube.
- The Zhumell Z10/StellaLyra 10”/Orion Skyline 10 is an affordable 10” Dobsonian packed with features and accessories, offering some of the best value for the money of any telescope of this aperture.
- The Celestron StarSense Explorer 10” Dobsonian’s lightweight base and StarSense Explorer technology make it quick to set up and a breeze to use, though the scope lacks anything but the most basic accessories and features to get you started out with and the price is a little steep.
- The Sky-Watcher 12″ Flextube Collapsible Dobsonian is essentially a scaled-up Skyliner 250P FlexTube but the collapsible tube provides significantly more space savings at this aperture. This can make a huge difference for portability, though the manual Sky-Watcher Dobsonians aren’t exactly known for having the most well-designed and smooth mounts. The GoTo version of the 12” FlexTube has the same features as the 10” model and is an even better choice.
- The Explore Scientific 12″ Truss Tube Dobsonian is extremely compact when disassembled and features an all-metal design, built-in fans and a dual-speed Crayford focuser. However, it comes with no usable accessories to start out with.
- The Orion SkyQuest XT10G GoTo Dobsonian is identical to the Skyliner 250P FlexTube GoTo Dobsonian in design and function apart from of course a solid tube, a dual-speed 2” Crayford focuser and a slightly different set of accessories.
- The Celestron NexStar Evolution 8” isn’t quite as capable as a 10” Dobsonian on account of its smaller aperture and long focal length, but it still has plenty of light-collecting and resolving power, is extremely compact, and features built-in WiFi and an onboard lithium battery as well as some astrophotography capabilities which a GoTo Dobsonian cannot attain.
- The Orion SkyQuest XX14i Dobsonian’s huge 14” primary mirror blows away the views through a smaller 10” or 12” while the IntelliScope digital setting circles help you to find your way around the sky. However, despite its truss tube, it is quite big and you should really have a smaller telescope to accompany it.
- The Orion SkyQuest XX12G Dobsonian has the same GoTo mechanics and features as the Sky-Watcher GoTo Dobsonians but its full truss tube makes it more portable than the 12” FlexTube, albeit more complicated to assemble.
- The Skywatcher Skyliner 250P FlexTube GoTo features the same overall design and features of the 10” model but of course with additional aperture, while still remaining fairly portable and convenient, if a bit heavier than the Skyliner 250P FlexTube GoTo or the manual 12” model.
Aftermarket Accessory Recommendations
A shroud is required for any truss or collapsible tube telescope, including the Skyliner 250P FlexTube. 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 necessary to see anything at all. A telescope without a shroud is also more prone to dew, frost, and fogging of the secondary mirror, and a shroud keeps dust and dirt out of your optics as well – don’t forego picking one up.
To fully utilize the wide range of useful magnifications and field of view options with the Sky-Watcher Skyliner 250P FlexTube GoTo Dobsonian, you should also consider purchasing at least a couple of additional eyepieces. The provided pair of 3-element 1.25″ eyepieces is adequate for acquainting yourself with the basics of astronomical observation, and the 120x magnification offered by the 10mm ocular allows for reasonable planetary views. However, access to a wide range of magnifications is essential for observing a variety of celestial objects. For low-power viewing with the Skyliner 250P FlexTube, a 28mm UWA (43x) or 32mm SWA or PanaView (38x) eyepiece will provide about the lowest useful magnification and widest possible field of view that can be used with this relatively fast scope. A coma corrector such as the Explore Scientific HRCC or Baader MPCC will clean up the coma present at the edges of your low-power field of view with this scope, too.
For intermediate magnifications between the included 25mm and 10mm eyepieces, we recommend a 16mm Ultra Wide Angle (UWA) eyepiece (75x magnification) or, as a more cost-effective alternative, a 15mm redline/goldline ocular (80x magnification) appropriate for the Skyliner 250P FlexTube GoTo Dobsonian. You could also replace the stock 10mm ocular with a 10mm UWA (120x) or 9mm redline (133x magnification) for sharper and more immersive views. For higher power, we recommend 7mm (171x) and 4mm (300x) UWA or planetary eyepieces for high magnification views of the Moon, planets, and double stars with the Skyliner 250P FlexTube GoTo Dobsonian.
It is also worth considering the replacement of the Skyliner 250P FlexTube GoTo Dobsonian’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 enable 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 allow for more precise collimation by properly aligning a collimation tool with the telescope and focuser body. At f/4.7, accurate collimation is essential – investing in a Cheshire collimation tool is a wise and relatively minor investment that will greatly simplify the process of accurately collimating the Skyliner 250P FlexTube GoTo Dobsonian.
Lastly, a narrowband Ultra High Contrast (UHC)/OIII nebula filter can significantly enhance your views of nebulae, such as the Orion Nebula, when using virtually any telescope, including the Skyliner 250P FlexTube GoTo Dobsonian. Moreover, it provides enough contrast improvement to reveal previously invisible nebulae, such as the Crescent Nebula or the Crab Nebula, with the Skyliner 250P FlexTube GoTo Dobsonian under dark skies. A 2″ filter will attach to an aftermarket threaded 1.25″ adapter, such as the one mentioned earlier, thus ensuring compatibility with either size eyepiece.
What can you see?
The Skyliner 250P FlexTube GoTo Dobsonian of course puts up the bright and bold views of any good 10” Dobsonian under good conditions – provided you get a shroud for the tube, that is. Performance on deep-sky objects is inevitably going to be limited by whatever light pollution conditions you find yourself under – only the darkest skies will allow you to see decent detail in galaxies while severe light pollution will wreck nebulae even with a filter, and obscure globular star clusters too. The same goes for moonlight – if the Moon is more than a thin crescent it’s as bad as the worst city lights. Open star clusters are fairly resilient, however, and those such as the Double Cluster, M35, or M45 are spectacular from almost any sky. Globular star clusters like M13 and M15 can almost always be resolved into individual stars with the Skyliner 250P FlexTube GoTo Dobsonian at high magnifications and the scope’s GoTo system will find them easily, as well as keep them centered at high power.
While appearing as washed-out smudges to the beginner or under bad conditions, with time and effort you’ll be able to see plenty of detail in galaxies with a 10” Dobsonian under good skies. The Skyliner 250P FlexTube GoTo Dobsonian will be able to reveal H-II regions and hints of spiral arms in M51 and M33, dust lanes in M82 or M31, and many irregularly shaped galaxies, interacting pairs, and huge groups, including the vast Virgo Cluster containing hundreds of visible members.
Nebulae are also delightful with the Skyliner 250P FlexTube GoTo Dobsonian. The Orion Nebula (M42), the Lagoon (M8) and the Swan (M17) dazzle even from suburban skies while a UHC filter enhances the view. Dark skies will allow you to see the Veil Nebula supernova remnant with the aid of a UHC filter, along with the Rosette Nebula, the Flame Nebula, and perhaps even the Horsehead. Smaller planetary nebulae like the Cat’s Eye or the Blue Snowball reveal emerald and deep blue colors too, with intricate details visible with high magnification and good seeing. Double stars are also plentiful and many feature contrasting colors, often requiring very steady conditions (and of course good collimation) to split apart at high magnification.
10” of aperture is often considered the “sweet spot” for high-power lunar and planetary viewing before diminishing returns kick in as atmospheric conditions rarely allow the utilization of the full resolving power of much large instruments. The Skyliner 250P FlexTube GoTo Dobsonian will show you the phases of Mercury and Venus easily, along with dark markings and polar ice caps on Mars under favorable conditions and of course a wealth of fine detail on the Moon which never disappoints. Jupiter’s vivid cloud belts, storms, and of course the Great Red Spot are a delight, and you can watch the four Galilean moons (themselves easily visible in the 9×50 finder) transit and cast inky-black shadows; during transits their disks are more readily apparent and you may be even able to resolve some hints of shading on Ganymede, the largest of the four. Saturn’s rings delight with the Cassini Division visible within; when the rings are at a wider tilt (not until after 2030) you may also be able to make out the Encke gap in the rings too. Saturn shows cloud bands like Jupiter’s but they are dull, uniform and less distinct, while a half dozen moons can be seen next to it as star-like points, the brightest being gold-colored Titan, which is vaguely apparent as not-quite-a-star on account of its angular size, just barely resolved with the 10” FlexTube under very steady conditions. You can also tell that the asteroid Vesta is out of round under similar conditions at very high magnfications; it is around the same angular size and color as Titan.
Uranus’ teal-green disk is resolved with the Skyliner 250P FlexTube GoTo Dobsonian with possible hints of cloud detail, while up to 4 of its moons might be visible if you can spot them amid its glare; our guide on observing Uranus goes into more detail. Neptune is often just a bluish fuzz but its moon Triton is fairly obvious next to it, while Pluto is just barely within the grasp of the Skyliner 250P FlexTube GoTo Dobsonian’s light-gathering power under dark skies as it continues to grow dim; the dwarf planet is still visible as a star-like point if you know where you are looking.
The Skyliner 250P FlexTube GoTo Dobsonian’s fairly large aperture and motorized tracking capabilities make it an excellent choice for planetary astrophotography. It is on par in performance with many of the popular catadioptric telescopes among lunar and planetary imagers. To get the sharpest possible images, a color high-speed CMOS camera like the ZWO ASI224MC in combination with a 5x Barlow lens is ideal; with just a few minutes of footage, amazing shots can be achieved of the Moon and planets. Deep-sky astrophotography is not viable with an alt-azimuth mounted, unguided telescope such as the Skyliner 250P FlexTube however.