The Optical Tube
The 12” FlexTube Dobsonian is a 12” (300mm) f/5 Newtonian reflector with a focal length of 1500mm. At f/5, coma is present with wide-angle 2” eyepieces, though the bigger concern is other aberrations like edge-of-field aberrations with many cheaper eyepiece designs like Erfles, Kellners, and so forth. Collimation is fairly critical at f/5, and the 12” FlexTube has a primary mirror that is easily adjusted without tools, while the seldom-adjusted secondary mirror requires a hex key. No collimation tools are provided with the 12” FlexTube.
The 12” FlexTube has great optics and can theoretically handle magnifications up to 600x, assuming you have good seeing conditions. However, under many circumstances, you’ll be hard-pressed to be able to take advantage of half that magnification or see a noticeable improvement over a typical 10” in resolving power, all other things being equal. The main gains are on deep-sky objects. The primary mirror cell sits on a 9-point floatation support to prevent the mirror from bending under its own weight and producing deformed images.
The hallmark feature of the 12” FlexTube’s optical tube assembly is, of course, the collapsible tube. When extended, the tube is 55” long; collapsed, it’s 36” long. This is a big difference of 19″ compared to the 8″ and 10″ models, which only saved 11″. When the telescope is collapsed from 55″ to 36″, it can fit in a closet under clothes or across the back seat of a car, which it couldn’t do when it was extended. The FlexTube extends in seconds; you just unlatch a few knobs, extend the poles until they hit their stops, and tighten the knobs again. Provided you extend the poles all the way, the scope holds collimation fairly well, even after repeated assembly and disassembly.
When collapsed, the 12” FlexTube is fairly easy to carry; numerous carry handles are built into the tube, and it only weighs 46 lbs (20.9 kg), 2 lbs lighter than a comparable solid-tubed 12” OTA, and of course significantly less awkward to pick up and move around.
Like the other FlexTube Dobsonian sold by Sky-Watcher, the 12” FlexTube uses a 2” single-speed Crayford focuser, which lacks any compression ring and requires tools to adjust tension. It also requires an extension tube for use with most eyepieces. Rather than give you a 2” extension tube and 1.25” adapter, however, Sky-Watcher provides a 2” extension tube and a separate 2” to 1.25” extension tube/T-adapter; using 2” filters or quickly swapping from 2” to 1.25” adapters requires obtaining a regular 2” to 1.25” adapter with filter threads.
The Sky-Watcher 12” FlexTube Dobsonian includes two “Super” Plossl 1.25” eyepieces: a 25mm providing 60x magnification and a 10mm providing 150x. These eyepieces are fine, but a large Dobsonian like the 12” FlexTube benefits from wide-angle eyepieces designed for fast telescopes.
For a finder, the 12” FlexTube comes with a right-angle, correct-image 9×50 finder scope. This finder has an image that’s neither inverted left-right nor flipped upside down, matching the orientation of star charts and astronomy apps. It also shows stars a few magnitudes fainter than what you can see with your unaided eyes alone, a few bright deep-sky objects, and it has a field of view of about 6 degrees, similar to many binoculars. However, looking down the perpendicular eyepiece can be a little confusing when trying to coarsely aim the telescope, and many people end up having to augment the 9×50 finder with a zero-power red dot or reflex sight for initial aiming and then use the 9×50 for fine adjustment.
The Sky-Watcher 12” FlexTube uses a Dobsonian mount of the same design as the other Sky-Watcher manual Dobsonians. You assemble the mount – made out of ¾” laminate-covered particle board – from a flat-packed box with a pile of included wood screws and a hex key. While theoretically able to be disassembled, the particle board cannot maintain the threads cut into it by the wood screws if you take it apart and put it back together multiple times, so assembly is essentially permanent. The sides of the base feature additional 90-degree braces to keep the mount stiff, and there is little to no weight optimization, the ground board is a solid circle instead of a triangle, and the base has no cutouts—only a single, poorly positioned handle. The base is claimed to be 38 lbs, but it’s in all likelihood heavier than that (we were unable to get a sufficiently wide scale for our sample). The weight and bulk of the 12” FlexTube’s base are far more of a bottleneck for transporting it than the collapsed optical tube.
For azimuth, the 12” FlexTube glides on 3 plastic pads against its laminate-covered base to provide smooth motions as with any good Dobsonian. The 12” FlexTube uses the same flawed altitude bearing design as the other Sky-Watcher Dobsonians, however—the undersized, cylindrical bearings ride on plastic cylinders, with bicycle handles acting as “brakes” or clutches to adjust friction. Thankfully, compared to the smaller FlexTubes, the bearings are still bigger, and thus balance issues with heavy eyepieces and accessories are less likely to occur, but the design is still annoying. In extreme cases of top-heaviness, you’re forced to lock the bearings in place or add counterweights to the back of the tube, however, which is not ideal.
Should I buy a Used Sky-Watcher 12” FlexTube Dobsonian?
The 12” FlexTube is a great telescope, and there is little to go wrong with a used one. A damaged or missing base can be replaced, though expect to pay less for one. Small dents in the tube are not a concern as long as they don’t get in the way of the optics or FlexTube system. As usual, be sure to check that the reflective coatings on the mirrors of the 12” FlexTube are not corroded or otherwise damaged, though a little dust or fingerprints are fine and fairly easy to clean.
The 12” FlexTube is our top pick in its price range, but there are certainly reasons to go with other 12” Dobsonians or a smaller instrument depending on your budget or needs.
- Manual Scope (Best Value): The Apertura AD12/Zhumell Z12/Orion SkyLine 12 has a massive solid tube, and is considerably less portable than the 12” FlexTube. However, it’s cheaper, less complex, and comes with more features like a dual-speed focuser and a built-in cooling fan.
- Manual Scope: The Apertura AD10/Zhumell Z10/Orion SkyLine 10 has the same features and accessories as the AD12/Z12 but in a considerably smaller form factor which is more friendly for portability.
- Partially Computerized Scope: The Celestron StarSense Explorer 10” Dobsonian is a little more lightweight than the AD10/Z10, and features Celestron’s StarSense Explorer technology to help you navigate the night sky with little in the way of extra setup time or complexity. However, it includes fewer accessories to get you started.
- Manual Truss Tube Scope: The Explore Scientific 10” Hybrid Dobsonian is extremely compact, able to fit in a fraction of the space of a normal 10” Dob or the 12” FlexTube. However, the included accessories are not very good and the scope needs some modifications to work well.
- Manual Truss Tube Scope: The Orion SkyQuest XX12i Dobsonian is more compact than the 12” FlexTube thanks to its true truss design and more weight-optimized base, though the base is similarly bulky in frame. It features Orion’s IntelliScope encoders to help you find deep-sky objects as well, though the system is not as intuitive or quickly setup as the Celestron StarSense Explorer, which unfortunately is not presently offered with a 12” scope.
- Manual Truss Tube Scope: The Explore Scientific 12” Truss Tube Dobsonian is even more portable than the 12” FlexTube due to its full truss tube design, is stiffer, and has a dual-speed 2” Crayford focuser. However, assembly is more complicated, included accessories are nonexistent and the scope needs some DIY modifications and upgrades to work well.
- Manual Truss Tube Scope: The Sky-Watcher 10” FlexTube GoTo Dobsonian offers fully motorized GoTo and tracking capabilities with a more portable form factor than the manual 10” and 12” FlexTube scopes. Newer versions can be controlled with your smartphone or tablet remotely, and the telescope can be aimed manually with no disruption to its GoTo and tracking capabilities.
- Computerized Scope: The Celestron NexStar Evolution 8 is far less capable in light collecting or resolving power compared to a 10” or 12” Dobsonian but is very compact and features a built-in battery along with full motorized tracking and pointing capabilities, controlled via your smartphone or tablet.
- Partially Computerized Truss Tube Scope: The Orion SkyQuest XX14i Dobsonian is similar in overall weight and bulk to the 12” FlexTube but its larger aperture gives you 36% more light collecting area for brighter and better deep-sky views, while the Intelliscope object locator can assist you in aiming at these faint fuzzies.
- Computerized Truss Tube Scope: The Orion SkyQuest XX12G Dobsonian is a little more compact than the 12” FlexTube thanks to its true truss trube design and is able to be aimed freely manually with its dual encoders as well as via the built-in motorized tracking and GoTo system.
- Computerized Truss Tube Scope: The Sky-Watcher 12” FlexTube GoTo Dobsonian is basically identical to the manual version, and can be used manually, but offers fully motorized GoTo and tracking, with the newest version being controlled via your smartphone or tablet. The base can also be disassembled for transport.
Aftermarket Accessory Recommendations
Getting a shroud for the 12” FlexTube Dobsonian is very important. You can sew it yourself or buy one ready-made to fit onto the telescope. It’ll keep stray light out of the optics as well as prevent your mirrors from dewing up.
To complement the 12” FlexTube’s provided 9×50 finder, we’d recommend considering a Telrad or Rigel QuikFinder to attach to the tube next to it. Either project a red “bullseye” reticle with degree circles onto a plastic window, making for an easy “zero-power” aiming experience.
A true 1.25” to 2” adapter is something you won’t regret buying for the 12” FlexTube, as it saves a lot of hassle when it comes to switching from 1.25” to 2” eyepieces and allows you to use 2” filters. A brass compression ring adapter will also prevent ugly set screw marks from appearing on your 1.25” eyepieces. The Astromania twist-lock unit is a bargain, while discerning astronomers may want the Glatter Parallizer for a better fit.
Extra eyepieces at this price range are really up to individual discretion, but at the minimum, we’d recommend something in the 5-8mm range for high magnification and a good 2” wide-angle eyepiece for the 12” FlexTube Dobsonian. The Apertura 32mm SWA (47x) provides a much wider true field of view and lower magnification than the stock 25mm unit, while the Explore Scientific 6.5mm 82-degree eyepiece (231x) makes for a great high-power ocular. These are just some of the many options available, however, and we encourage you to do your research on what type of eyepieces and what assortment is right for you.
Lastly, a good UHC nebula filter, like the Orion UltraBlock, will improve your views of emission and other nebulae with any telescope, regardless of your viewing conditions. The 2” UltraBlock in conjunction with a quality 2” to 1.25” adapter can be used with your 1.25” oculars, and thus it’s a good idea to go with the 2” version to future-proof yourself even if you don’t plan on getting any 2” eyepieces right away.
What can you see?
The 12” FlexTube’s large aperture makes it a great choice for observing deep-sky objects. You’ll have no problem seeing thousands of galaxies under dark skies and details like spiral arms and dust lanes in many such as M51 and M82, as well as resolving individual stars in the Andromeda Galaxy and even being able to view extragalactic globular clusters. Under light-polluted skies, however, galaxies are mostly just going to be washed-out smudges, making the portability of the 12” FlexTube a huge advantage. We’ve curated a list for you of the top 50 galaxies to observe under good conditions with the 12” FlexTube and smaller instruments.
Open star clusters like the Double Cluster show lots of spectacular star colors in the 12” FlexTube, regardless of viewing conditions. Globular star clusters can be resolved into individual stars even under sub-par and light-polluted skies; the 12” FlexTube shows the different morphologies of many globulars like dust lanes, bright cores, and out-of-round shapes in clusters like M13, M15, and M92 respectively. Bright nebulae like Orion (M42) and the Lagoon (M8) are jaw-dropping and show greenish colors within, while you can also make out the shape of the Swan Nebula (M17) and even outline the Pillars of Creation in the Eagle Nebula (M16) under dark skies – a UHC filter helps. A UHC filter also brings out the huge Veil supernova remnant and its spectacular detail, as well as the dim Helix planetary nebula and Crab supernova remnant (M1). Smaller planetary nebulae show beautiful greenish, turquoise, and bluish colors with fine detail and sometimes central stars within, like the Cat’s Eye, Blue Snowball or Ring (M57) even under so-so skies; good seeing is just as important as low light pollution for observing detail in these objects.
The 12” FlexTube is great for viewing the Moon and planets, too. Expect to resolve the phases of Mercury and Venus, ice caps and dark markings on Mars, and thousands of details on the Moon. The cloud belts and Great Red Spot of Jupiter look fantastic, and you can resolve its moons’ disks easily when seeing permits, along with their shadows when they transit, and even some features on Ganymede and Io. Saturn’s rings and the Cassini Division within are gorgeous, and the Encke Gap in the rings is also visible under very good conditions. Half a dozen moons accompany Saturn in a 12” telescope, and you can see some cloud banding on Saturn itself along with the orange-gold disk of Titan. Uranus’ blue-green disk can be resolved, along with at least a couple of its moons and some faint cloud details, while Neptune’s blue disk is often fuzzy but its moon Triton is readily apparent. Lastly, Pluto is a fairly easy catch in the 12” FlexTube under dark skies, appearing as a star-like point. Our guides on observing the planets with telescopes go into more detail on what you can expect to see with the 12” FlexTube as well as different instruments of varying aperture.