The Optical Tube
The Explore Scientific 12″ Truss Tube Dobsonian is a 12″ (305mm) f/5 Newtonian reflector with a focal length of 1525mm. The relatively fast f/5 focal ratio brings coma into play with wide-angle 2″ eyepieces, though the more significant concern is other aberrations such as edge-of-field distortions with many budget eyepiece designs like Erfles, Kellners, and so on. Unlike smaller Dobsonians, the 12″ Truss Tube’s primary mirror is thin enough to require a robust flotation mirror cell. The primary mirror rests on three triangles, each with three points, to distribute its weight and prevent sagging under its own weight. Rollers gently resting against the side of the mirror similarly distribute the horizontal load and prevent astigmatism from appearing when the telescope is aimed nearer to the horizon.
A battery-powered cooling fan system, composed of not one but two fans – blowing sideways for optimal ventilation and to prevent the formation of a “boundary layer” of trapped warm air in the mirror box – significantly reduces the primary mirror’s cooldown time.
I’ve observed that the 12″ Truss Tube Dobsonian boasts exceptional optics, theoretically accommodating magnifications up to 600x, assuming optimal seeing conditions. However, most of the time, it may be challenging to utilize half of that magnification or discern a noticeable enhancement in resolving power of the 12” Truss Tube over a standard 10″ Dobsonian, all other factors being equal. The more important benefits lie in observing deep-sky objects, where the 44% increase in light-gathering power over a 10” scope becomes apparent.
The 12″ Truss Tube Dobsonian’s “tube” – if you can call it that – can be disassembled into several sections. The primary mirror box houses the mirror, cell, and attached altitude bearings, which, while removable, are more practical to leave in place. Eight truss poles connect to the mirror box corners and join at the top of the tube, where the skeletal upper tube assembly is attached, containing the focuser, secondary mirror, and mounting points for a finder. The mirror box has built-in handles, and the top of it unfolds on a hinge when the scope is in use. However, this design choice means that dust, bugs, etc. can intrude through the handles, and there is no baffling around the corners of the mirror box; you’ll want to come up with an additional, more tightly fitting cover for the primary mirror, and perhaps cut out some sheet plastic to improve stray light control.
A well-optimized truss tube Dobsonian like the Explore Scientific Truss design or a premium model has eight poles that form a triangular shape, rendering the telescope remarkably rigid—more so than the 3- and 6-pole designs commonly seen in commercial scopes. The truss poles easily detach entirely from the upper and lower sections of the telescope, and the lower end is optimized to be as low-profile as possible with big altitude bearings (which have other advantages we’ll go into), leaving only the compact lower and upper tube assemblies. This is in contrast to the designs seen from many other manufacturers, where the tube collapses into one massive and slightly shorter assembly, or where the trusses merely replace a section of the tube, leaving you with a big lower cylinder that’s still quite awkward and bulky.
To fine-tune the collimation of the 12″ Truss Tube Dobsonian’s primary mirror, you use a provided hex wrench tool to adjust three screws on the primary mirror’s sides, which hold the entire mirror cell suspended beneath—essentially the opposite of the conventional method, where the primary rests on three spring-loaded knobs at the rear. This design not only allows for adjustments while you’re looking through the focuser with a Cheshire or similar collimation tool but also offers greater rigidity, ensuring better collimation from the start. The downside is that it is all too easy to accidentally bang the wrench into the primary mirror if you are not careful!
The secondary mirror of the 12” Truss Tube is attached to a rather unusual spider with vanes that are not parallel in all axes, making a + shape, but rather with an X-shaped orientation instead. This produces slightly worse diffraction spikes on the planets and brightest stars, but has a few advantages. The offset shape prevents deformation from overtightening the vanes and is more rigid, while also keeping the size of the upper tube assembly down by having the secondary collimation screws attached with a rather unusual 90-degree perpendicular orientation to the usual.
Adjusting the scope’s secondary collimation can be a little confusing at first, but is actually a lot easier to do while looking through the focuser if you’re using a Cheshire. This design was also the only way that the 12” Truss Tube manages to fit the upper tube assembly inside the rocker box(!) by keeping it as short as possible when the scope was dismantled. There is no cover provided for the secondary mirror by default, so you’ll want to make a rigid one or use a soft cloth bag.
The upper tube assembly of the 12” Truss Tube is a skeleton of bent aluminum, and, unsurprisingly, lets a lot of stray light in. You will want to cut some sheet plastic and bend it to fit inside the upper tube assembly to prevent glare and moisture from wrecking your view. The baffle directly opposite the focuser is simply inadequate on its own.
Finally, the focuser on the 12” Truss Tube is a 2” linear bearing Crayford focuser, which has a smooth metal track on the main bearing to improve rigidity. An extension tube and 1.25” adapter – both with compression ring fittings – are also provided. By default, the 12” Truss Tube’s focuser is positioned at a 90-degree angle to the ground, meaning it is perfectly horizontal. As a consequence, the telescope’s eyepiece is frequently not only low but also at an awkward angle. Removing and reinstalling the truss couplers on the upper tube assembly with a screwdriver allows you to rotate the other upper tube assembly components 45 degrees, resolving this issue.
No eyepieces are included with the 12” Truss Tube, so you’re expected to pick up your own aftermarket ones. For aiming the telescope, a red dot finder is provided. This red dot finder, the same low-quality unit included with most other Explore Scientific telescopes, employs a non-standard shoe design unique to Explore Scientific, making it challenging to use alternative finders. The finder has a slightly tinted window, likely due to its original intended production for non-astronomical purposes. Aligning the bracket with the telescope, and keeping it that way, can also be problematic. I’d recommend replacing it as soon as possible with a quality reflex sight or finder scope.
The Explore Scientific 12″ Truss Tube Dobsonian, as with any Dobsonian, pivots on laminate-covered bearings riding on Teflon pads in an alt-azimuth configuration, that is, up and down and side-to-side. Instead of the particle board that is usually used for the rocker box, however, the whole base is made out of thin sheet metal riveted together like the mirror box. Likewise, the rocker has built-in handles to easily pick it up and carry it around,
For altitude movement, a pair of substantial, semi-circular bearings are affixed to the sides of the 12″ Truss Tube Dobsonian’s mirror box. The large diameter of the bearings ensures that the telescope remains balanced when you attach a heavy eyepiece, finder, or other accessory to the front end, although counterweights can be added to the rear end in extreme cases. The bearings, coated in textured glassboard, slide smoothly against the Teflon pads on the rocker sides; there’s no need to adjust clutches or springs, and motion doesn’t become “stuck” when making minor position adjustments at high magnification. In the case of an extreme balance issue, there are additional holes to adjust the forward-backward positioning of the bearings on the mirror box a little bit to compensate.
For azimuth (side-to-side) movement, the rocker box base is covered in the same glass board material, smoothly rotating on a trio of Teflon pads instead of inexpensive roller bearings or the often overly adhesive or jerky motions of melamine gliding on nylon pads common in many other mass-produced Dobsonians. I’ve successfully tried applying paste wax and bar soap to the bearings to enhance the already buttery smoothness of the scope’s motions.
Should I buy a Used Explore Scientific 12″ Truss Tube Dobsonian?
The Explore Scientific 12” Truss Tube Dobsonian is almost entirely made out of metal, and any damage to it is thus very obvious even at a glance – but hard to fix. Make sure that the scope has no obvious damage to the mechanical components and that the mirror coatings appear bright and free of obvious corrosion or fungus; a bit of dust or dirt can be cleaned, however. Severe damage to the coatings will require a recoat, which at this aperture size can be cost-effective if you get a deep enough discount on a used unit but will still cost several hundred dollars.
The Explore Scientific 12” Truss Tube Dobsonian is our second-highest ranked scope in its price range, after the Sky-Watcher 12” FlexTube Collapsible, which is only ranked higher due to its ease of use and provided accessories which can benefit beginners. It is otherwise best-in-class. Nonetheless, here are some alternatives for you to pursue.
- The Apertura AD12/Zhumell Z12/Orion SkyLine 12 boasts a huge solid tube and is notably less portable than the Explore Scientific 12″ Truss Tube Dobsonian or other truss/collapsible scopes of this aperture. However, it is more affordable, less intricate in its assembly process, and includes a decent accessory bundle to get you started too.
- The Apertura AD10/Zhumell Z10/Orion SkyLine 10 shares the same features and accessories as the AD12/Z12 but in a considerably smaller, more portability-friendly design. A 10” is also a lot cheaper than a 12” and leaves you more room in the budget for accessories or to spend on trips to dark sky sites which might easily make up for the difference in performance.
- The Celestron StarSense Explorer 10″ Dobsonian is somewhat lighter than the AD10/Z10 and incorporates Celestron’s StarSense Explorer technology to help you navigate the night sky with minimal extra setup time or complexity. However, it comes with fewer accessories to begin with.
- The Explore Scientific 10” Hybrid Dobsonian has the same basic truss tube design and advantageous mechanical aspects of the 12” Truss Tube, but with only a single-speed focuser, a less compact upper tube assembly, and of course a smaller 10” aperture.
- The Sky-Watcher 12” FlexTube Collapsible Dobsonian features a collapsible FlexTube design to reduce the tube length enough to fit in an ordinary vehicle, and includes a basic but adequate set of accessories to get you started. It’s not nearly as compact as Explore Scientific or Orion truss tube scopes, but considerably easier to assemble.
- The Orion SkyQuest XX12i Dobsonian exhibits a compact form similar to the Explore Scientific 12″ Truss due to its truss design. However, its upper and lower assemblies are taller, and the base is considerably bulkier. Orion’s included IntelliScope encoders will also help you to locate deep-sky objects, although the system is not as user-friendly or quick to set up as the Celestron StarSense Explorer, which unfortunately is not currently available paired with a 12″ Dobsonian option.
- The Sky-Watcher 10″ FlexTube GoTo Dobsonian provides full motorized GoTo and tracking capabilities in a more portable design compared to the manual 10″ and 12″ FlexTube scopes. Updated versions enable remote control via your smartphone or tablet, and the telescope can be aimed manually without interfering with its GoTo and tracking functions.
- The Celestron NexStar Evolution 8 has significantly less light-gathering and resolving power than a 10″ or 12″ Dobsonian, but it is extremely compact and features a built-in battery along with comprehensive motorized tracking and pointing capabilities, which can be controlled through your smartphone or tablet.
Aftermarket Accessory Recommendations
In addition to a set of a few eyepieces and a better finder than the finder provided, the Explore Scientific Truss Tube 12” Dobsonian absolutely requires a fabric shroud. You’ll also need to put plastic to line the scope’s open-frame upper tube assembly either entirely or at least as a baffle directly across from the telescope’s focuser.
Without a shroud and appropriate baffling installed in the upper tube assembly, 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 shroud also mitigates the formation of dew and frost on your secondary mirror and will tend to keep your optics cleaner too.
Naturally, a telescope is useless for observation without eyepieces, and the Truss Tube 12” Dobsonian includes none whatsoever, so we recommend starting with at least two or three third-party oculars for this scope. You may also want to consider investing in a coma corrector, such as the Explore Scientific HRCC, Baader MPCC, or Tele-Vue Paracorr II. While these devices can be expensive, they provide sharp stars to the field of view’s edge when used with this telescope and any high-quality wide-angle eyepiece.
Ideally, you’ll want a high-quality low-power eyepiece and a high-power eyepiece at the minimum for the 12” Truss Tube. A 30mm or 24mm Explore Scientific 82-degree (51x or 64x, respectively) or a Meade 28mm PWA (55x) would be our pick for a low-power, wide-field option. Complement this with a good 12-16mm ocular, such as the Meade 16mm PWA or 16mm Nagler Type 6 (95x) – and/or a 13mm Nagler (117x) or a 14mm Explore Scientific 82-degree (109x), and you’re good to go.
For medium to high power with the 12” Truss Tube, a 9mm goldline/redline (169x) works well; alternatively, the 8.5mm Explore Scientific 82-degree (179x) presents a wider and sharper view. Depending on your atmospheric conditions, additional eyepieces for up to 300-500x such as the 6.5mm and 4.5mm ES82 LER eyepieces (235x and 339x, respectively), or similar quality wide-field, short focal length oculars, might be worth picking up. Magnifications above 300x are unlikely to be useful often with this scope, as even the slightest disturbances in atmospheric conditions will rarely permit a consistently sharp image at such high power.
As mentioned, the red dot finder provided with the Explore Scientific Truss Tube 12” Dobsonian is of very poor quality, making aiming it at deep-sky objects somewhat difficult. As such, we would strongly recommend replacing it with a better zero-power finder, such as a Telrad or an Explore Scientific ReflexSight. Alternatively, a 9×50 right-angle, correct-image finderscope is a decent pick, and many Dobsonian owners like to employ both to locate deep-sky objects.
Finally, a narrowband Ultra High Contrast (UHC)/OIII nebula filter can substantially boost your viewing experience of many nebulae, such as the Orion Nebula, when employed with nearly any telescope, including the Explore Scientific Truss Tube 12” Dobsonian. This filter also augments the visibility of planetary nebulae by lowering the apparent brightness of stars at the eyepiece without impairing that of the nebula, increasing contrast, and making it a breeze to find them at low power hiding amidst a dense background of stars. Additionally, it delivers enough contrast improvement to reveal detail in objects that may have been previously invisible entirely, like the Veil Nebula, or Flame Nebula, which are just a few examples of what can be seen with the Explore Scientific Ultra Light 12” Dobsonian under suitably dark skies.
What can you see?
The large aperture of the Explore Scientific 12″ Truss Tube Dobsonian makes it an excellent choice for observing deep-sky objects. Under dark skies, you’ll have no issue viewing thousands of galaxies and discerning details like spiral arms and dust lanes in objects such as M51 and M82, as well as resolving individual stars in the Andromeda Galaxy and even observing extragalactic globular clusters. However, under light-polluted skies, galaxies will mostly appear as washed-out smudges, making the portability of the 12″ Truss Tube Dobsonian a significant advantage so you can use it where it works best – under dark skies far away from city light pollution, or at least the worst of it.
The Explore Scientific 12″ Truss Tube Dobsonian also resolves detail in numerous planetary nebulae, displaying captivating green, blue, or yellow-gold colors and intricate details. These nebulae are so bright that they look impressive even under suburban and city skies. Some, like the Blinking Planetary Nebula, have easily visible white dwarfs in their centers as well, while others do not. Emission nebulae such as Orion and the Lagoon appear stunning with the 12″ Truss Tube Dobsonian, particularly under dark skies and with a UHC filter. On perfect nights, you can barely discern the famed Pillars of Creation within the Eagle Nebula, M16, too. With the 12″ Truss Tube Dobsonian, you’ll also have the opportunity to resolve many of the globular star clusters scattered about the night sky, such as M13 and M15, into individual stars at high magnification. And even under city skies, you can observe plenty of open star clusters, many of which showcase hundreds of colorful stars.
A high-quality 12″ Dobsonian is also ideal for observing the Moon and planets. You can expect to resolve the phases of Mercury and Venus, the ice caps and dark markings on Mars, and countless details on the Moon. Jupiter’s cloud belts and Great Red Spot appear stunning, and you can easily resolve its moons’ disks when seeing allows, along with their shadows during transits and even some features on Ganymede and Io. Saturn’s rings and the Cassini Division within are breathtaking, and the Encke Gap in the rings is also visible under very good conditions. Saturn’s cloud bands and a blue-gray polar region, home to the planet’s hexagonal polar storms, are visible. About half a dozen of Saturn’s moons can be seen alongside the ringed planet, with Titan appearing larger than a point source and displaying a golden-yellow hue.
Under optimal conditions, Uranus’ greenish disk may exhibit subtle cloud markings, and its four brightest moons—Ariel, Umbriel, Oberon, and Titania—are faintly visible with the 12″ Truss Tube. Neptune, with half the angular size and fainter than Uranus, is clearly resolved as a bluish disk, and its largest moon, Triton, is more easily seen than Uranus’ moons due to its larger size and more reflective surface. Pluto can also be glimpsed as a faint, star-like point under dark skies with an instrument of this aperture.