The Optical Tube
The Explore Scientific 12″ Ultra Light 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″ Ultra Light’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.
The 12″ Ultra Light Dobsonian boasts exceptional optics and can theoretically accommodate magnifications up to 600x, assuming optimal seeing conditions. However, most of the time, it may be challenging to utilise half of that magnification or discern a noticeable enhancement in resolving power of the 12” Ultra Light 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″ Ultra Light 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-optimised 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 optimised 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″ Ultra Light 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” Ultra Light 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” Ultra Light 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” Ultra Light is a skeleton of bent aluminium, 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” Ultra Light 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” Ultra Light’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” Ultra Light, 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. We recommend replacing it as soon as possible with a quality reflex sight or finder scope.
The Explore Scientific 12″ Ultra Light 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″ Ultra Light 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. Applying paste wax or bar soap to the bearings further enhances the already buttery smoothness of the scope’s motions.
Should I buy a Used Explore Scientific 12″ Ultra Light Dobsonian?
The Explore Scientific 12” Ultra Light 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 pounds
Under £1200 Range
- The Sky-Watcher Skyliner 300P FlexTube Dobsonian isn’t nearly as compact as the Explore Scientific 12” Ultra Light or another proper truss tube scope of this aperture, but it’s simple to set up and use as well as including an acceptable set of starter accessories too.
- The StellaLyra 12″ f/5 Dobsonian provides similar views to the 12” Ultra Light and outstanding value for those who aren’t bothered by its large solid tube and fully manual design. With a decent set of starting accessories and a straightforward solid-tube format, you won’t need to worry about buying extras like a shroud, finder, or eyepieces to begin observing.
- The StellaLyra 10″ f/5 Dobsonian offers exceptional value for a 10″ Dobsonian as with its larger 12” counterpart, boasting an extensive array of features and accessories that set it apart from its competitors. Its performance matches that of collapsible or truss-tubed 10″ Dobsonians without necessitating additional purchases, such as a shroud.
- The Explore Scientific 10″ Ultra Light Dobsonian has the same design as the 12” Ultra Light but in a more portable, albeit less powerful, 10” aperture package.
- The Celestron StarSense Explorer 10” Dobsonian is a lightweight and portable option among 10″ Dobsonians, utilising Celestron’s StarSense Explorer technology with your smartphone to assist in locating deep-sky objects. However, much like the Explore Scientific scopes, few useful accessories are provided to give you a satisfactory observing experience without additional purchases.
Over £1200 Range
- The StellaLyra 16″ f/4.5 Dobsonian delivers significantly enhanced performance compared to the 12″ Ultra Light due to its larger aperture, while its 6-pole truss tube can be disassembled into relatively manageable subcomponents. This telescope is a behemoth but will provide you with spectacular views of deep-sky objects.
- The Sky-Watcher Skyliner 350P FlexTube Dobsonian is considerably heavier, wider, and taller than a 12″ telescope. Although its 14” primary mirror doesn’t offer a huge improvement over the 12″ Ultra Light in performance, it remains superior for deep-sky observation if you can tolerate the bulkiness of the huge FlexTube assembly and beefy particle board Dobsonian base.
- The Sky-Watcher Skyliner 300P FlexTube GoTo Dobsonian is heavier and pricier than the all-manual 300P but includes fully motorised tracking and GoTo features, as well as the Sky-Watcher FreedomFind encoder system for manual aiming. It is also roughly the same physical size as the non-GoTo 300P FlexTube.
- The Sky-Watcher Skyliner 250P FlexTube GoTo Dobsonian is slightly more compact than a traditional solid-tube 10″ Dobsonian, offering fully motorised tracking and GoTo capabilities, and shares the same FlexTube design as the other telescopes in Sky-Watcher’s range.
Aftermarket Accessory Recommendations
In addition to a set of a few eyepieces and a better finder than the finder provided, the Explore Scientific Ultra Light 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 Ultra Light 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.
At the very least, you should acquire a high-quality low-power eyepiece and a high-power eyepiece for the Ultra Light 12” Dobsonian. A 28mm Ultra Wide Angle (UWA) eyepiece, yielding 54x with the Ultra Light 12” Dobsonian, would be a good choice. For high magnification, a 10mm UWA (153x) or similar eyepiece would be our top pick. A 7mm UWA (218x) or planetary eyepiece along with a 4mm UWA (381x) are ideal for higher magnification, and for medium power, a 16mm UWA (95x) is an excellent choice. You’ll also need a collimation tool, such as a Cheshire, for accurate collimation of the Ultra Light 12” Dobsonian.
As mentioned, the red dot finder provided with the Explore Scientific Ultra Light 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 Ultra Light 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″ Ultra Light 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″ Ultra Light 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″ Ultra Light Dobsonian also resolves detail in numerous planetary nebulae, displaying captivating green, blue, or yellow-gold colours 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 centres as well, while others do not. Emission nebulae such as Orion and the Lagoon appear stunning with the 12″ Ultra Light 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″ Ultra Light 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 colourful 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-grey 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″ Ultra Light. 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.