The Optical Tube
Optically, the Ursa Major 8″ is an 8” (203mm) f/5.9 (1200mm focal length) Newtonian reflector. With an f/5.9 focal ratio, coma is virtually non-existent, most inexpensive wide-angle eyepieces function at least acceptably at providing sharp views, and collimation tolerances are not ludicrously demanding. The primary mirror is made out of BK7 glass, and a 12v DC-powered fan is attached to speed up the mirror’s acclimation to cool ambient nighttime temperatures. Adequate cooldown is necessary for the primary mirror to provide sharp images at high magnifications, and this can take some time even with the fan on.
The optical tube of the Ursa Major 8″ is fairly characteristic for an 8” Dobsonian; it features a slender-walled steel tube held together with robust metal castings at both ends, extending approximately 4 feet (1.2m) in length, just adequately compact to fit across the rear of the majority of vehicles. This is also equivalent in length to a typical 6” or 10” Dobsonian, so if you cannot afford the 8” or desire to opt for a larger model, the 6” Ursa Major variant or numerous 10″ telescopes will be equally portable and exhibit similar overall handling characteristics (all possessing focal lengths of approximately 1200 mm).
The focuser on the Ursa Major 8″ is GSO’s single-speed 2” Crayford design. This employs a metal drawtube against rollers to seamlessly manoeuvre the eyepiece/accessories back and forth with no gears or any kind of increments for ultra-precise focusing accuracy. It is feasible to upgrade to a dual-speed model by either substituting the entire unit or procuring an upgrade kit, but this is not absolutely essential. The focuser utilises a compression ring to secure your eyepieces instead of a screw, so your eyepiece barrels will not accumulate irksome dents and scratches over time.
The Ursa Major 8″ primary mirror is straightforward to collimate without the need for tools, although the included lock bolts can be slightly perplexing and are actually removable. The secondary mirror necessitates a hex key (supplied) for adjustment, but this is infrequently required, if at all, and fitting thumbscrews will lead to diminished alignment accuracy and increased collimation shift over time.
The Ursa Major 8” Dobsonian includes a 25mm Plossl eyepiece for 48x, a 9mm Plossl for 133x, and a 9×50 finder scope for aiming. The included 25mm Plossl eyepiece provides 48x magnification and nearly the maximum field of view that the scope can achieve with a 1.25″ ocular. Manufactured by GSO, like the rest of the telescope, this eyepiece is sharp and offers a reasonable 55-degree field of view, decent contrast and glare control, and sufficient eye relief to be somewhat comfortable for those who wear eyeglasses.
The 9mm Plossl has limited eye relief, requiring you to place your eye against it to view the entire field of view. With an apparent field of view of around 45 degrees, it is somewhat on the narrow side, and this is particularly noticeable when compared to the 25mm ocular. Nonetheless, it is sufficiently sharp and appropriate for observing smaller targets such as the Moon, planets, double stars, globular star clusters, and planetary nebulae. However, it should be noted that objects will appear to drift across the entire field of view more rapidly than with a comparable wide-angle eyepiece.
The image through a typical straight-through finder scope like the provided 9×50 unit is inverted upside-down and, naturally, magnified by 9x in comparison to what is visible with the naked eye, but its 50mm aperture also reveals stars fainter than those perceivable with the unaided eye alone —along with some of the brightest and most obvious deep-sky objects. This is particularly advantageous if you reside in an area plagued by light pollution and are unable to discern many stars with the unaided eye. The primary drawback of the straight-through finder, however, is that it can be somewhat uncomfortable to peer through, especially when aimed high in the sky and its crosshairs are also somewhat challenging to discern at night.
The Ursa Major 8″ Dobsonian employs a fairly standard Dobsonian mount. For azimuth (side-to-side) motion, the telescope swivels on a laminate that glides against a trio of plastic pads. To pivot in altitude (up/down), the Ursa Major 8″ Dobsonian utilises plastic cylindrical bearings positioned atop plastic pads.
The altitude bearings on the Ursa Major 8″ Dobsonian, much like the majority of consumer Dobsonians, are undersized, potentially leading to balance issues when heavy accessories, such as large eyepieces, are mounted at the front. To counterbalance the overly light motions of the undersized bearings, the Ursa Major 8″ Dobsonian employs springs to firmly pull the bearings against the base, thereby increasing friction. This system functions remarkably well. Even at 400x magnification, it is easy to aim the Ursa Major 8″ Dobsonian, and the balance remains consistent with all but the heaviest eyepieces and accessories in the focuser. However, should the scope become sufficiently unbalanced that the springs are insufficient, you’ll need to attach counterweights to the back of the tube to compensate, and all this extra weight may result in sticky altitude motion.
Similar to most mass-market Dobsonians, the Ursa Major 8″ Dobsonian’s base requires self-assembly using a hex key, akin to assembling cheap flat-packed furniture. The base is made from particle board, making it vulnerable to damage, particularly from moisture, over time. Fortunately, constructing a replacement from plywood is both simple and cost-effective—a plywood base is not only lighter, but can also be further optimised for other design improvements if you build one yourself.
On the side of the Ursa Major 8″ Dobsonian’s base, there is an eyepiece rack, which we would advise against using, as it is all too easy to accidentally drop and scratch an eyepiece, kick dirt onto it, or simply leave your eyepieces exposed to dew as they remain out in the open. It also can be easily bashed into the side of the base and damage it.
Should I buy a Used Ursa Major 8” Dobsonian?
A pre-owned Ursa Major 8” Dobsonian could be a prudent choice, as acquiring a second-hand model can save you funds that can be allocated towards purchasing supplementary accessories (desperately required since the scope is accompanied by a minimal number of extras). However, it is vital to scrutinise any used Dobsonian for damage to the mirror coatings or to the particle board base. Replacing a damaged base is generally straightforward and cost-effective if you can source some Teflon/PTFE and some good plywood, as well as gain access to a few basic carpentry tools to build it. Conversely, damaged mirror coatings can pose a more significant issue. If the mirror appears dull, it may merely necessitate cleaning, but if there are signs of fungal or chemical corrosion, pinholes, or a see-through appearance to the coating, the mirror might require recoating, which can be costly. Before reaching a decision, it is crucial to weigh the expense of recoating against purchasing a brand-new telescope. In all likelihood it probably won’t be worth recoating the primary mirror of the Ursa Major 8” unless you practically got a used unit for free.
It is not usually essential to be concerned about dents on the tube of a second-hand Dobsonian, as they are frequently inevitable and typically do not influence the light path. In instances where dents do affect the optics, they can often be eliminated using tools akin to those employed for removing dents on a vehicle or even with a plunger or hammer.
The Ursa Major 8” Dobsonian is an extremely capable telescope that offers excellent value for its price. However, one may want to consider a more compact 6″ Dobsonian or more powerful 10-12″ Dobsonian telescopes, as one of these alternatives may better align with your goals, storage requirements, and budget.
- The StellaLyra 8” f/6 Dobsonian offers a few additional features the Ursa Major 8” lacks, such as a dual-speed focuser, a 2” wide-angle 30mm eyepiece instead of a 25mm Plossl, and a 9×50 right-angle finder. This makes it an even better deal for the money than the Ursa Major.
- The Celestron StarSense Explorer 8” Dobsonian, like the Ursa Major 8” Dobsonian, comes with few features besides a minimalist focuser and accessory set. However, it does have a lightweight base and features Celestron’s StarSense Explorer technology.
- The Bresser Messier 8″ Dobsonian is a comparable alternative to the Ursa Major 8″ Dobsonian. It offers a similar level of performance, but with an upgraded mount. The Bresser Messier Dobsonian design features balance adjustments and large altitude bearings that result in even smoother movements than the Ursa Major. Despite these features, it falls short in terms of accessories when compared to the Ursa Major 8″ Dobsonian, as its included finder/eyepiece are quite low quality and the solar filter is not all that special.
- The Sky-Watcher Virtuoso GTi 150P is an exceptionally transportable telescope, complete with a fully motorised mount equipped with GoTo technology. This technology can be operated using a smartphone app or manually directed by hand. Although its aperture is more diminutive compared to the 8” Dobsonians in this price range, its compact design renders it an outstanding choice for individuals who prioritise easy transportation and user-friendliness. The manually operated Heritage 150P is also an excellent option if you can forgo the computerised functionality.
- The StellaLyra 10” f/5 Dobsonian possesses the remarkable features of its 8” equivalent, such as its superior quality focuser and additional accessories, but it also benefits from its more substantial aperture, which collects 36% more light than an 8” Dobsonian telescope. Despite the increase in aperture, the overall dimensions and weight of the telescope remain only marginally larger due to the comparable proportions of the base and tube.
- The Bresser Messier 10” Dobsonian has the same features, design, and accessories as the 8” model with a larger aperture and more or less the same pros/cons but better performance due to the larger primary mirror.
- The Explore Scientific 10” Ultra Light Dobsonian exhibits a more streamlined, all-metal design in stark contrast to the bulky tubes and particle board bases of most commercial solid-tubed Dobsonians. This makes it an ideal option for those seeking a more easily transportable telescope, and it’s far more compac than standard 8” Dobsonians like the Ursa Major. Nonetheless, it demands more assembly and offers a smaller range of accessories. Conversely, the 10″ aperture provides superior performance and is guaranteed to deliver astonishing views of the night sky.
- The StellaLyra 12″ f/5 Dobsonian is a formidable telescope that boasts an extraordinary selection of accessories, offering excellent value for your investment, much like its smaller StellaLyra Dobsonian counterparts. However, manoeuvring its solid tube may prove difficult without a dolly, and a sizeable vehicle is essential for transporting it to dark sky locations. If your budget permits, a pricier collapsible or truss 12″ model could be a more suitable choice for simpler transportation and assembly.
- The Explore Scientific 12″ Ultra Light Dobsonian features a refined truss design, substantial altitude bearings, and all-metal components, ensuring a compact and lightweight storage solution with smooth movements devoid of springs, clutches, or other aids. Additionally, it is equipped with a dual-speed 2” Crayford focuser and delivers all the performance expected from a 12” telescope. Regrettably, it lacks supplementary accessories or electronic features, and collimation along with the assembly of the many fiddly parts of the truss tube can be a little more time-consuming if you’re not used to telescopes.
- The Sky-Watcher Skyliner 300P FlexTube isn’t significantly lighter than a solid-tubed scope of its aperture but is certainly more compact, and unlike a truss tube scope, there are no removable parts to worry about.
- The Celestron StarSense Explorer 10” Dobsonian offers a lightweight base and the same StarSense Explorer technology as its 8” counterpart, though it of course comes up short with a single-speed focuser that’s not easy to upgrade to a dual-speed unit along with no other features or accessories.
Aftermarket Accessory Recommendations
The Ursa Major 8” Dobsonian’s included pair of eyepieces serve as a solid starting point; however, an extensive range of magnifications is crucial for examining various celestial objects. A respectable yet reasonably priced 2″ super wide angle (SWA) eyepiece, like the 38mm OVL PanaView (providing 32x magnification), offers the lowest magnification and virtually the widest achievable true field of view that the Ursa Major 8″ Dobsonian can attain, making it ideal for initially locating your target and delivering awe-inspiring views of expansive open star clusters.
We also recommend a 9mm goldline/redline (133x magnification) eyepieces to supplant the Ursa Major’s stock 9mm Plossl ocular, as it boasts substantially longer eye relief, a more expansive and immersive apparent field, and superior interior blackening for enhanced contrast and diminished scatter on bright targets. A 2x Barlow lens, combined with a 9mm eyepiece for 266x or alternatively a dedicated 4mm or 5mm planetary eyepiece (300x or a more conservative 240x, respectively), will yield the highest practical magnification with the Ursa Major 8″, providing your atmospheric conditions allow for it. A Cheshire collimator is also a must-have for the Ursa Major 8″ Dobsonian in order to accurately collimate it for the sharpest possible views.
Last but by no means least, a narrowband UHC/OIII nebula filter is highly advisable for nearly any telescope, including the Ursa Major 8″ Dobsonian, to amplify contrast on nebulae such as Orion and expose hitherto unseen detail and structure. This is particularly beneficial when observing under light-polluted skies.
What can you see?
The Ursa Major 8” Dobsonian, much like any reputable 8” Dobsonian, is a versatile instrument capable of delivering views of a wide array of celestial objects. Even under light-polluted conditions, this telescope can unveil an extensive variety of open star clusters, such as M11 and M35, individual stars within globular clusters like M13 and M15, and numerous double stars, many of which present a challenge to separate at high magnifications and/or offer vibrant colouration.
The quality of a telescope’s views of most deep-sky objects is heavily reliant on the degree of light pollution in your vicinity. Suburban light pollution can essentially eliminate galaxies altogether and substantially impair nebulae, while highly light-polluted skies will render observing anything but star clusters a frustrating endeavour. For optimal viewing of galaxies and nebulae with any telescope, moderately dark skies are necessary, and the quality of your observations will improve as your sky conditions become more favourable as well as with plenty of experience at the eyepiece over time, allowing you to learn more observing tricks and get used to looking at “faint fuzzies” or taking advantage of the split seconds of steady air for high-resolution views of the planets.
With the Ursa Major 8” Dobsonian, you can observe hundreds of vibrant open star clusters and several dozen globular clusters, most of which can be resolved into individual stars using a high-magnification eyepiece. Nebulae such as the Orion Nebula (M42) are detailed and bright even under subpar skies and are further enhanced by a UHC filter. The Veil Nebula is spectacular under dark skies with a UHC or Oxygen-III filter, and you can also explore numerous colourful planetary nebulae. You should be able to see nearly all of the galaxies in the Messier catalogue, and under optimal conditions, you can begin to discern a few features in brighter and more detailed galaxies. The spiral arms of M51 begin to be revealed with an 8” scope at medium magnification, while the dust lanes in M82, M64, and the enormous Andromeda Galaxy (M31) are strikingly apparent due to their high contrast.
Moreover, the Ursa Major 8″ Dobsonian can deliver outstanding views of Solar System objects, encompassing the Moon’s myriad craters and towering peaks, as well as the phases of Mercury and Venus. The telescope has the capacity to unveil a smattering of dark markings on Mars, accompanied by the planet’s polar ice caps and any dust storms, although it is necessary to await Mars’ favourable proximity to Earth to distinctly discern these surface features. Jupiter’s intricate cloud belts, the striking Great Red Spot, and its four prominent moons are all discernible through this telescope. These moons present themselves as tiny disks alongside their pitch-black shadows when transiting in front of Jupiter, resulting in an eclipse over the planet’s cloud tops.
Saturn’s rings and the Cassini Division within them are effortlessly resolved by the Ursa Major 8″ Dobsonian, the latter necessitating a high-power eyepiece in conjunction with the planet’s cloud bands. Additionally, one should anticipate observing several moons encircling the ringed planet, the most prominent being Titan. Uranus, appearing as a bluish-green disc, is readily visible in the Ursa Major 8″ Dobsonian’s 9×50 finderscope. Although a few of its moons can be detected with an 8″ telescope, a larger instrument is, in reality, required to observe them for all but the best observers and conditions. Neptune can be identified using the 9×50 finderscope but appears as a deep blue blur at best through the telescope, although its moon Triton is relatively simple to spot alongside it. Pluto demands dark skies for any visibility, and realistically, an 8″ telescope is no longer sufficient to observe it, as the dwarf planet continues to fade as it recedes from the Sun.