The Optical Tube
The Star Discovery 150i is a 6” (150mm) f/5 Newtonian with a 750mm focal length. These scopes are quite popular, particularly among beginners in visual astronomy or astro-imaging. For example, the Heritage/Virtuoso GTi 150P, also from Sky-Watcher, is a 6” f/5.
However, the Star Discovery 150i diverges from the majority of common 6” f/5 Newtonians in terms of its primary mirror cell. The primary mirror cell of the Star Discovery 150i is made entirely from plastic and features a closed back, which means the mirror cannot cool down as quickly as it would in a regular, open cell. More concerning is that there are no collimation adjustments available. This means that if your scope is out of collimation, there isn’t much that can be done to rectify it besides send it back to the manufacturer. There are some DIY solutions for fixing the collimation using the screws that hold the cell to the tube walls, but these methods are only effective for minor adjustments and are not practical to tweak in the field.
The 150i appears to hold collimation reasonably well over time, but potential buyers should be cautious. The similar plastic, non-adjustable mirror cell of the smaller Heritage 100P and 114P tabletop Dobsonians is barely acceptable but those telescopes are much smaller and the mirrors weigh much less, as well as both telescopes being a much lower-priced option where compromises are more expected. The 150i’s secondary mirror can be adjusted for collimation quite easily, though it’s unlikely you’ll have to do so often.
The Star Discovery 150i’s focuser is a 1.25” rack-and-pinion design, made entirely out of plastic. This is a common focuser used on many lower-priced telescopes. While it functions adequately, the plastic focuser may exhibit some wobble if you attach a heavy camera or eyepiece, and of course limits the maximum field of view as well as preventing the use of a coma corrector with this scope. Replacing the focuser with a high-quality Crayford focuser necessitates a level of comfort in making modifications to the telescope and removing the optics, but this can be done with relative ease.
As is common with most quality tripod-mounted telescopes, the Star Discovery 150i attaches to its mount with a Vixen-style dovetail bar. If you did want to use the telescope on an equatorial mount, you would probably be better off removing the bar and putting the telescope inside a set of 179mm tube rings attached to a dovetail for more rotation and balancing adjustment options.
The Star Discovery 150i comes with two eyepieces and a red dot sight. The provided eyepieces are 1.25” 3-element “Super” eyepieces, which are variously described as either a reverse Kellner or Konig optical configuration. Both eyepieces feature glass optics with anti-reflection coatings, although the body and housing of each eyepiece are almost entirely plastic. With the 150i’s focal ratio of f/5, you’re inevitably going to see some coma at the edges of the field of view at low power, the 25mm (30x) Super eyepiece is fairly sharp closer to the centre of the field and offers comfortable eye relief. The 10mm (75x) also performs reasonably well. Both are more comfortable to use than a typical Plossl eyepiece, but slightly inferior in sharpness and glare control compared to a well-made Plossl or wide-angle ocular as would typically be expected to be supplied with a scope of this price.
For manual aiming as well as aligning the GoTo mount, the Star Discovery 150i is provided with a standard battery-powered red dot finder, which works well.
The Star Discovery 150i’s mount is fairly standard as far as altazimuth GoTo mounts these days go – internals with simple servo-driven motors and gears that may or may not be plastic, an 8-AA battery power pack and 12v port for power, and stainless steel legs. However, the Star Discovery mount adds a notable improvement with its dual optical encoders. these encoders allow you to move the scope manually without ruining the GoTo alignment. This is great if you or someone else accidentally move the telescope, if you just want to move around the sky manually, or if you want to save power by manually moving the scope to the general area of sky you want to go to and then using the GoTo to fine-tune your pointing. This same tech is used on Sky-Watcher’s GoTo Dobsonians. The entire mount is controlled via the SynScan app which can be installed for free on your smartphone or tablet, though you can also pair it with an app like SkySafari, directly control the mount through a PC, or purchase a SynScan hand controller and plug that in to control the mount. The 150i’s mount works well, and can be used with other telescopes thanks to its Vixen-style dovetail saddle, though the provided 150mm f/5 optical tube is about as large and heavy of a load as you can realistically use with the mount without running into issues with stability.
Should I buy a Used Sky-Watcher Star Discovery 150i?
A used Star Discovery 150i will need to be checked to make sure that the computerized mount is fully functional, the mirror coatings are in good condition, and all plastic parts are still in one piece. The fragility of the focuser and mirror cell cannot be understated. Additionally, a used 150i or any other reflector’s primary mirror appears to have signs of fungus, chemical corrosion, pinholes, or a transparent coating, the mirror may require recoating. Unfortunately, this process is not cost-effective for the Star Discovery 150i, and it is very difficult to properly remove or squarely re-install the primary mirror from its all-plastic mirror cell.
Dents on the tube of a used reflecting telescope usually don’t matter and can be easily removed, but make sure that the 150i’s spider and primary mirror cell are still in one piece if you come across a dented unit, as they are plastic and easily cracked.
An 8” Dobsonian or manual tabletop scope would probably be a better pick than the Star Discovery 150i. We’ve assorted a few options for these as well as some other computerized instruments for your convenience.
- The Sky-Watcher Heritage 130P is a little smaller than the Star Discovery 150i, but includes a sharp 130mm f/5 primary mirror that’s easily adjusted for collimation and a simple tabletop Dobsonian mount, all at a remarkably low price tag.
- The Sky-Watcher Heritage 150P has identical optics and accessories to the Star Discovery 150i, but features a collapsible tube for better portability, a simple tabletop Dobsonian mount, and of course the ability to adjust the primary mirror for collimation (and a collimation tool included, too).
- The Ursa Major 6” f/8 Planetary Dobsonian is a straightforward and affordable 6” f/8 Dobsonian, which sits on the ground without the need for a table or tripod. It has an all-metal (albeit 1.25” only) Crayford focuser, includes a decent pair of eyepieces, and of course puts up fabulous views.
- The Sky-Watcher Skyliner 150P Classic Dobsonian is similar to the Ursa Major 6” f/8 Planetary Dobsonian but features a 2” rack-and-pinion focuser, as well as a friction-tensioned base rather than springs for the altitude bearings which may or may not be an advantage.
- The Bresser Messier 5” Dobsonian possesses optics comparable to the Heritage 130P, albeit within a more simplistic solid tube design. It should be noted, however, that the accessories provided with this model are rather mediocre in quality even by the low standards at its price point.
- The Sky-Watcher Virtuoso GTi 150P has exactly the same optics, accessories and features as the Star Discovery 150i without any of the cons (or a tripod, though one can be attached) with a collapsible tube, computerized tabletop mount with FreedomFind encoders, and all of the performance of the Heritage 150P from which it borrows its tube and accessories.
- The Ursa Major 8″ f/6 Dobsonian is a simple 8” Dobsonian, which boasts an impressive amount of light gathering area – almost twice that of a 6” scope like the 150i, with 30% more resolving power too. Despite this, its weight and bulk are only marginally greater than a 6″ f/8 Dobsonian. A 2” Crayford focuser and a decent set of accessories are also provided.
- The Bresser Messier 6″ Tabletop Dobsonian features the same optics as the Star Discovery 150i but sports a 2” all-metal rack-and-pinion focuser along with a tabletop Dobsonian mount.
- The Bresser Messier 6” Planetary Dobsonian is a high-quality 6” f/8 Dobsonian with an excellently-designe mount and 2” focuser – though its long 1200mm focal length gives it a narrower maximum field of view than 1.25”-only 6” f/5 reflectors.
- The Celestron Astro Fi 130mm has less aperture than the Star Discovery 150i, but features a (technically) 2” focuser which is much more well-constructed and partially metal, and a primary mirror which can be adjusted for collimation.
- The StellaLyra 8″ f/6 Dobsonian is a premium 8” Dobsonian, complete with a range of additional features other models lack such as a dual-speed 2” Crayford focuser, integrated cooling fan, and an abundance of high-quality accessories to assist you in getting started. The 10″ StellaLyra model offers a similarly exceptional experience, with minimal extra bulk but even greater light-collecting and resolving capabilities.
- The Celestron StarSense Explorer 8” Dobsonian makes use of the innovative StarSense Explorer technology to aid in pinpointing celestial targets in the night sky with the help of your smartphone. Although it lacks a variety of additional features and accessories, it is an excellent choice for those desiring computerised functionality. Furthermore, it delivers superior performance and stability in comparison to the Star Discovery 150i, all while maintaining a similar price point.
- The Bresser Messier 8″ Dobsonian boasts sharp 8″ optics, a top-notch 2.5” hybrid rack-and-pinion focuser, and silky-smooth bearings, all available at a reasonably affordable price. Despite these advantages, the eyepiece and finder scope that accompany the telescope are of somewhat inferior quality when compared to those provided with competing 8” Dobsonian models, such as the StellaLyra, Sky-Watcher, and Ursa Major scopes.
- The Sky-Watcher Skymax 127 AZ-GTi is an excellent option, offering the high-quality Skymax 127 Maksutov-Cassegrain optical tube mounted on a versatile AZ-GTi mount and tripod. The motorized tracking GoTo mount can be pushed around the sky by hand or operated via your smartphone and features FreedomFind encoders just like the Star Discovery 150i’s mount. The Skymax 127 is comparable in planetary performance to the 150i but noticeably inferior for deep-sky viewing.
Aftermarket Accessory Recommendation
There are several affordable and highly beneficial accessories worth obtaining for the Star Discovery 150i in order to fully optimise its performance. A wide-angle low-power eyepiece with a wider field stop and slightly better correction of edge-of-field aberrations than the stock 25mm (30x) Super eyepiece, such as the 25mm Omegon FlatField ED or Celestron X-Cel LX, is a good choice.
For medium power, a 16mm Ultra Wide Angle (UWA) eyepiece (41x) is an immersive and sharp option, while a 15mm redline/goldline (47x) serves as a decent budget alternative. Additionally, a 10mm UWA eyepiece (75x), or a 9mm redline/goldline (83x), delivers sharper views, a wider field of view, and a more comfortable experience compared to the Star Discovery 150i’s stock 10mm ocular. A 7mm UWA or planetary eyepiece (107x) provides the optimal magnification for viewing the planets and splitting double stars even on a fairly turbulent night.
For the highest usable magnification with the Star Discovery 150i, a 4mm planetary eyepiece or a 4mm UWA (188x) is a solid choice. Due to the lack of collimation adjustments and the limitations of typical atmospheric conditions, we would not recommend using above 200x magnification with the 150i, even though it is technically capable of handling up to 300x magnification on bright targets.
Lastly, a narrowband Ultra High Contrast (UHC)/OIII nebula filter can significantly improve your views of nebulae, such as the Orion Nebula, when using almost any telescope, including the Star Discovery 150i. Furthermore, it provides enough contrast improvement to reveal previously invisible targets, such as the Crab Nebula and Veil Nebula supernova remnants, when using this telescope under dark skies.
What can you see?
The Star Discovery 150i delivers outstanding performance on Solar System objects provided it is collimated well enough. You can easily see the phases of Venus, as well as those of Mercury. Neither planet has any surface features that an amateur telescope can display. Mars reveals its polar ice cap and any ongoing dust storms with ease, while a few dark markings can be seen on its surface when the planet is close to Earth and conditions are favourable. Jupiter’s moons can be seen effortlessly at any magnification; high power shows their discs and dark shadows when they transit across Jupiter, along with Jupiter’s colourful pastel cloud belts, festoons, and the Great Red Spot. Saturn’s rings are visible along with the razor-thin Cassini Division within them; its linear, dull cloud belts are noticeable; and a handful of dim moons appear around it as star-like pinpoints with the Star Discovery 150i. You can see Uranus and Neptune as fuzzy greenish and bluish splotches, but the Star Discovery 150i lacks the necessary aperture to reveal any of Uranus’ moons, while Neptune’s moon Triton is difficult to spot at best with a 6” telescope and Pluto is entirely out of reach owing to its faintness.
The quality of your deep-sky viewing with any telescope will be heavily influenced by the level of light pollution in the skies where you observe. Under very light-polluted conditions, you are limited to star clusters and bright nebulae, while faint nebulae are invisible and galaxies appear as washed-out smudges at best. Thankfully, the Star Discovery 150i excels for observing large open star clusters, thanks to its wide field of view, regardless of the sky conditions. Objects like the Double Cluster in Cassiopeia or the Wild Ducks Cluster (M11) in Scutum are great targets, as is the famous Pleiades cluster (M45) in Taurus. Under most viewing conditions, you can also start to resolve the brightest globular clusters, such as M13, M15, and M22, into individual stars with the Star Discovery 150i, though you’ll need to use 100x magnification or higher to do so easily. The Orion Nebula of course looks fantastic, particularly with a UHC nebula filter, and you can observe the Veil Nebula with a filter under dark skies too. There are also quite a few planetary nebulae, such as the Cat’s Eye or Blinking Planetary, which begin to reveal various shades of blue and green colouration with the 150i as well as fine detail when conditions are favorable.
Under dark skies, the Star Discovery 150i is capable of providing decent views of galaxies. You can discern dust lanes in large, brighter galaxies like M31, M64, and M82, and you can identify that galaxies like M51 and M101 have spiral arms, even though the arms themselves might be just beyond reach. The Virgo Cluster displays dozens of members, and numerous galaxy groups can be observed as well. And, of course, there are plenty of double stars you can split with this telescope at high magnification with steady skies and adequate collimation accuracy.
The Star Discovery 150i’s mount is an alt-azimuth design. Its tracking is accurate enough and the tripod is stable enough for planetary imaging and visual astronomy, but the plastic focuser and lightweight mount can’t really hold a heavy camera for astrophotography or track in an equatorial configuration to let you take long exposures. You could do planetary imaging with the Star Discovery 150i, but to achieve the optimal focal ratio (f/25 to f/30) and image scale, you’ll need either a 5x Barlow or to stack multiple 3x/2x Barlow lenses. You will also need to make sure the scope is collimated, the focuser doesn’t sag, and obtain a suitable high-speed planetary imaging video camera.