The Omni XLT 120mm Optical Tube Performance
The Omni XLT 120 is a 120mm (4.7”) f/8.3 refractor. At this aperture and focal ratio, there is a somewhat-significant amount of chromatic aberration, but lunar and planetary views are still excellent. You will notice color fringes on anything brighter than 3rd magnitude, but high-power views are no problem, and images will not “break down” the way they do with faster or cheaper achromats.
The optical quality is top-notch, especially for an inexpensive achromat. The lens cell is also collimatable should the need arise. There is zero plastic anywhere in the scope.
The Omni 120 has a 2” all-metal, single-speed rack-and-pinion focuser. Even though it works well enough, it can be hard to get the focus right at high magnifications and can slip when carrying heavy loads. I would suggest getting a GSO Crayford focuser or even a Moonlite or Feathertouch focuser to replace it.
The Omni 120 attaches to its CG-4 mount using a pair of hinged tube rings and a Vixen dovetail bar. You can piggyback your DSLR or point-and-shoot camera (does anyone still own those?) using the included ¼ 20 captive screw/knob on one of the rings, and shoot wide-field astrophotos if the CG-4 mount is equipped with a motor drive.
About the Supplied Accessories
The Omni XLT scopes are all fairly Spartan when it comes to the included accessories: simply a 25mm Plossl and a 6×30 finder, as well as a 1.25” prism star diagonal in the case of the refractors. The 25mm Plossl has a special long-eye relief design and works pretty well for low power, while the 6×30 is uncomfortable to use and provides rather dim images. The included star diagonal is really nice and is all you need if you want to stick with 1.25” eyepieces.
About the Omni CG-4 Mount
The Omni CG-4 is a professional-grade mount based on the Advanced VX. It should not be confused with the older, black CG-4, which has legs made of extruded aluminum and cheap plastic parts. It is more or less an Advanced VX stripped of electronics and with thinner tripod legs (1.75” vs. 2”), and since the electronics are the main thing people complain about with the VX, what you are left with is a high-quality, all-metal mount capable of carrying up to 20 pounds of payload.
You can upgrade the CG-4 with single-axis or dual-axis motors for hands-free tracking and slewing, as well as a polar scope for precise polar alignment.
The CG-4’s only flaw is the short tripod legs. With a much shorter refractor (such as my FC-76), the eyepiece is a little low even with the tripod legs extended all the way, and with the Omni 120, it’s near the ground. You can, however, buy or make a mount pier extension, which will make you much more comfortable at the cost of a little more weight and, of course, less portability and ease of use.
Should I buy a used Omni XLT 120?
Absolutely! There’s not much to go wrong with a used, all-manual refractor – just make sure the objective lens is in good shape and that the mount moves smoothly.
Alternative Recommendations
The Celestron Omni XLT 120 is certainly a nice telescope, especially for a fairly inexpensive refractor so large, but beginners might want something with more aperture and free of chromatic aberration, while those looking for a “grab n’ go” telescope will probably desire a smaller telescope with a less bulky mount or awkwardly long tube.
Under £600
- The Apertura AD8/Zhumell Z8/Orion SkyLine 8 offers just under triple the light gathering power of the Omni XLT 120 and nearly double the resolving power, free of the troublesome chromatic aberration that plagues cheaper achromatic refractors like the XLT 120. The magnitude-brighter views are a huge help with viewing deep-sky objects. The AD8 also includes a 2” dual-speed Crayford focuser, built-in cooling fans, and a variety of high-quality accessories like a 9×50 right-angle correct-image finder scope, 2” 30mm wide-angle SuperView eyepiece, and of course a stable, lightweight, and easy-to-use Dobsonian mount that doesn’t require polar alignment, adjusting slow-motion knobs, or locking any clutches to aim around the sky.
- The Explore Scientific 10” Hybrid Dobsonian offers more than double the resolving power and 4.5 times the light-collecting area of the Omni XLT 120, providing far brighter and sharper images than the Omni XLT 120 can possibly achieve. The collapsible truss tube allows the scope to fit in a fairly small space but still assembles in minutes with no tools, and the scope’s mount is buttery smooth and easy to aim around the sky. There’s only a single-speed 2” Crayford focuser and only a basic (and low-quality) red dot finder and low-power 1.25” eyepiece are included – but the same is true of the Omni XLT 120 and many other otherwise high quality scopes anyway.
- The Orion SkyQuest XT8 provides the same buttery-smooth Dobsonian mount of the AD8 (if not better due to some slight design differences such as the use of spring-tensioning) and equally bright and crisp views, easily beating out the Omni XLT 120 in performance in almost every aspect. However, the included accessories are a bit lax, and when you consider that the AD8/Z8 is only a bit more expensive, the argument for purchasing is a bit weak.
- The Sky-Watcher Virtuoso GTi 150P offers a slight uptick in light gathering and resolving power compared to the Omni XLT 120, with the perks of a much wider field of view and freedom from the annoying chromatic aberration thanks to its Newtonian optical design. The Virtuoso GTi 150P’s collapsible tube and tabletop Dobsonian mount make it lightweight and portable, and the mount features fully computerized tracking and GoTo capabilities that can be ready in minutes and is controlled entirely via your smartphone or tablet. You can also aim the telescope manually (even when powered on) – or simply purchase the cheaper, all-manual Heritage 150P, which is identical apart from the lack of electronics. The cheaper Virtuoso GTi 130P and Heritage 130P share the features of their respective 150mm counterparts, but are cheaper – though quite a bit smaller in aperture and thus less capable than a 150mm instrument, without much of an increase in portability, if at all.
- The Celestron Astro-Fi 130 has essentially the same light-gathering and resolving power as the Omni XLT 120, but of course lacks chromatic aberration since it is a reflector, and the shorter focal length allows for a wider possible field of view. This telescope comes with a pair of decent eyepieces and its mount features fully motorized tracking and GoTo, all controlled with the SkyPortal or SkySafari app on your smartphone or tablet. However, the Astro-Fi cannot be aimed manually and is kind of expensive considering the fairly small aperture and relative lack of features/accessories, particularly when compared to the Virtuoso GTi tabletop Dobsonian telescope offerings from Sky-Watcher.
£600-£850
- The Apertura AD10/Zhumell Z10/Orion SkyLine 10 has over double the resolution and sharpness of the Omni XLT 120, with the same great features and accessories of the AD8/Z8 package like a dual-speed Crayford focuser, 2” wide-angle SuperView eyepiece, 9×50 right-angle finder, and of course the stable, easy-to-use and portable Dobsonian base. A 10” solid-tubed Dobsonian is just as portable and compact as an 8” with a negligible difference in weight or difficulty in transporting it yourself.
- The Celestron StarSense Explorer 8” Dobsonian is a bit bare-bones when it comes to features and accessories – the 2” Crayford focuser is only a single speed, the finder is a cheap red dot unit, and only a single 25mm 1.25” Plossl eyepiece is provided – but this scope features ergonomic improvements over other 8” Dobsonians in the form of cutouts in the base and carry handles on the tube, as well as Celestron’s StarSense Explorer technology, which turns your smartphone into a computerized object locator in just minutes, helping to find “faint fuzzy” deep-sky objects. Being an 8” Dobsonian, the StarSense Explorer 8” of course blows away the Omni XLT 120 in sharpness, light-collecting area, and ease of use, too.
- The Sky-Watcher 8” FlexTube Dobsonian, besides of course its massive 8” of aperture, has a collapsible strut tube, which reduces its length and thus allows the telescope to fit in a smaller space for storage or transport – helpful if you’re trying to fit into a closet or a small car, for instance. It includes a pair of decent 1.25” eyepieces, a single-speed 2” Crayford focuser, and a 9×50 straight-through finder. However, the 8” FlexTube requires a shroud to keep stray light and moisture out of the optics during use, and is actually quite heavy; the struts don’t reduce the telescope’s weight at all compared to a traditional, solid-tubed instrument.
Aftermarket Accessory Recommendations
The Omni XLT 120 only comes with one eyepiece, and it’s safe to say it’ll benefit from additional ones. What exactly you purchase is up to you, but we’d recommend at least a 6mm goldline (167x) to start for high-magnification lunar, planetary, and double star viewing. Along with good eyepieces like the Meade 5000 UWAs, the Explore Scientific 68– or 82-degree, or the Baader Hyperion or Morpheus series, a 2″ star diagonal might also be useful for making low-power views wider.
What can you see with the Celestron Omni XLT 120?
The Omni XLT 120 is primarily a lunar and planetary instrument, but its aperture is big enough to show you a fair number of deep-sky objects too.
Within the Solar System, the Omni XLT 120 is capable of showing you a wealth of detail on the Moon, along with, of course, the phases of Mercury and Venus. Mars’ polar ice caps and a few dark areas are visible when the planet is close to Earth, which occurs for a few months out of every two years. Jupiter’s cloud belts, the Great Red Spot, and various fainter festoons and swirls are visible in its turbulent atmosphere, and the 4 Galilean moons circling the planet are visible as tiny disks when they eclipse and transit Jupiter every so often. Saturn’s rings, the division in them, and several moons can be spotted, along with some low-contrast cloud belts. Uranus and Neptune are merely bluish dots, devoid of atmospheric features or visible moons.
The 120mm aperture is just barely enough to start resolving globular clusters’ individual stars, such as those in M13, M15, M92, M22, M2, or M3. The better the skies you have and the more skilled you are at observing, the better your chances are. There are a lot of planetary nebulae that can be seen, like M27, M57, the Cat’s Eye, and the Blinking Planetary. Open clusters look great, and benefit from the wide fields that can be provided with the modest focal length of the Omni 120. You’ll also be able to see most of the Messier galaxies and many of the brighter NGC objects, some of which will show details like dust lanes or spiral arms under dark skies. Bright emission nebulae like M42, the Orion Nebula, and M8, the Lagoon Nebula, look great even under light-polluted skies, filled with stars and faint wisps of gas.
Astrophotography Capabilities of the Scope
While the chromatic aberration of the Omni 120 isn’t severe, it would certainly be a nuisance for imaging, particularly deep-sky astrophotography; you would be much better off for either. And even with dual-axis drives and autoguiding, the CG-4 cannot track accurately enough for deep-sky astrophotography with the Omni 120 optical tube, mostly due to the strain placed on it by the high weight (12.5 pounds for the OTA plus at least 5 for the camera/guide scope, and the mount only has 20 pounds of capacity). Thus, the Omni XLT 120 is really not an astrophotography telescope.
