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/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. While adequate, it is often a bear to focus precisely at high magnifications and can slip when carrying heavy loads. I would recommend replacing it with a GSO Crayford focuser, or even a Moonlite or Feathertouch focuser.
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 is 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
Not to be confused with the older, black CG-4 which uses extruded aluminum legs and cheap plastic parts, the Omni CG-4 is a professional-quality mount based on the Advanced VX. 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 greatly increase comfort at the expense of a slight weight increase and of course a decrease in portability/convenience.
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.
We’d recommend taking a look at a multitude of alternatives to the Omni XLT 120 at its price point, all of which have significantly more aperture and functionality.
- Apertura AD10/Zhumell Z10/Orion SkyLine 10 – More than double the aperture means 4x the light gathering and 2x resolution, no chromatic aberration, more user-friendly, vastly superior accessories.
- Orion XT8i – More aperture means more light gathering and resolution, computerized pointing system, no chromatic aberration.
- Celestron NexStar 6SE – A bit more aperture, significantly more compact, full GoTo mount, and no chromatic aberration.
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. A 2” star diagonal might also be useful for enabling wider low-power views, along with nice eyepieces like the Meade 5000 UWAs, Explore Scientific 68- or 82-degree, or Baader Hyperion or Morpheus series.
What can you see?
The Omni XLT 120 is primarily a lunar and planetary instrument, but its aperture is big enough to show you a fair amount 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. Numerous planetary nebulae such as M27, M57, the Cat’s Eye, and the Blinking Planetary are visible. Open clusters look great, and benefit from the wide fields that can be provided with modest focal length of the Omni 120. You’ll also have no trouble seeing most of the galaxies in the Messier catalog and many of the brighter NGC objects, some of which will present detail such as 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.
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.