The Optical Tube Of Omni XLT 150
Unlike older equatorially-mounted 6” Newtonians which were typically f/8 to f/10, the Omni XLT 150 is a fast, short f/5. This focal ratio produces a wider field and lower power with a given eyepiece, but does require significantly more precise collimation. Also, it will have coma with a 2” wide-field eyepiece, though at f/5 you don’t strictly need a coma corrector to use the scope.
The optics in the Omni XLT 150 are of decent quality, though you won’t get the same kind of planetary performance a longer Newtonian provides as the secondary mirror is larger in size relative to the primary mirror, decreasing contrast.
The Omni 150 is collimatable as with any good Newtonian telescope, but Celestron doesn’t provide any collimation tools with the scope. A collimation cap, which is really all you need for most telescopes, can be easily made by putting a small hole in a 35mm film canister lid (which perfectly fits in the scope’s 1.25” adapter) and cutting off the canister’s bottom. Fine tuning can be done by centering and defocusing a bright star (preferably Polaris as it doesn’t move) and adjusting the primary until the secondary mirror appears exactly centered.
The Omni 150 comes with a high-quality 2” Crayford focuser and a 1.25” adapter. This focuser is abnormally tall, as Celestron has taken pains to prevent the drawtube from intruding on the scope’s light path – a frequent problem with small scopes. Being all-metal and including a tension adjuster, it is adequate for even the heaviest eyepieces or cameras.
The Omni XLT 150’s tube rings, which allow for rotating the eyepiece/finder position and balancing the scope, are metal and are bolted to a Vixen dovetail which allows the scope to be put on almost any modern telescope mount. One of the tube rings has a ¼ 20 captive knob which allows you to piggyback your DSLR camera on for wide-field shots if the scope is equipped with a motor drive.
About the Accessories
The Omni XLT 150 comes with a single eyepiece, a 25mm Plossl producing 30x. You will really benefit from a good 2” wide-angle eyepiece (keep the focal length under 35mm, otherwise the exit pupil will be too large and you’ll lose light) and some 1.25” eyepieces such as the popular and cheaply-available “goldline” 66-degree eyepieces.
The Omni’s finder is a cheap 6×30 unit. While it does work, it’s not very comfortable and the images are quite dim. I recommend swapping it for a 50mm unit and/or getting a Rigel Quickfinder – a Telrad doesn’t have enough room on the scope’s optical tube.
Mount of Omni XLT 150
The Omni XLT 150’s mount is known as the Celestron CG-4, sometimes referred to the Omni CG-4 so as not to be confused with the cheap, older black units on extruded aluminum tripods that Celestron used to bundle with some of their scopes in the late 1990s/early 2000s.
The CG-4 is basically the same mount head as an Advanced VX or older CG-5, completely stripped of all electronics, with a shorter counterweight shaft and on a slightly smaller tripod (1.75” steel legs as opposed to 2”). Since the electronics and their housings are typically the most cheaply made parts of a modern, mass-manufactured equatorial mount, what you are left with is an extremely high-quality mount that even twenty years ago would’ve been priced as much as the entire Omni XLT 150 package.
To give you an idea of how well-made the CG-4 is, I trust and use my $2,000 Takahashi FC-76 refractor on it. That is how good it is.
The CG-4 can be upgraded to have a dual-axis motor drive, which is essential for astrophotography. The dual-axis drive also allows you to slew the scope in either direction in right ascension or declination at the push of a button, which is convenient for centering targets without jiggling the scope.
The CG-4 can also be upgraded with a polar scope for precise polar alignment – another astrophotography requirement.
Unusually, the CG-4 comes with knobs for slow motions rather than flexible slow-motion cables. I don’t really have a preference, but you can get some cables for almost nothing if you prefer them over knobs.
Should I buy a Used Omni XLT 150?
If the price reduction compared to buying new is fair, then go for it. Be sure to check that the mount motions are smooth and the mirrors aren’t corroded. Dust or dirt can be easily cleaned off. If the mount or counterweights are rusty or have chipped paint, they’ll still function like normal – but be prepared to offer less.
In lieu of the Omni XLT 150, there are several other good telescopes in its price range, with various advantages and disadvantages compared to it:
- Apertura AD8/Zhumell Z8/Orion SkyLine 8 – More aperture, simpler mount, better accessories, and focuser.
- Sky-Watcher 8” FlexTube – More aperture, simpler mount, slightly better accessories, and a collapsible tube.
- Celestron Astro-Fi 130 – Slightly less aperture but with an easy-to-use fully computerized mount controlled by your phone or tablet.
Aftermarket Accessory Recommendations
The main items we’d recommend adding to the Omni XLT 150 are a 6mm “goldline” eyepiece for higher magnification (125x) and a better finder of some sort. In addition to the 6mm, you may want goldlines or some other quality eyepiece in focal lengths of 9-10 and 14-16mm for more magnification options, such as the Meade UWA, Explore Scientific 82-degree, or Baader Hyperion lines.
For a finder, we’d probably recommend a Rigel Quikfinder, or an inexpensive red dot sight due to the Omni’s small tube. A 9×50 finderscope also works but is a bit heavy and not very comfortable to look through. There isn’t enough real estate on the tube to really accommodate a Telrad.
What can you see?
The Omni XLT 150’s fast focal ratio and thus wide field of view make it especially suited as a “rich-field telescope” for sweeping the Milky Way and observing large star clusters. Under dark skies, you can also observe a lot of dark nebulae in the summer Milky Way with the Omni XLT 150. The German equatorial mount design makes wide-field sweeping with the Omni XLT 150 a little awkward, but you can get used to it after a little while.
So, what else is there to see with the Omni XLT 150?
Inside the Solar System:
- The Moon – Craters down to about 2 miles in size, countless ridges, fault lines, valleys, mountains, etc.
- Mercury & Venus’ phases
- Mars’ ice cap and a few albedo features
- Jupiter’s bands, Great Red Spot, moons, and shadow transits
- Saturn’s cloud belts, rings, the Cassini division, half a dozen moons
- Uranus as a teal/turquoise dot
- Neptune as an azure-gray dot, with its moon Triton faintly visible
Outside the Solar System:
- ~50 globular clusters, ranging from the bright, partially-resolvable M13 to dim ones you’ll struggle to see at all
- A couple dozen galaxies with discernable details and a few hundred to a few thousand visible total, depending on your skies
- Hundreds of open clusters
- Dozens of planetary nebulae, some of which may show hints of color
- A dozen or two emission nebulae such as Orion, the Lagoon, the Swan, and the Trifid
- Thousands and thousands of double stars
- About 40 million stars in the sky are brighter than magnitude 14, the limit for a 6” telescope
With 6” of aperture, good optics, and a wide low-power field of view, you will take a long time to run out of targets.
The Omni XLT 150 is capable of decent lunar and planetary imaging with a webcam-style CCD, but you will need a quality 5x Barlow (expensive and of no use visually) to boost the image scale sufficiently, and of course a motor drive for hands-free tracking.
The Omni 150’s fast focal ratio and sturdy mounting do permit deep-sky imaging with a DSLR and the motor drive, but you do have limits. Without autoguiding, the scope is realistically limited to relatively short exposures and you will have to throw out some frames. You can purchase a hand controller that allows for autoguiding, but an autoguider and guide scope are expensive and add to the weight of the setup, straining the mount. That being said, a 6” f/5 Newtonian on a CG-4 is probably the best thing I could recommend at this scope’s price point for deep-sky astrophotography – just remember to have realistic expectations.