The Optical Tube
The Celestron Omni XLT 102AZ is an achromatic refractor with an aperture of 102mm, a focal ratio of f/6.5, and a focal length of 660mm. This shares optics and parts used in many other Celestron telescopes, such as the StarSense Explorer DX 102AZ, which uses an identical optical tube, and others like the AstroMaster 102AZ, which share optics but are different in construction.
As refractors get larger in aperture and faster in focal ratio, they tend to exhibit more chromatic aberration. The Omni XLT 102AZ is no exception, and it will show significant chromatic aberration on bright targets. The achromatic design of the XLT 102AZ, which uses two lenses made of different glass types to control chromatic aberration, is insufficient to fully eliminate this issue at a focal ratio of f/6.5. As a result, the Omni XLT 102AZ shows purple halos of chromatic aberration on bright targets like the Moon and planets and is noticeably softer at higher magnifications compared to other telescope designs or a longer focal ratio achromatic refractor. It still displays fairly pleasing views but runs out of steam at above 100x, and fine planetary detail is somewhat obscured.
The Omni XLT 102AZ is equipped with a 2″ all-metal rack-and-pinion focuser, which is able to handle even fairly heavy 1.25″ eyepieces. However, the telescope will not balance on a mount with a 2″ diagonal and eyepieces; the XLT 102AZ lacks a long dovetail bar or tube rings to allow it to slide along in the mount saddle to compensate for the rear-heaviness of a 2” diagonal and eyepieces. The focuser is also not as sturdy as is generally warranted for using 2” oculars anyway, and the mount would be best without having additional weight placed on it. You could purchase tube rings to allow the use of 2” accessories with the Omni XLT 102AZ if you wish, but between the rings, a 2” diagonal, and 2” eyepieces, you’d likely spend more than the telescope itself costs just on those upgrades.
The Celestron Omni XLT 102AZ, like many beginner telescopes sold by Celestron, comes with a 1.25″ Amici prism diagonal, which is designed for terrestrial viewing. This diagonal produces an image that is corrected for left-right orientation rather than being flipped like a mirror, as is the case with an astronomical star diagonal. Not only is such a feature completely unnecessary, but the Amici design causes more glare and a bright diffraction spike on many targets. In the case of the unit that comes with the kit, the plastic body and low-quality coatings cause more internal reflections. The poor optical quality of the prism itself hurts the quality of the image, and the prism is too small, so that the edges of the field of view are slightly darkened when using the 25mm Plossl eyepiece that comes with the kit. Replacing it is a must.
Celestron provides a single 25mm Plossl eyepiece with the Omni XLT 102AZ, which is a step up from the cheap 3-element eyepieces often included with beginner telescopes and has a sharp apparent field of view of 52 degrees, providing 26x magnification and a true field of about 2 degrees with the XLT 102AZ. There is some slight vignetting towards the edges of the field of view with the stock Amici prism diagonal.
For a finder, the Omni XLT 102AZ is provided with Celestron’s “StarPointer Pro” finder, a battery-powered reflex sight that is seemingly an attempt to compete with the Telrad. It features a bullseye reticle instead of a dot like a normal red dot sight but otherwise handles similarly to a standard red dot finder. The bullseye reticle may or may not be of any benefit to you, though the window you look through is a little bigger, which makes aiming more comfortable.
The AZ3 Mount
The AZ3 mount provided with the Omni XLT 102AZ is an alt-azimuth single-arm fork mount with the tube mounted off to the side of the altitude (up-down) axis rather than above it like a photo tripod. This avoids any possible balance issues when aimed high in the sky. The tripod is a sturdy affair made of extruded aluminum. The AZ3 uses a standard Vixen-style dovetail saddle to hold the optical tube in place and allows for coarse up-down and left-right aiming by simply pushing the telescope around the sky. Fine adjustments for pointing and tracking at higher magnifications can be made using the two slow-motion adjustment cables, which use gears to move the mount in small increments.
While the AZ3 mount is a little small for a mount capable of supporting the 102AZ’s optical tube, it generally works well and is even sturdier if you take advantage of the provided accessory tray to place weights to lower the center of gravity of the telescope. However, the backlash from the flexible slow-motion cables can be annoying when using high magnifications, and the mount may not be as intuitive to use as a simple Dobsonian mount.
Should I buy a Used Celestron Omni XLT 102AZ?
The Celestron Omni XLT 102AZ’s lack of plastic parts and simple refracting optics mean that it’s relatively hardy even in the hands of less-than-competent owners and resellers. Any damage to the objective lens, optical tube, or mount/tripod should be pretty obvious. Expect to pay less for a unit with missing accessories. The Omni XLT 102AZ is already a pretty great deal at its new MSRP; a used one should be a bargain.
The Omni XLT 102AZ is a pretty good telescope, but any reflector of equal or greater aperture is likely to prove superior.
- The Sky-Watcher Heritage 150P has 2.25x the light gathering ability and 50% more resolving power than the Omni XLT 102AZ, as well as being free of chromatic aberration and far more portable thanks to its tabletop mount and collapsible tube.
- The Sky-Watcher Heritage 130P isn’t as big and powerful as its larger sibling, but offers the same features – and plenty more capability than the Omni XLT 102AZ – at an even lower price.
- The Zhumell Z114/Orion StarBlast 4.5 Astro offers slightly more light-gathering ability and a wider achievable field of view than the Omni XLT 102AZ, without the complexity of the Heritage telescopes’ collapsible tubes.
- The Popular Science Celestron StarSense Explorer DX 100AZ has similar optics to the Omni XLT 102AZ but a lower-quality, all-plastic focuser. However, you get two eyepieces, and the AZ3 mount has Celestron’s StarSense Explorer technology added to assist you in navigating around the night sky.
- The Sky-Watcher Virtuoso GTi 150P is largely identical to the Heritage 150P but features the addition of fully motorized GoTo and tracking, all controlled via your smartphone or tablet. Sky-Watcher’s unique FreedomFind encoders also allow you to aim the telescope manually without interrupting the GoTo and tracking functionality. The Virtuoso GTi 130P offers the same upgraded capabilities with the Heritage 130P optical tube.
- The Orion SkyQuest XT6 has a rock-solid full-sized Dobsonian mount, a 2” Crayford focuser that can actually handle 2” eyepieces, and of course all of the light-collecting and resolving power of a good 6” reflector.
- The Celestron StarSense Explorer DX 102AZ is completely identical to the Omni XLT 102AZ apart from adding a second eyepiece, a simpler red dot finder, and the aid of Celestron’s StarSense Explorer technology to help you find deep-sky objects.
Aftermarket Accessory Recommendations
It is advisable to replace the 1.25″ Amici prism that comes with the Omni XLT 102AZ with a better star diagonal. A better diagonal can make a big difference in how well the Omni XLT 102AZ works for astronomical viewing by getting rid of problems like vignetting, glare, ghost images, and lack of sharpness that are often caused by the Amici prism. As previously mentioned, the telescope cannot balance with a 2″ diagonal, so a 1.25″ unit is the best option. The Celestron 94115-A Prism Star Diagonal is a good choice for this purpose.
A 32mm Plossl eyepiece provides a slightly wider field of view than the stock 25mm Plossl does with the Omni XLT 102AZ, yielding 20x magnification and a 2.6-degree angular field of view. More eyepieces would also be helpful if you want to get a higher magnification than the 25mm Plossl gives you. At the minimum, for planetary viewing, a 6mm “goldline” eyepiece will produce 108x with the XLT 102AZ, which is about as high as you should probably be using.
A UHC nebula filter like the Orion UltraBlock filter improves contrast on nebulae with any telescope. This opens up a lot of previously-invisible targets with the XLT 102AZ under dark skies and compensates for the effects of mild light pollution on some nebulae. You can get it in the 1.25” format, but if you plan on using a larger telescope, a 2” UltraBlock and a threaded 1.25” adapter will allow you to use the filter with the Omni XLT 102AZ and not have to worry about upgrading when you use a larger instrument and 2” eyepieces.
What can you see with Celestron Omni XLT 102 AZ?
The Omni XLT 102AZ is best suited for viewing large, bright deep-sky objects such as open star clusters and large nebulae. Many open clusters are visible throughout the night sky, and the Omni XLT 102AZ will render hundreds of colorful, pinpoint stars in them—even with fairly light-polluted viewing conditions. Most planetary nebulae are too small to distinguish clearly with the Omni XLT 102AZ, but a few, notably the Ring Nebula (M57), the Dumbbell (M27), and the Blinking Planetary, can be seen. With the aid of a UHC nebula filter and under fairly dark sky conditions, you can also see the Helix Nebula.
Large nebulae like the Rosette, the North America Nebula, or the Veil look fantastic through the XLT 102AZ with a UHC filter and dark skies, and the smaller but brighter emission nebulae like Orion (M42), the Swan (M17), and the Lagoon (M8) show up even in the suburbs unfiltered, though darker skies and a good UHC filter will give you the best possible views.
Galaxies require dark skies to be viewed well with any telescope, but through the Omni XLT 102AZ, most galaxies will appear as indistinct, featureless smudges regardless of how good your viewing conditions are. A few of the brightest galaxies, however, such as the Andromeda Galaxy (M31) or the Cigar Galaxy (M82), show dust lanes and companion galaxies through the XLT 102AZ under dark skies. The Omni XLT 102AZ’s 4” aperture is sufficient for you to see many groups of galaxies as well as larger clusters like the Virgo Cluster, though all of their individual members will appear as little more than “faint fuzzies” of varying shapes and sizes.
The chromatic aberration and small aperture of the Omni XLT 102AZ make it hard to see the moon and planets. The Moon is a delight in any telescope, with thousands of surface features visible, and the phases of Venus are obvious. You can just barely make out the phases of Mercury and the polar ice cap on Mars with the XLT 102AZ, along with any dust storms on Mars and possibly a couple of dark markings on its surface when it’s close to Earth. Jupiter’s four large moons and cloud belts are visible, along with smaller details in the clouds, such as the Great Red Spot.
On a good night with the Omni XLT 102AZ you can just about make out the disks and shadows of the four Galilean moons when they transit in front of Jupiter, provided you use high magnification. The rings of Saturn are, of course, spectacular, along with the razor-thin Cassini division within and some dull cloud banding on Saturn itself. You should also be able to see a few moons around Saturn. Uranus and Neptune appear as featureless teal and blue orbs as seen through the XLT 102AZ, barely distinguishable from surrounding stars of similar brightness. The moons of Uranus and Neptune are too faint to see with only a 4” telescope, as is Pluto; a 6-8” or larger telescope is required for any of these objects.