Celestron Omni XLT 102ED Telescope
By Chuck Hawks
In the beginning, Celestron telescopes came with orange tubes. Somewhat later and for many years thereafter, Celestron catadioptric (CAT), refractor and Newtonian reflector scopes were supplied with handsome, gloss black tubes with orange logos. Now it seems that Celestron has gotten away from their traditional black/orange motif.
The XLT 102ED reviewed here was supplied with what I would describe as a "gunmetal blue pearl" main tube. The front cell, focuser casting and mounting rings are painted white. The CG-4 mount wears a matching white finish, as does the tripod's head and leg locks. It is a good looking package, even if not recognizably Celestron. (Other Celestron scope lines come with white, black, gray, slate gray, orange and brass tubes.) Incidentally, the markings on the sides of the optical tube read, "Omni XLT100." "ED" is nowhere to be found and the objective diameter is understated by 2mm (according to Celestron's specifications), although the item number in fine print on the rear of the scope tube is correct. I guess +/- 2mm is "close enough for government work" in Red China and the poor serf who stencils the logos on the tubes at the factory (the scope is manufactured by Synta Optical, who now owns Celestron) probably could not tell ED optical glass from the bottom of a Coke bottle.
The Omni XLT 102ED represents the apex of Celestron's refractor telescopes and is the most expensive. There are larger 120mm and 150mm refractors in the line, to be sure, but their optical formula does not include an aspheric, extra-low dispersion (ED) glass element (according to Celestron's ad copy) that is supposed to deliver near apochromatic (APO) performance.
The bottom line is that this four-inch refractor is about as good as it gets in reasonably affordable refractors. A true, three element apochromatic scope of similar size will usually cost well over twice the $1000 MSRP of the Omni XLT 102ED without an equatorial mount. I have read comments online that claim the XLT 102ED delivers about 90% of the performance of a similar size APO scope for less than half the price. We will see.
To begin, here are the features of the Celestron Omni XLT 102ED:
Omni XLT 102ED. Illustration courtesy of Celestron International.
Here are the technical specifications:
The Omni XLT 102ED is built on a seamless aluminum main tube and uses heavy-duty cast aluminum front and rear cells for structural rigidity. There is no plastic, fiberboard, or other substitute materials used in its construction. The inside of the tube is blackened and baffled. Dual one-inch wide rings are used to attach a Vixen style dovetail that mates the scope to the CG-4 equatorial mount. The focuser is a smooth, 2" Crayford type, superior to the rack and pinion type supplied with less expensive refractors.
Refractors are probably the most durable and trouble free of the three common types of telescopes. (The others being the pure reflector and the catadioptric scope that combines a glass front element with two mirrors.) Quality refractors, especially those using ED glass or an APO objective lens group, produce images of legendary sharpness and contrast.
Relatively trouble free operation and sharp, contrasty views are important plusses for an amateur telescope. Unlike telescope designs using mirrors, refractors have no secondary obstruction to degrade resolution. Refractors also avoid most of the coma inherent in reflector telescopes. Coma is an optical aberration that causes point sources of light, such as stars, to become crescent shaped near the edge of the field of view.
Of course, refracting telescopes also have drawbacks. Refractors are relatively long (their light path is straight, not folded) and expensive for their light grasp. (It takes more grinding and polishing to make a glass element with two finished sides than a mirror with one finished side.) The common achromatic refractor with a two spherical element objective lens is going to produce some color fringing when viewing bright objects, usually in the green/yellow part of the spectrum. This is because it takes a minimum of three conventional optical glass elements to focus all three primary colors of light (red, green and blue) to the same point. Color fringing can be minimized by incorporating an ED glass element, fluorite lens element, aspheric elements, three or more spherical elements, or some combination of the above in the objective lens system, but doing so substantially increases the price of the telescope. This reduction of chromatic aberration is the primary justification for the substantially higher MSRP of the Omni XLT 102ED ($999.99) compared to the otherwise similar Omni XLT 102 achromat ($543.99).
Celestron Omni XLT series telescopes come with a solid CG-4 German equatorial mount incorporating ball bearing races in both axis and includes a dedicated tripod with 1.75-inch diameter, two section, stainless steel legs. This high quality mounting system is an improved version of the Celestron Super Polaris German equatorial mount and it makes the Omni XLT series scopes a true long-term investment and a best buy.
The mounting system is at least half the value of any amateur telescope and its importance is usually overlooked by first time buyers. One of the things an amateur astronomer learns with experience is the crucial importance of a solid, accurate mounting system. Celestron's CG-4 mounting system should satisfy the needs of any visual astronomer.
Motor Drive Set DA (item #93522) is available for the CG-4 mount at a modest cost. (The MSRP is $137.99, but at this writing it retails for only $95 from Optics Planet.) This includes a battery pack that takes four "D" cells, drive motors for the mount's right ascension (sidereal rate) and declination axis, and a hand controller with multiple slew rates. The motor drive set is essential for astrophotography and convenient (although certainly not necessary) for visual observation.
Another handy mount accessory is the Polar finder CG-4 (item #94223), a small finderscope that mounts in the axis of the CG-4 equatorial mount and simplifies polar alignment. I had previously used a similar set-up for a Celestron C-8 scope on a Super Polaris German equatorial mount and found it worth its modest $61.99 MSRP.
When you purchase any new telescope, you are going to need a set of oculars (eyepieces). The supplied 25mm Plossl medium magnification ocular is handy, but you will need to augment its capabilities with shorter focal length (higher magnification) and longer focal length (wider field of view) oculars. Since I have owned a fair number of telescopes, I had a reasonable selection of 1.25-inch oculars available for this review. These include a 40mm Celestron Plossl (22.5x), 32mm Celestron Plossl (28x), 25mm Celestron X-Cel (36x), 17mm Celestron Plossl (53x), 14mm Televue Radian (64x), 12mm Celestron Ortho (75x), 9mm Burgess/TMB (100x), 7mm Burgess/TMB (128.5x) and a Celestron Ultima 2x Barlow lens.
The Celestron XLT 102ED telescope that is the subject of this review was well packaged to survive the rigors of shipping and handling and it arrived in what appeared to be perfect collimation. One of the great things about refractors is their durability and ability to shrug off handling and transportation jolts that would put a Newtonian reflector, or even a lot of CAT's, out of service.
A couple nights of backyard observation with the XLT 102ED quickly revealed that the scope is fully functional, but not perfect, as supplied. The mount and scope are good, but the 6x30, straight through, inverted image finder had to go. Crawling around on the ground trying to peer through the finderscope when the telescope is pointed high in the sky is no fun, not to mention hard on my aging back. To replace the supplied finderscope, I ordered a StellarVue F2 Deluxe red dot finder ($52) and F2D mount ($14) that fits the Vixen type mounting shoe cast into the rear cell of Celestron (and most other mass produced) refractors. Another handy finder accessory is a green laser, such as the Celestron Laser Finder Kit.
I joined several of the Astronomy and Photography Online staff (Jim Fleck, Bob Fleck, Rocky Hays and Gordon Landers), who happen to be amateur astronomers as well as recreational shooters, at the Izaak Walton Gun Range south of Eugene, Oregon. All shooting ceases at sundown and it is a membership facility, so when we arrived at 10:00 PM, we had the place to ourselves. It is located near the suburbs of Eugene and the sky glow from the city intrudes on the northern horizion. In addition, the range is surrounded by mature fir trees that cut off the view at lower elevations, so it is not an ideal observing site. However, it is close to where most of us live and the relatively large open space provided by the 200-yard rifle range allows an unobstructed view of the night sky above about 30 degrees elevation.
We had three telescopes of similar aperture for comparison. The telescopes included a Meade 114, 4.5-inch, 900mm focal length, Newtonian reflector; Stellarvue Raptor SVR-105, 4-inch, 735mm focal length, APO refractor; and the Celestron Omni XLT 102ED (4-inch / 900mm). The Stellarvue and Celestron scope's focusers can accommodate two-inch oculars, but most owners will use 1.25-inch visual accessories, as we did in all three telescopes for this review. It is worth noting that the Stellarvue Raptor 105 has a carbon fiber main tube and an air spaced apochromatic triplet objective. It costs about $2600 without a mount and, as StellarVue states, "this is not a 'compromise' telescope." (A full review of the SVR105 Raptor can be found on the Astronomy and Photography index page.)
Setting-up the XLT 102ED was straightforward. I transported the scope in three main sections, the optical tube (with finder removed), mount (with counterweight removed) and tripod legs (with center tray removed). Upon reaching the observing site, set-up the tripod and attach the accessory tray. Place the CG-4 mount on top of the tripod and, using the big plastic knob, tighten the threaded center shaft to hold it in place. Attach the counterweight. (I used only the seven pound weight to balance the scope and had previously marked its proper location on the shaft with a Sharpie.) Last, place the telescope on the mount and tighten the main and safety thumb screws that secure the telescope's Vixen dovetail mounting rail to the mount. Slide the finder scope and mount into its shoe and tighten the thumb screw.
My simplified alignment technique for visual observing is to level the tripod with the north leg in line with the north star and the mount centered in azimuth and set for the correct latitude (44 degrees in this case). That is all you need to do with a German equatorial mount to minimize declination adjustments for enjoyable viewing. You should only have to make a minor adjustment with the declination slow motion control maybe once every 15 minutes or so. If you do not have Motor Drive Set DA you will, of course, have to use the right ascention slow motion control to counter the rotation of the earth.
The other two scopes were on alt-azimuth mounts and required even less set-up time. The movements of the Meade alt-azimuth tripod mount were jerky and made manually aiming the telescope difficult. In addition, it lacks slow motion controls. Dual axis drive motors and a hand controller to slew and aim the telescope were installed, but we ran out of batteries (10 AA cells are required!) and could not use them. In addition, this mount is far too flexible and quite sensitive to vibration.
On the other hand, the Stellarvue APO refractor was mounted on an outstanding Stellarvue alt-azimuth MG head and aluminum surveyor's tripod that provided single-handle slow motion adjustments for both altitude and azimuth. This rock solid alt-azimuth mount makes casual viewing a real pleasure. It costs about $500 from Stellarvue, but is worth the price if you want a really good alt-azimuth mount. The excellent Stellarvue 9x50mm, right-angle finder scope mounted on the SVR-105, which features a correctly oriented view, is not exactly inexpensive, either. Both the mount and finder scope are extra cost accessories, since the scope was purchased as an optical tube. We won't even mention the array of deluxe oculars assembled for use with this telescope, except to say that it is a very impressive system.
Having set-up the scopes, we were ready to observe some commonly viewed objects. Unfortunately, tall trees obscured almost all of the ecliptic from our observing site. The only planet available was Jupiter, and then only for a relatively brief time. We mostly concentrated on relatively bright deep sky objects that were essentially overhead. I used the 40mm Plossl ocular in conjunction with the Celestron's finder scope to locate objects, taking advantage of that ocular's relatively wide field of view, then switched to shorter focal length oculars for increased magnification when applicable.
GLOBULAR CLUSTER M 13 - Found in the Hercules constellation, this is the finest globular cluster (magnitude 5.9) visible from the northern hemisphere. It is an impressive object when viewed through practically any small telescope and the XLT 102ED was no exception. M 13 looked good through the 40mm Plossl and increasingly better in the 32mm, 25mm and 17mm oculars. The 17mm Plossl gave an adequately bright view and individual stars at the fringe of the cluster could be seen.
Switching to the Stellarvue APO refractor, the best view seemed to be at about 100x magnification, which produced a sufficiently bright and impressively detailed view. Taking that as a hint, I switched to the 14mm Radian ocular in the Celestron (64x) and was rewarded with increased detail. This proved to be the optimum choice for M 13 among the oculars at my disposal on this particular night. The XLT 102ED could not match the performance of the Stellarvue APO in terms of magnification, but it was pretty darn good.
The Meade reflector was used only with a 26mm Meade Super Plossl ocular and, while the cluster could be clearly identified, it failed to resolve any individual stars. The alternative oculars available for the Meade were the two "Modified Achromats" supplied with the telescope, a 25mm and a 9mm. Neither had sufficient resolution to be very useful, so we stayed with the 26mm Super Plossl, a good general-purpose ocular.
RING NEBULA M 57 - The famous Ring Nebula (magnitude 9.7) in Lyra is the remains of a star that went super nova and left a "smoke ring" in space. (Actually, an expanding cloud of incandescent gasses.) After locating the Ring as a fuzzy spot with the 40mm Plossl ocular, switching to the 25mm X-Cel and using averted vision revealed a tiny ring. Dropping down in focal length to the 17mm Plossl showed the ring clearly, although still small in size, and with about optimum brightness. Having had success with the 14mm Radian on M 13, I also tried it on the ring. The result was a larger but dimmer image, as you might expect. Both the 17mm and 14mm oculars were useful, but we gave the 17mm the nod due to its greater brightness.
The view of the ring at similar magnification looked much the same through the Stellarvue scope, as there is no fine detail that can be resolved by a small telescope. The Meade reflector showed a round fuzzy area with its 26mm Super Plossl, but lacked the resolution and magnification to show the ring shape.
ANDROMEDA GALAXY M 31 - This giant elliptical galaxy is an impressive sight through binoculars or any decent telescope. It appears as an oval patch of light with a slightly brighter center. No individual stars can be resolved, but its size and brightness (magnitude 3.5) are unmistakable.
In the case of the XLT 102ED, the 40mm Plossl, 32mm Plossl and 25mm X-Cel all gave good views. Given the sky glow from nearby Eugene, which masked the true size of M 31, the 32mm and 25mm oculars were my favorites, although the bright center of the galaxy could be seen through the 17mm Plossl.
The four inch Stellarvue Raptor provided similar views at similar magnifications, while the 4.5-inch Meade reflector and 26mm Super Plossl combination did a good job on M 31, its best performance of the night. Credit that to its somewhat greater light grasp.
"DOUBLE DOUBLE" IN LYRA - Easily split with binoculars, the components of this double star are themselves binary stars and make a good resolution test for small telescopes. The components are magnitude 5.1-6.0 at three arc-seconds and 5.1-5.4 at two arc-seconds. Both refractors could split the component binary pairs, but the higher magnification oculars available for the Stellarvue Raptor gave it the edge, possibly assisted by its APO triplet objective lens.
The XLT 102ED could handle more magnification than I had anticipated. The 12mm Ortho, 9mm Burgess/TMB and 7mm Burgess/TMB all cleanly separated the binary pairs and more magnification was better. The 7mm provided the best split. I tried the 12mm in the 2x Ultima Barlow lens (equivalent to a 6mm ocular) and that combination also worked well. Overall, an impressive performance from a four-inch telescope with a theoretical limiting resolution of 1.37 arc-seconds.
WHIRLPOOL NEBULA M 51 - Located off the end of the handle of the "big dipper," this is a broadside view of a spiral galaxy, magnitude 8.1. An eight-inch or larger telescope has sufficient light grasp to reveal its spiral shape, but it appears as a faint, fuzzy spot in small telescopes and that is how it appeared in all three of ours. The 40mm Plossl, 32mm Plossl and 25mm X-Cel all revealed M 51 in the Celestron ED refractor, but lacked any discernable detail.
"DOUBLE CLUSTER" - These two open clusters, NGC 869 and NGC 884, lie in close proximity to each other between Cassiopeia and Perseus and provide a beautiful view in small telescopes. It takes a wide field of view to encompass both clusters, so the 40mm Plossl and 32mm Plossl provided the best views in the Celestron. The star images were sharp and bright through all oculars, but I preferred the 32mm ocular's field of view.
JUPITER - Jupiter and its four big moons is the largest and most impressive planetary system in our solar system. Since it is very bright, it allowed the use of high magnification oculars, although it was a warm night and rising air currents caused a lot of image degradation. We had to look for details as the image swam in and out of focus. Never the less, the XLT 102ED provided very impressive views of the big planet, particularly for a relatively small aperture telescope. A good refractor truly gets all there is to be had from its available aperture.
As usual, I aimed the scope at the planet using the 40mm Plossl. The noteworthy views of Jupiter really started with the 17mm Plossl ocular and got better from there, as the magnification was increased. The 17mm showed the two central cloud bands and all four moons. Jupiter looks like a miniature solar system to me and the 17mm portrays that image quite well. Switching to the 14mm Radian gave the first tentative glimpse of four cloud bands and the great red spot, which I call the "great pale spot," using averted vision. The 12mm Ortho was a slight improvement over the 14mm, again using averted vision. I also tried the 12mm with the Ultima Barlow lens (effectively a 6mm eyepiece) and the scope handled the magnification well, providing a decent view of four cloud bands and the great pale spot when the atmosphere cooperated. There was some loss of sharpness, inevitable with any Barlow lens, but overall this Celestron Ultima is a good one.
Even better, however, were the last two oculars I used, the 9mm and 7mm Burgess/TMB's. These are sold as planetary oculars. The 9mm clearly showed four cloud bands and the oval shape of the great pale spot when the atmospheric shimmer momentarily abated. It would be hard to ask for a better view of Jupiter from a four-inch telescope. However, being an optimist, I went for the 7mm and was rewarded by the most impressive view of Jupiter I had yet seen. During the times of relative atmospheric stability, the great planet looked like one of those striated marbles floating in the blackness of space. The oval shape of the great pale spot was clearly visible and four to six cloud bands could be resolved, depending on the seeing condition at the moment. I could detect no color fringing with any of the oculars I tried, from 40mm to 7mm. All in all, an impressive performance and it was with regret that I watched Jupiter slip behind the trees.
It is worth noting that, in stable seeing conditions, the Omni XLT 102ED provides sharp star images with any good ocular. At no point was I bothered by chromatic aberration. In addition, the scope's single speed Crayford focuser comes with large, rubber covered knobs and operates smoothly with almost no play. It is a nice scope to focus and to use.
The CG-4 mount and tripod are steady and vibrations dampen quickly, even at full leg extension. (The way I used it.) When balanced correctly, which is very easy to accomplish, the mount slews almost effortlessly and the smooth acting slow-motion control knobs make it easy to center an object in the field of view. The RA and Declination locks are positive (turn clockwise to lock). It's a good mount for visual astronomy.
In the field, the XLT 102ED easily outperformed the 114mm Meade Newtonian on most subjects and the Celestron CG-4 mount is far superior to the flimsy Meade alt-azimuth mount. (Of course, it also costs a lot more.) The Stellarvue Raptor 105 APO was optically superior to the Celestron, but certainly not three times better, as its price might suggest. However, it is a fact that to extract the absolute top level of performance from almost anything requires a far greater expenditure of time, effort and money than it takes to achieve, say, 90% of that level of performance. In that light, the price of the Stellarvue is not exorbitant, but it probably costs more than all but the most serious amateur astronomers are willing to spend.
For the rest of us, the Celestron Omni XLT 102ED delivers good optics on a solid, functional equatorial mount at a price we can potentially afford. Whether it offers 90% of the overall performance of the SVR-105 APO, I could not precisely say. I can say that it offers similar performance at a slightly lower level and for a considerable reduction in price. The Celestron Omni XLT 102ED is a well designed, well made, reasonably priced, durable and versatile telescope system that requires minimal care and represents a solid, long-term investment.
Copyright 2009, 2010 by Chuck Hawks. All rights reserved.