Celestron Omni XLT 150 Review

Celestron Omni XLT 150 Review

by Joe Bergeron

Celestron’s line of low-cost Chinese import telescopes, the Omni XLT, has intrigued me since its introduction. The reason is simple, if slightly ridiculous: the colors. The deep-blue-and-white color combination is very appealing to me (obviously). In June 2007 I noticed that Oklahoma astronomy dealer Astronomics had a slightly used example of one of the cheapest models, the $400 Omni XLT 150 reflector, available for a total of $338. According to them, their customer had returned it in favor of something smaller. Since it’s easy to blow more money than that on a single eyepiece, I decided to satisfy both my curiosity and my liking for blue telescopes and white mounts by ordering it.

The scope arrived promptly in two boxes of substantial size. Both were double boxed, which was good, since they had suffered at the hands of FedEx. The inner boxes and their contents were in perfect condition. The larger and heavier of the two boxes contained the mount and tripod, which are identical for each of the four Omni XLT models (the others being a 102mm f/10 refractor, a 120mm f/8 refractor, and a 127mm SCT). The other box, considerably lighter in weight, contained the optical tube, rings, dovetail bar, and accessories. Everything was packed well and sensibly, and all parts were present.

Since I am a wizened scholar of telescopes and their ways, nothing in those boxes was a mystery to me. I quickly assembled the telescope without referring to the manual. For those needing it, the documentation is comprehensive, well-written, and useful. Also included is a basic version of “The Sky” planetarium software, if you happen to use Windows and have nothing better already.

The assembled telescope is very handsome. The tube color is blueberry, slightly metallic and sparkly. The mount and tube fittings are finished in a thick, glossy, snow-white paint. I was not disappointed with this aspect of the telescope.

The main tube is made of thin, rolled steel with a prominent seam opposite the finder mount. It's out of round along this seam, being a bit flattened there. This is apparent when the scope is mounted in the rings. The tube color is very efficient at heating up in sunlight. The tube interior has a nice flat black coating. The entire optical tube assembly shows considerable thought and expertise in its design and execution. The 150 is f/5. With its 150mm (5.9”) aperture and 750mm focal length, the tube is a short 27 inches in length. The optics arrived in a state of pristine cleanliness. The primary mirror has a neat center ring to assist in collimation. Both mirrors have Celestron’s XLT multi-layer reflective coating, which should produce the brightest images possible for a reflector of this aperture. Popping in a Cheshire eyepiece revealed the scope to be out of collimation. Since it was easy to look through the Cheshire and reach the collimation knobs at the same time, I was able to align it in a few seconds. The collimation knobs were a bit difficult to turn, and might defeat a child. The rear cell also has locking screws to assist in holding collimation. Some sort of membrane covers the back of the mirror, presumably to prevent dust from migrating into the tube.

The diagonal mirror appeared to be properly placed and aligned, according to my sight tube. This mirror is glued onto a four-vane spider which appears to be well designed. The vanes are very thin. Collimation requires the use of an Allen wrench in three small screws. The 1.25-inch focuser is pretty good. Initially, racking it in and out produced an alarming crunching sound, which I believe was caused by a bit of Styrofoam packing material caught in the rack. The large knobs, with their rubber gripping surfaces, are TeleVue-esque in appearance. Eyepieces are secured with two set screws. This arrangement looks and feels cheap after years of using only brass compression ring fittings, but as I have observed so many times, one cannot have everything.

The little 30mm finder is mounted on a tall stalk and uses two-point alignment with a third spring-loaded point of contact. The finder has good eye relief and a focusing objective, and appears to be quite decent if you can make do with a small straight-through finder. I replaced it with a 50mm RACI finder to make finding things easier and more comfortable.

The main telescope’s plastic dust cap has a 40mm off-axis pull-out aperture, for those times when you want to project the sun’s image onto a card, or when you want to guess at what Galileo might have seen with his tiny telescopes (your view will be much better than his though).

One small issue was that one of the tube mounting rings was slightly rotated on the dovetail bar, meaning it was angled on the tube. Attempting to loosen the bolt which attached the ring to the bar was a challenge. Synta appears to hire one of those gigantic Chinese basketball players to tighten these bolts. It would not budge, and my Herculean (by way of Napoleon Dynamite) efforts eventually sheared the bolt. I replaced it and was able to correct the issue.

The CG-4 equatorial head is a cute little handful, a functional miniature version of a German equatorial mount. It would be perfect for a smaller telescope, but I suspected it would prove marginal with the 150, or with any of the Omni XLT scopes except the stubby little 5” SCT. This is a matter of small concern to me, for reasons which I’ll reveal later. The head is well designed and constructed. It has provision for a polar alignment scope which is not included. The little setting circles are primarily decorative. The right ascension circle is loosely mounted. The manual knobs provided for the slow motion controls are actually decent, big and mounted on metal stalks.

The stainless steel tubular tripod, with its 1.75” diameter legs, should be fine with any of the Omni XLT scopes, and looks especially imposing with the dinky CG-4 head mounted atop it. Since I tower above the landscape at 6’2”, the scope’s height on this tripod works well for me, but in some positions it would be inaccessibly tall for a child.

Finally, the scope comes with a generic 25mm eyepiece with a 50 degree apparent field of view. It seems pretty sophisticated for what is essentially a freebie eyepiece, with good eye relief and a screw-up eye guard for adjusting it. With this scope it delivers 30X and a field of 1.6 degrees. With a 5mm exit pupil, this is as low a power as I would wish to use on this scope.

The telescope makes a good first impression as it stands gleaming in the sunlight. First light took place on a night of the full moon, which was somewhat limiting. I will add that this scope looks beautiful in the light of a full moon. The blue of the tube is visible, and the white parts appear luminous.

My first impression of the mount was correct: marginal for a scope of this size. Slight touches and adjustments produced vibrations lasting a few seconds, the exact time depending on the attitude of the scope. However, the scope remained usable at powers up to 214X. It moved smoothly and freely, and the manual slow motions were also smooth. I did not find it too difficult to overlook the shakiness of the mount, and I am not inclined to be overly forgiving about such things.

Focusing on stars revealed an unfortunate, needless drawback of the scope’s design. The focuser is tall, which is inappropriate for a small, fast reflector. The constricted light path and the large distance between the eyepiece and the diagonal mirror result in vignetting. If the eyepiece focused at a point closer to the diagonal, field illumination would be improved. As it is, full illumination is barely achieved only in the very center of the field, if at all. This kind of design is all too common in commercial telescopes. There is no real reason for it. The telescope tube should be a couple of inches longer, and the focuser a couple of inches shorter, to avoid this issue. I suppose the tall focuser with its long travel is used to maximize the scope’s compatibility with various accessories. It does this at the expense of optimal visual performance. How much difference this actually makes at the eyepiece is hard to determine, but I suspect the effect is minimal.

Despite this, the scope produced some pleasant views, though bright star images at high power were nowhere near as clean as they are with my refractors. Fainter stars cleaned up better. At 214X, Epsilon Lyrae was resolved into four decent Airy disks surrounded by fairly neat diffraction rings. At 107x, M13 presented a powdering of faint stars despite the illumination of the full moon. Using a 22mm Panoptic eyepiece, I saw sharp stars occupying the central 80% of the field, typical for an f/5 reflector.

I devoted most of my time to studying Jupiter at 214x. Jupiter, with its mass of low-contrast detail, is a critical test of any telescope. Any scope which cannot perform well on this planet is of little interest to me. That night’s seeing was mostly good, with an occasional flicker of a moon or shifting of the disk. The planet’s edge was usually sharp. Detail, however, was elusive. I got only occasional glimpses of substantial detail. The shadow of Europa was sometimes visible. Overall it was an uninspiring view, but I decided not to let a single look at the low-riding planet form my evaluation of the scope’s planetary capabilities, especially without comparing it to a telescope of known quality at the same time.

Star testing revealed no astigmatism, but did suggest a zonal error, with a dark ring on one side of focus replaced by a bright ring on the other.

The focuser stiffened up in the cool temperatures. I will be among the multitudes who remove the thick paste that passes for grease on these Chinese focusers and replace it with something better. The little finder was sharp, but I had to disassemble the eyepiece to remove some conspicuous bits of plastic debris. It is highly prone to dewing under my local conditions.

Late on the following night, which was damp and dewy following the passage of a storm front, I was out again. Jupiter looked as before, hinting at potential detail. Diffraction spikes off the big planet were visible, but not overly obtrusive. The telescope must have been better cooled, because it did better with stars. At 214x I got a neat split of the close double Pi Aquilae (Rebchis), with a separation of about 1.4 seconds of arc. Deneb at this power appeared pointy and fairly well defined, and it even star tested better, with no sign of the zone I thought I saw on the previous night.

The telescope was also much steadier. Now we come to the primary reason I bought this telescope: I wanted the tripod. I also have an Orion Skyview Pro mount, which I use with my Astro-Physics Stowaway 92mm refractor. It’s an older mount, with an older tripod, its legs only 1.5” in diameter, and several inches shorter than the current tripods, even when fully extended. I knew the newer, taller tripod would make this refractor more comfortable to use. Therefore I bought the Omni. I swapped its tripod for my older one, resulting in two hybrid mounts: a superior blend of the SVP head and the Omni tripod, and a fusion of the CG-4 head with the old SVP tripod, which is more in keeping with the petite size of that mount anyway. The penguin-colored combination of the nice SVP head with the Omni tripod gave the 150mm reflector a stable home which pays little heed to raps, taps, and touches. Clearly, the CG-4 head isn't up to that task, at least if you approach it as someone used to steady mounts. The CG-4 will seem luxurious to someone upgrading from a department store scope.

On the third night of testing I conducted a “comparo”, specifically one involving the ancient feud between reflectors and refractors. Yes, it was 6-inch reflector vs. 6-inch refractor, going scopo-y-scopo. One caveat: my big refractor, an Astro-Physics f/9 EDT, is actually a 155mm, while the Omni is of course a 150. Therefore the little guy started out with a slight aperture deficit. I could have evened things out by making an aperture mask for the refractor, but I was too lazy. I used only Nagler and Radian eyepieces to make these comparisons.

When I set these two scopes up beside each other, I was reminded of Tolkien’s Middle Earth, with an eager little hobbit jumping up and down next to a tall Elf Lord while crying “I can do that! I can do that too!” And you know what? The cheap little reflector could do most of what the exalted refractor could do. Most of it...

The first thing I looked at was the crescent of Venus. Not surprisingly, except for the diffraction spikes in the Omni, both scopes produced about the same view. Moving to Saturn, only a degree or so away, revealed greater differences. At similar powers, the refractor view was significantly brighter and sharper. No surprises so far. Jupiter got the most eyepiece time by far. Using both scopes at around 200x, there was, at first, little to choose between the two. The views were marred by high, fast seeing, meaning both scopes looked about the same, except for the refractor again being brighter. Contrast was a non-issue. I could not discern much of a difference between them. I estimate the Omni has a central obstruction of 25% or more (I’m not going to stick a ruler down there to find out for sure). That does not appear to be enough to cause a major reduction in image contrast. I decided to look at other objects while Jupiter climbed.

One good target was the unequal double star Delta Cygni. The 155mm EDT is a sublime splitter of doubles, and this one is no exception, rendered as a perfect little pair of jewels. Yet the Omni hung right in there, also producing a neat split, though not quite as clean as the refractor. I tried a couple of deep-sky targets, even though the sky was already growing lighter due to the nearness of moonrise. Here the refractor showed its superior light grasp, offering sweet views of M13 and the Ring Nebula which were quite a bit brighter, and a bit clearer, than those in the Omni. Yet the views in the Omni were respectable and enjoyable, in the same league, just somewhat dimmer and not as pristine.

With Jupiter at the meridian, I returned to it. Now, with the planet barely above the worst effects of seeing, the 155 EDT pulled ahead, offering views of Jupiter which were impeccably crisp. The view in the Omni was softer-edged, but impressively, it revealed nearly the same detail as the extremely high-end refractor. The Great Red Spot was making a transit, and was visible with similar ease in both scopes. I was more impressed by the similarity of the views than by their differences. I decided to add a third telescope to the mix, my 92mm A-P Stowaway refractor, probably the most optically perfect telescope I own. Its view of Jupiter was much dimmer than either of the other two scopes, of course, but the detail revealed was not far behind the Omni. In overall performance, the $400 Omni was midway between these two fabled refractors, and closer to the big one than the little one.

With my mounts spread thin, I put the Stowaway on the CG-4 for this three-way shootout. It worked okay, but was distinctly more wiggly than the SVP. I would prefer to limit the CG-4 to the smallest scopes, such as an 80mm refractor, a 90mm Maksutov, or a small solar scope such as the Coronado PST.

I then turned all three scopes to Izar, Epsilon Bootis. All showed this fairly close, unequal double beautifully. The Omni was the worst of the three, but not by any great margin. This time I thought the view in the Stowaway was best, as its fat, hard Airy disks displayed the star colors nicely.

For the fourth and final night of my test, I used the Omni to view various deep-sky objects during the brief window between the end of twilight and the onset of moonrise. At 95x, I had a nice view of M51, with a suggestion of a swirly pattern. At the same power, M81 and M82 were two bright lights barely fitting in the same field of view, two galaxies with very different personalities. With an OIII filter in place, the Dumbbell Nebula was big and bright, with traces of wispy detail across its face. Using Radian eyepieces for 95x and 62x, stars were sharp across the entire field. The cluster M71 in Sagitta was a dense, pretty enhancement of the Milky Way star field. Backing off to 34x with the 22mm Panoptic eyepiece, the entire Coat Hanger asterism fit into the field, a pattern of bright, sharp stars.

My pond-side observing spot is hardly a prime dark sky site. I was interested in trying the Omni at a true dark-sky location. My chance came at the 2007 Stellafane convention, where I viewed the following objects, most in Sagittarius:

NGC 6522 and 28: 2 similar small blurs at 94x, separated by about 20'. 22 is about 1 mag brighter and much more concentrated. It has a mag 13 star just NE ofd the core (seen at 150x). 28 is more irregular and shows glimpses of a few very faint stars.

NGC 6540: Small faint elongated irregular spatter of very faint stars. It seems to have a partner, a larger open cluster to the SSE, about 7' away. 94x

NGC 6544: Nice little glob at 150x, elliptical with a few stars resolved.

NGC 6553: Large glob of fairly low surface brightness, not very condensed, a bit flattened, hint of very many faint stars, mag 12 star near NW edge, diameter about 2.5". Nice object.

NGC 6568: At 62x a large, patchy, scattered group, about 15' across, has arcs or incomplete loops, looks like a bird with curved outspread wings. About 30 stars look like members.

NGC 6583: Small ghostly group, very rich, like 7789 but much more distant, and probably obscured by dust.

NGC 6723: Surprisingly bright and easy given it was only about 8 degrees above the horizon. 94x. About 3' diameter. Round, granular, with a few fugitive stars visible.

NGC 6726-27 (CrA) Very near to above. A wide unequal double surrounded by an obvious puff of nebulosity condensed around the stars. A closer, equal pair is about 15' away. 62x.

NGC 6642: Small, faint, round, faint star to the NW, at seen at 64x. At 94x, contains 1 or 2 points. Not a difficult object.

NGC 6638: An easy, small, condensed, neat little thing containing a hint of some coarse sparkles. 94x.

The Omni XLT 150 costs less than 10% of what I paid for the large A-P refractor in 1994, yet it offers about 70% of the visual performance, in a package that weighs less than half as much. I could transport and use this scope without giving much thought to preserving my investment. Today you could pay more for counterweights and tube rings for a 6-inch apochromat than the total cost of the Omni. It is a surprisingly serious stargazing tool for very little money. No observer need be embarrassed to use such a telescope. A diligent observer could occupy himself for years with this scope. It would be fine for a Messier Marathon, and quite feasible for logging the Herschel 400 from a dark site. It was a lot more pleasant to use on the stable, driven SVP mount than on the supplied CG-4, but it’s still useable in its native form.

For my small investment of $338, I not only got the taller tripod I wanted, but a fine 6” reflector as well, plus a neat, though limited, little equatorial mount. This was the third 6” f/5 reflector I’ve owned, so I have a good idea of what to expect from them. This one gave up nothing to its two predecessors.

I foolishly sold that original scope for some pittance two or three years after I bought it. Having missed it ever since, I bought a new OTA for a modest price from High Point Scientific at the end of 2014. The newer scope has the advantage of a substantial 2" Crayford focuser, which has me considering it for imaging as well as for eyeballing things.