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Celestron Omni 127

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Celestron Omni 127 Review

The Situation

Some years ago, I lived in a rural location under relatively dark (mag 5.5) skies. At the time, I owned an Orion XT10 Dobsonian, and the views through it were gorgeous. Unfortunately, the tube was very heavy and awkward, which made me reluctant to move it any farther than out into the yard and back again. (I'm a 5' 6" physically active woman, but definitely not an athlete.) When I took a new job, I sold that house and moved to a new state in a location much closer to a large city. I decided that the XT10 would see relatively little use there and reluctantly sold it, replacing it with a much more portable Orion Short Tube 80.

After yet another move to the Boston, MA south shore suburbs, a mild case of aperture fever set in. I knew that I didn't want to go back to the days of hauling a large Dob around, no matter how impressive the views might be. But observing conditions at my third-floor apartment balcony were adequate for good lunar and planetary views, and the observing field of the South Shore Astronomy Club offered skies around mag 5, despite issues with high humidity.

I agonized between the Orion Skyview Pro 127mm Maksutov-Cassegrain for $699 and the Celestron Omni XLT 127 Schmidt-Cassegrain for $629. In the end, the ease of collimation, lighter weight, and greater versatility of the Omni 127 won out. It was frustrating not being able to find much in the way of reviews on the Omni 127, and I hope that the review you are now reading will help remedy that lack!

The telescope and mount were ordered from Adorama Camera, given its good reputation, relatively nearby location, and free shipping. Although 7-10 day shipping was chosen, the order was placed Wednesday night and the scope was on my doorstep when I arrived home from work on Friday. (After several good clear nights of lunar observing with the ST80, it was raining for the first time in days when the Omni arrived...)

Telescope Arrival and Assembly

The scope and mount were well packaged. All parts were double-boxed and some small items were triple-boxed. Although there was very minor damage to the outermost boxes, there was no damage at all to the inner boxes, scope, OTA, or accessories.

The CG-4 Mount

After using an EQ-1 equatorial mount to support the ST80 for several years, the CG-4 is a revelation. Now, the EQ-1 is by no means a bad mount. It's designed to be inexpensive, lightweight, and highly portable and serves that purpose well. It is adequate for supporting a lightweight scope such as the ST80. But its damping time is very poor, making high-power focusing a tedious guessing game.

The CG-4 is a much heavier mount, but it is still highly portable. I have no trouble picking up the entire mount/OTA assembly in one piece and moving it across a room, although I doubt that I will carry it through doors or down stairs that way. There is scarcely any plastic anywhere in the mount; only the slow motion knobs, dovetail locking knobs, and setting circles are plastic. Even the clutch release handles are metal. I am confident that this mount could handle an OTA heavier heavier than a C5.

Although the mount shipped with two counterweights, only the lighter 4 lb weight was needed to balance the small 6 lb OTA. Acceptable balance is easy to achieve. The instructions are clear, and include pictures showing the names of each component and drawing lines to the named component. This is very helpful when the instructions tell you to remove or tighten a particular know or bolt; you know immediately which one is being discussed.

Slow motions are very smooth. Those who complain of the Synta "glue" problem would be very happy with the CG-4 lubrication as shipped. There is a slight amount of backlash in the slow motions, but it does not interfere with star hopping at all. Above 200X, the slow motions are no longer very slow, and a gentle hand is needed.

The C5 Optical Tube Assembly

Fundamentally, the Omni 127 is the latest incarnation of the classic C5.

The lightweight OTA is incredible. Not having any prior experience with owning SCT's (although I have looked through those belonging to others at star parties), I didn't realize just how easy it would be to carry around the bare 6 lb OTA. It's only marginally heavier than the 3.8 lb ST80. I wish that Celestron would make a custom case available for the OTA; I would gladly pay extra for one. Since they include one with the C5 spotter scope, which is essentially the same OTA, they must already have designed and manufactured the case. Why not sell it separately to Omni 127 owners and have an additional profitable item?

I was somewhat disappointed that the scope shipped with a 6x30 straight-through inverting finder, which is not adequate for my light-polluted skies. So at the same time the Omni was ordered, a 9x50 right-angled correct-image finder was ordered from Orion. If you live in suburbia or, worse, an urban location, plan on purchasing a 9x50 finder separately. (I absolutely love this finder and recommend it highly.) The standard dovetail on Orion finders fits perfectly in the finder dovetail slot on the Omni, so installation is trivial.

It is not easy to sight down the finder or primary optical tube to get things pointed in the right general direction. If you are used to long, slow refractors or Newtonians, this may require some adjustment on your part. After struggling to star-hop with the right-angled 9x50 finder, I ordered a Rigel Quik Finder, which made a big difference in my ability to locate objects.

The focuser moves very smoothly, with just enough friction to remain in place.


The only planet in the sky at present is Jupiter. I have looked at it many times with the ST80 using powers anywhere from 25X to 133X. But the resolving power and violet haze overwhelm the view and show little but the main equatorial belts and Galilean satellites.

First light in the C5 was on a very hazy, overcast night with only a few objects showing. I was able to view Jupiter at 208x as a quick test, but the planet's image was quite soft and fuzzy, with no way to separate the effect of the haze from any miscollimation or the natural diffraction fuzz of the large central obstructions.

A few nights later the seeing and transparency improved. I was able to set up the ST80 and C5 side by side and compare the views of Jupiter. Eyepieces were chosen to yield approximately 78x. The planet looked quite similar in both, but there was substantial chromatic aberration seen in the ST80 and none in the C5. There may have been marginally more detail visible in the C5, but the difference was quite subtle. Moving up to 139x softened the view substantially, which probably says more about our hazy, blurry New England nights than about the scope.

It does appear that the C5 arrived slightly out of collimation. Defocusing on Arcturus at moderately high power yielded very clean, concentrenic Airy disks and rings. However, 2nd magnitude stars just barely out of focus showed that the rings are not *quite* perfectly concentric. I made a crude artificial star and tweaked collimation slightly. Rotating one secondary screw about 1/16 of a turn is all it took, so it clearly wasn't too far off as shipped.

Any new observers thinking about buying the Omni 127 (or any other SCT) need to realize that the scope MUST be properly collimated to see its potential. If you aren't certain how to do it, seek help from your local astronomy club. It is not a difficult process, and it is critically important.

Deep Sky

My eyepiece assembly is very modest, including only inexpensive 1.25" eyepieces. I hoped that the slow (f/10) focal ratio of the C5 would not tax them as much as faster wide-field scopes do, although in practice they have generally done well in the ST80 (f/5). My set includes a 32mm Orion Highlight Plossl, 25mm and 10mm Sirius Plossls that came with the ST80, a University Optics 16mm Konig, and 9mm, 7mm, and 6mm University Optics Abbe Orthoscopics. The 1250mm focal length of the C5 is identical to that of the XT10 I once owned, so the eyepieces originally purchased for it are also a good fit for the C5. An Orion Shorty Barlow 2X completes the set.

With Sagittarius well-placed from my balcony, I turned the scope on M22. Despite local light pollution, I was able to see both guide stars and the globular itself in the 9x50 finder. (Although I was able to find the globular in the ST80, it was invisible in the 6x30 finder.) At 78x in the C5, it was clearly visible but unresolved. At 139x, it began to resolve into stars around the edges. It was quite an attractive sight, and would no doubt have been more more so under a dark sky.

The CG-4's slow motions and rock-steadiness make star hopping very simple. M28 was easily found at 78x.


I am a fairly serious lunar observer with a shelf full of lunar atlases (both Rukl and Hatfield), the complete set of Clementine Lunar Digital Image Mosaic (LDIM) CD's, Patrick Moore's "New Guide to the Moon", Charles Wood's "The Modern Moon: A Personal View", etc, etc. It was the limitations of the ST80 for lunar observing that really pushed me into purchasing a larger scope with more resolving power and no significant chromatic aberration. The night I ordered the new scope, the ST80 had failed to show any sign of the Harpalus/Mare Humorum rilles at 133x, despite good seeing and near-optimum terminator location. So how would the C5 stack up?

I had owned the new SCT for several weeks before getting reasonable transparency and a Moon in position to be viewed from the balcony. Mediocre seeing left the image boiling at 208X (6mm Ortho), but the amount of detail visible was impressive. Dropping to 139X (9mm Ortho) improved the image steadiness while preserving most of the detail. The Alpine Valley was prominent, with shadows bring the Alps into good relief. Cassini was quite impressive, and the terminator fell just west of the famous Ptolemaeus/Alphonsus/Arzachel group. Wrinkle ridges were visible on Imbrium. The difference in resolving power vs. the ST80, even at the same magnification, is very evident. If you are seriously interested in looking at detail on the lunar surface, I recommend against small aperture scopes (i.e, 80mm or less). There's much more to see at 4 - 5".


A modest 8" Dob may still be in my future, opening up the DSO window a little wider and giving better views of globular clusters. However, since I am so often seeing-limited, I am not sure that more aperture will significantly improve planetary views here on 90% of observing nights. Hopefully we will be in a position to buy a house next year, which will help with storage and ease of carry-out and setup. But the C5 will continue to have a role as my portable, equatorially-mounted mid-sized scope even if a Dob joins the chorus.

Cathy James


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