CELESTRON OMNI XLT 150R

Back in my high school days, when I became an enthusiastic amateur astronomer, my friends tagged me with the unique nickname “Chromabs”(short for “chromatic aberration”) in deference to my fascination with refracting telescopes. Because there were no commercially available apochromats in those days, blind (please forgive the pun) acceptance of chromatic aberration was a fact of life.

When I graduated from a fine 60mm Unitron refractor to a homemade 6-inch, f/15 beast with a very nice Edmund Scientific-sourced objective lens, I had learned to ignore the subtle color fringing produced by these otherwise razor-sharp instruments.

As I moved on through a series of reflectors and catadioptric scopes and, eventually, ED and triplet refractors, I still maintained a soft spot in my heart for the ungainly, long-tube achros.

But the physical length of an f/15 achromat of any significant size was its real undoing, as the massive mount required to hold such a telescope pretty much ruled out easy portability. Yet, what about a large short-focus Fraunhofer achromat for rich-field observing? Orion (Synta) and Celestron were producing some very competent 100mm and 102mm, f/5 and f/6 scopes, and the Orion 120mm, f/5 impressed me with both its image quality and sensible price.

In the 150mm (6-inch) range, however, the Celestron and Meade f/8 6-inchers and f/6.5 unit from Antares, although quite good optically, simply weighed too much to permit the use of a small, portable mount. Though Skywatcher of Canada did offer a Synta 150mm, f/5 achromat, American observers had no easily obtainable alternatives – until now.

THE CELESTRON OMNI XLT 150R


The Celestron OMNI XLT series has been around for some time, but it has been expanded recently to include a 100mm, f/9 ED refractor and a 150mm, f/5 achromat.
All scopes in the OMNI series utilize the Celestron CG-4 mount, a smaller, less robust version of the CG-5 used in their Advanced Series. Coupled with a sturdy adjustable steel-tube tripod with 1.75” legs, the mount is finished in an attractive off-white with the standard Vixen/CG dovetail slot for attaching a variety of optical tube assemblies. Slow-motion controls grace both the polar and declination axes, and a DC-powered motor drive is an available option.

Immediately, one has to question the mount’s ability to handle a 6-inch refractor, albeit one of short focal length – but it actually does work reasonably well – for the Omni XLT-150R weight but 16 pounds and is only 34 inches in length with the dew cap in place. This renders the scope smaller and lighter than almost all 5-inch refractors and even some 4-inchers: real possibilities for grab-and-go capability.

The 150R’s optical tube is finished in the characteristic Omni dark metallic blue with off-white accents to match the mount provided. Aesthetics are usually a rather individual matter, but I found the result quite tasteful and attractive. A somewhat stiff but reasonably smooth 2-inch rack-and-pinion focuser with 1.25” adapter fills the back end along with a dovetail mounted 6x30 finder. A 1.25” prismatic star diagonal and uninspired 25mm eyepiece are provided as is a press-fit dew cap and a plastic dust cap with a removable 115mm center section for stopping down what is a very fast telescope. The included tube rings and attached Vixen/CG dovetail for mounting the tube assembly are the typical Synta units also used by Orion.

With the beefy achromatic doublet and its heavy cell up front, the weight of the 150R is clearly biased in that direction, but offsetting the 8-inch mounting dovetail allows the scope to look relatively symmetrical on its mount. Adding a 2-inch star diagonal and heavier finder also aids in creating a balanced appearance. With the two counterweights provided, the telescope is easily brought into perfect harmony. The entire assembled configuration weighs less than 50 lbs.

Under the stars, the mount functions fairly well, but only to a point. With the tripod legs extended to about 90 percent of their full travel, damping times after a sharp rap on the tube were in the 3.5 to 4-second range: not at all bad at low and medium magnifications, but a bit dicey at powers over 100x or so. This same mount is rock-steady with the little C-5 SCT, but the much heavier and longer refractor pushes it to the limit.
Motions of the mount are fairly smooth, but there is a bit of sticky inertia to overcome followed by too much eagerness to move rapidly and easily. I ended up attaching the telescope to a Universal Astronomics Unistar (heavy) mount atop a Celestron tubular steel tripod. This combination provided a genuine grab-and-go set-up that proved to be light and sturdy.
My suggestion to Celestron: offer the scope as an OTA only or couple it with the heavier CG-5 equatorial mount.


OPTICAL TESTING


Frankly, I had very low expectations for a mass-produced, short-focus achromat. Surely, there would be gobs of false color and poor correction of other aberrations.
But what a shock! Although the obligatory chromatic aberration was present, the telescope star tested remarkably well. Using a #58 (green) filter while targeting Polaris, I found that the optics were in good collimation, showed no astigmatism, and (though mildly undercorrected) showed sharp, distinct fresnel rings. The bright outer ring, dark gap, and well-defined inner rings attested to rather impressive optical quality. Comparing the 150 to my Orion 120mm short-tube refractor, I found the larger scope to be a tad better optically.


LUNAR and PLANETARY VIEWING



One can hardly expect a short-focus achromat to be optimized for lunar and planetary observation, but I did try a number of targets to see if the XLT 150R could serve up a decent image. An observer’s personal tolerance for the ravages of chromatic aberration has a lot to do with the telescope’s acceptability in this role. I, for one, can handle a moderate violet halo around bright objects, but excess red is truly bothersome.
The crescent Venus in the evening sky (a tough test of color purity) was an ugly sight with a 5mm Orion Ultrascopic (150x) in the drawtube. Gobs of purple, red, and violet surrounded the planet like a shimmering kaleidoscope. However, the actual disc of the planet remained sharp and nicely defined. Trying a variety of filters to improve the image, I found that a Sirius MV-1 reduced the false color by about 60 percent. A less aggressive MV-20 was less effective, and a Baader Semi-APO filter seemed to do no good at all. Because the scope is well corrected in the yellow-green areas of the spectrum, I tried a W 56 (green) filter out of desperation. Voila! Although the planet took on the monochrome green appearance of a 1980’s computer monitor, the chromatic halo disappeared completely. Amazing! Suddenly, the bright “cusp caps” of the planet could be glimpsed, and its disc looked APO-sharp.

Similar results were found to be the case with Saturn. The ringed planet displayed a lot less CA even without filtration, but the W 56 brought it into perfect focus which revealed both equatorial bands, the ring shadow, and hints of the Cassini Division. The Omni has enough light grasp to accommodate the loss of brightness with the filter. With the aperture stopped down to 115mm, the image was even better, but a bit too dim at anything over about 150x.

Lunar observation again showed the effectiveness of the green filter. The first quarter orb displayed a wealth of sharp, high-contrast detail at 107, 125, and 150x. Plato and the Alpine Valley stood out in bold relief, and the lunar highlands were impressively defined. TeleVue Type 6 Naglers seemed to give the best results here, but the TMB Planetary Series oculars also performed well. Some of the cheap generic Plossls were noticeably inferior as they were compromised by the steep light cone. With the W 56 removed, chromatic aberration became a distraction at anything more than about 50x. But if the observer doesn’t mind a “green cheese” moon, the filter works near miracles.


DOUBLE STAR OBSERVATION

In trying to resolve stellar duos with the Celestron, I found that the scope actually produced more of a reddish halo than one of violet. This effect was particularly apparent on stars in the deep yellow to red color range. Just the same, I did find out that the telescope was quite capable of resolving doubles pretty close to its .76 arc-second theoretical limit.

The now widening (but still very close at 1.2 arc-sec) Gamma Virginis, though it was bathed in a ruddy haze at 214x, was resolved with a sliver of dark sky between the components – not very pleasing to the eye, but still impressive. With the W 56 filter in place, however, the pair became tightly defined and very sharp. A fairly clean airy disc and thin first diffraction ring around each star was even better at 268x with a Takahashi 2.8mm LE in the optical train.

Similar results were obtained with Castor, Izar, and Gamma Leonis: Unfiltered: resolved but colorful and messy; with W 56 filtration: very nice but monochromatic.

I suspect that the scope may resolve doubles close to or at Dawes’ limit, but it would need a night of tranquil air to take on the challenge.


DEEP-SKY PERFORMANCE

Six inches of short-focus refractor certainly has the potential of generating some terrific, high-contrast images of a variety of deep-sky objects, and the stubby Celestron delivered as expected. With a light throughput approaching 100 percent and no central obstruction, the XLT 150 provided some memorable deep-sky views and both low and medium magnifications. On objects of 5th magnitude or less, the scope’s inherent false color ceased to be a factor, and galactic star clusters, globulars, and diffuse nebulae were unexpectedly bright and cleanly defined.

The Double Cluster in Perseus (NGC 869/884) was a stunning vista of sparkling points at 83x using a 9mm Televue Nagler Type 6, the telescope’s significant light grasp affording a bright and sharp view. However, compared to my Takahashi FS-152 fluorite APO, the Celestron fell short in image brightness, contrast, and intensity. This, of course, is to be expected, but it does point out that an APO of similar aperture will concentrate more light of all wavelengths in a tight pinpoint and penetrate somewhat deeper in magnitude.

(I did check the focal plane of the OMNI XLT to determine if its interior baffles or focuser were cutting into the light cone and found that it might lose a tiny fraction of its aperture with a 2-inch star diagonal – perhaps about 2 mm – hardly significant.)

Globular clusters were also impressive with the Celestron refractor, M 13 being beautifully resolved to the core at 107 and 125x. M 92 and M 5 also displayed nice resolution as did M 10 and M 12 of the central Ophiuchus trio.

Galaxies were a satisfying treat. M 82 easily revealed its irregular structure and dark mottling, and the Sombrero (M 104) showed its bright nuclear bulge and prominent dark lane.

The real strength of the Omni, however, is in the wide-field views that it affords. Pop in a TV 19mm Panoptic yielding 40x, and the false color all but disappears when scanning the rich star fields of Cygnus and Cepheus. I have no doubt that the scope is capable of creating some marvelous views when it gets a shot at the rich Sagittarius and Scorpius star concentrations and the bright diffuse nebulae in that area of the sky. If the observer can accommodate the large exit pupil at even lower magnifications, this refractor promises some truly awesome possibilities.


THE H-ALPHA MACHINE

One of the primary reasons I purchased the Omni was my previous experience with a Collins I3 image intensifier coupled to a narrow-band hydrogen-alpha filter. This combination had given me some magnificent views of some faint and elusive diffuse nebulae with both small refractors and larger Newtonians. The aperture and fast optics of the Celestron scope seemed like the perfect match for some impressive vistas.

On a dark, moonless night with a naked-eye limiting magnitude of 5.5, I used the I3/H-alpha set-up at magnification of 50x with the 150R. The resulting views were spectacular. Because of the narrow H-alpha bandpass, the image intensifier was able to operate at close to its maximum 50,000x efficiency, darkening the sky background to pitch black and generating outstanding contrast. The Great Orion Nebula was so bright and detailed that it was enough to cause a loss of dark adaptation, albeit its monochrome green coloration. The normally elusive Horsehead and the surrounding IC 434 were easy to pick out (no imagination necessary!), and the nearby Rosette Nebula displayed many of the finely layered details that it normally reveals only in photographs. For moderately wide-field intensified H-alpha observing, the Omni is an ideal instrument.


IN CONCLUSION

The Omni XLT 150R has turned out to be a better-than-expected performer. If the prospective user realizes that it is not an all-purpose instrument, but rather a specialized telescope that does quite nicely in its own deep-sky niche, it is more than satisfactory. The unavoidable chromatic aberration that plagues it for high-power planetary viewing is pretty much a non-factor when sparkling open clusters, galaxies, and glowing diffuse nebulae are the targets. Yet, when stopped down to f/6.5 or more and the appropriate filters are used, the scope does serve up some reasonably pleasing views of the moon, planets, and multiple stars.

The Omni’s light weight and short tube allow for excellent portability when coupled to the standard (though marginal) CG 4 mount or a sturdy alt-az set-up. It is a telescope that is very easy to carry around the back yard or be transported to a dark sky site.

Aesthetically, it conveys that “cool” look that is bound to gather a crowd at star parties – and, for the most part, it will deliver what it promises.


Clear and steady skies!
Larry Carlino