Celestron C6-S Schmidt-Cassegrainian

In the realm of Schmidt-Cassegrainian telescopes, a notable gap in the otherwise popular 6-inch range has existed since the demise of the Criterion Dynamax 6 over twenty years ago. Celestron has now rectified that oversight by introducing their C6-S SCT, resurrecting a size they briefly offered decades in the past.

The newly-introduced C6-S is the first Celestron SCT to be manufactured overseas, and it fills a void in the product line-up that promises widespread appeal because of its combination of capability, light weight, and compact dimensions. The scope is offered as an optical tube assembly alone, a mounted CG-5 equatorial version, or as a full-fledged CG Go-To with computerized acquisition and tracking. Attractively priced, these new Celestrons cost less than Maksutov-Cassegrainians of comparable aperture. But how good are they?

Certainly, this 6-incher (actually a tad less with a clear aperture of 150mm) creates a favorable first impression. The tube is finished in a very attractive and uniform gloss black, with complementary textured black cast aluminum front and rear cells adding a professional touch. The scope came in a form-fitting Styrofoam cocoon with the 1.25-inch visual back and bracket for the 6x30mm finder already attached. A gold anodized CG dovetail rail runs almost the entire length of the tube for easy balancing on a host of equatorial and altazimuth mounts. Insert the matching black 6x30 finder into the bracket, and the scope is ready to go.

Strikingly small, this SCT weighed in at a mere 8.6 pounds on a produce scale with the finder and visual back attached. With a length of only 14.5”, it’s an obvious candidate for grab-and-go or air travel duty, as a lightweight mount is able to provide adequate support. The Celestron CG-5 mount is positively overkill, but it provides almost instantaneous damping and a set-up almost impervious to vibration and wind.

Optically, the tried-and-true Celestron formula of a short focus primary mirror and overall f/10 focal ratio has been translated into the 6-inch size. StarBright XLT coatings maximize light throughput, but a wickedly large 37.1 percent central obstruction is bigger than that of most SCT’s and almost all Mak-Cass scopes. Given these parameters, I was a bit apprehensive about the instrument’s performance capabilities, particularly on the planets and double stars, though the excellent Celestron 5 acquits itself well with an even larger central obstruction.

As it turns out, I need not have worried; the image quality generated by the C6 is comparable to the best SCT images I have seen with 5 and 8-inchers. The foreign manufacture of the scope seems to have retained the normally fine Celestron quality control and consistency. However, two minor problems were immediately apparent: the 1.25-inch visual back stubbornly refused to come loose when I wanted to attach a thread-on 2-inch star diagonal, and the telescope was noticeably out of collimation – running my unbroken string of 8 STRAIGHT factory-fresh misaligned SCT’s to a new record! The obstinate visual back was removed with a sharp tap from a mallet, finally revealing the threads that support a myriad of Celestron, Meade, and aftermarket accessories. The Celestron 2-inch diagonal, f/6.3 focal reducer; Meade f/3.3 reducer for photography and imaging; and just about anything else, all fit nicely.

Recollimation of the C6 was no particular problem, and I imagine that Bob’s Knobs to simplify the process are already available or soon will be.

PERFORMANCE TESTS

While this new Celestron has no direct competitor, the Orion 150mm, f/12 Maksutov-Cassegrainian is similar. My recent testing of the Orion scope allowed for some direct comparisons, and I could also draw upon several 4 and 5-inch APO refractors, a C5 SCT, and a 6-inch achromat for an impromptu “shootout” under the stars.

With the C6 perfectly aligned, star testing using Polaris showed a very nicely corrected optical system. Intra and extra-focal images were nearly identical with diffraction rings close to the ideal textbook appearance expected in a highly obstructed telescope. There was no evidence of astigmatism, spherical aberration, or rough optical surfaces. I would estimate the overall correction to be on the order of 1/7 to 1/8 wave. In focus, the image of the North Star at 180x (using a Celestron zoom eyepiece) displayed a hard, sharp airy disk with a well-defined first diffraction ring. Secondary rings came and went with changes in seeing conditions, and the nearby stellar companion was a fairly bright pinpoint. Unfortunately, the image shift prevalent in many SCT’s was also the case here. Although the moving-mirror focuser was very smooth and had a solid feel, the image migrated some 20 arc-seconds (about the current diameter of Mars) in a gentle linear motion at one point in the focusing range. Certainly not too bad, but it could be better.

With Mars climbing out of the turbulent low-altitude air, the Red Planet provided an attractive target and test subject. Under average seeing conditions (5 to 6 on a 10-point scale), the planet’s disk presented a fine sight at 167x with a 9mm TeleVue Nagler Type 6. The C6 revealed the bright north polar hood, limb brightenings, and fairly good contrast between the Martian maria and surrounding light pinkish desert areas. With a #21 orange filter, contrast was further improved, and the Hesperia “gap” between Mare Cimmerium and Mare Tyrrhenum was clearly visible in moments of steady seeing. A Takahashi FS-102 fluorite APO refractor showed essentially the same level of detail, but with superior contrast and slightly less brightness at the same magnification. An Antares 6-inch, f/6.5 achromat, despite its good optical quality, fell behind the Celestron, as its contrast was compromised by a purple”wash” of chromatic aberration, though it had the ability to bear power well. The 5-inch Takahashi FS-128, as expected, decisively trumped the other scopes in both the quantity of detail visible and the ease in discerning it. No real surprises here, but the C6 certainly didn’t embarrass itself in the company of some tough competition.

With the moon just a few hours before first quarter, the telescopes were turned to our natural satellite to generate some very pleasing views. I’ve always believed that a truly good scope will provide an “I’m in orbit” experience where the optics seem to disappear. On this count, the C6 delivered. At 50x, using a 30mm Celestron Ultima eyepiece, the lunar features stood out in stark contrast with no hint of spurious color or image softness. Earthshine was easily seen even when the moon was moved about and out of the field-of-view. The telescope’s effective baffling and well-controlled light scatter made for a very impressive view. With the magnification boosted to 224x with a Meade 6.7mm UWA, the richness of the lunar terrain was startling. Even at this power, the image remained bright, and tiny details in the Hyginus rill were sharp and well-defined. The Altai Scarp loomed large and impressive, and the lunar terrain north of Eudoxus and Aristoteles took on an almost three-dimensional appearance.

A day later, the Straight Wall became visible near the terminator and displayed its subtle irregularities and the elusive Rima Birt nearby. The view was strikingly similar to that afforded by the Orion 150mm Mak-Cass at similar magnification. Initially, I had the impression that the Orion scope had slightly better contrast, but now it seems that the significantly brighter image of the Celestron gave an erroneous read. These scopes are so close in revealing lunar and planetary detail that it’s difficult to choose one over the other – both are very good.

The compact Celestron also did well in resolving multiple star systems. The classic test, Epsilon Lyrae, showed a close but clean split of all four components at just 60x with a generic 25mm Plossl. At 150x, with a similarly inexpensive 10mm Plossl, the four tight airy disks were nicely defined and surrounded with clean but prominent first diffraction rings. These rings approached invisibility in the 4 and 5-inch APO’s, but the actual resolution was no better. Interestingly, the f/10 SCT showed almost no sensitivity to eyepiece cost or design: everything from a $20 Kellner to a TeleVue Nagler worked well.

More challenging doubles, Zeta Aquarii and Delta Cygni were also resolved without much difficulty. The latter’s faint, close companion was the greater test as mediocre seeing conditions hampered the split, but moments of steady air showed the tiny secondary just outside of the first diffraction ring with the Celestron zoom cranked close to its maximum at 180x. Though Antares is gone until next spring, I suspect that the scope will handily reveal its companion when seeing conditions are favorable.

Though larger telescopes are much preferred for deep-sky observing, the C6-S did provide some very pleasing views. The StarBright XLT coatings help a great deal here as they push light throughput to just about the maximum this design can achieve. Junk the cheap 1.25” star diagonal provided with the scope and replace it with a quality high-reflectivity 2-inch unit, and the Celestron transmits a lot of starlight with minimal loss. The consequent edge over both the Celestron 5 and Orion 150mm Mak-Cass is very noticeable on almost all objects.

Though certainly no RFT with its 1500mm effective focal length, the C6-S mated well with an Orion 2-inch, 40mm Optiluxe eyepiece. The resulting 37.5x generated an actual field-of-view of some 1.6 degrees – enough to frame the Pleiades and their soft blue nebulosity. In similar fashion, the Andromeda galaxy, M 31, spread from one end of the field to the other, enclosing both satellite galaxies and revealing a good deal of nebulosity and a hint of dark lanes. The background remained quite dark, and vignetting, if present at all, was subdued.

At 79x, using a TV 19mm Panoptic, the scope resolved M 13 to the core on a dark, transparent night; it partially resolved some of the lesser globulars such as M 2 and M 15, and dug into their cores when the power was upped to the 175x range. In comparison to the other telescopes, the C6 overwhelmed both the Celestron 5 and 4” APO in magnitude penetration. By counting stars is the beautiful Double Cluster in Perseus, I was able to determine that the C6-S provided about the same light grasp as the 5-inch Takahashi APO, and ,surprisingly, almost the same as the 6-inch achromat. Stars were hard, sharp pinpoints in the SCT, but not quite the equal of the burning stellar intensity that only a high-quality APO seems to generate.

Overall, I’m very impressed with the C6-S. This is a telescope with fine optical quality, good fit and finish, and great versatility. Its compact size and light weight make it a natural for those who have limited space or who travel by air. It features enough aperture for some very satisfying deep-sky views and the resolution and contrast needed for worthwhile lunar, planetary, and double star observation. APO refractor fanatics ( and I’m one of them) will still hunger for that last increment of perfection that only an unobstructed optical system can provide, but this little SCT will please many observers and imagers. Those who prefer tank-like ruggedness and relative freedom from collimation might opt for the Orion 150mm Mak-Cass – it’s a good telescope as well.

As for me, the ability of the C6-S to use a light mount and to accommodate just about every SCT accessory (including the 2-inch variety) known to humankind is the clincher.
And at $699 with a CG-5 equatorial mount, it qualifies, in my opinion, as a real bargain. If they are all this good, Celestron has a worthy successor to the C5 and a real winner.

As for my example – it’s a keeper.


Clear and steady skies,
Larry Carlino

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