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Home > Reviews > Telescopes > Cats > Celestron 8 XLT

Celestron 8 XLT
By Lawrence Carlino - 9/20/2004

Celestron C8-S XLT –
The CAT with (at least) 9 lives

From its inception in the 1960’s, the Schmidt-Cassegrainian telescope (SCT) has been a favorite instrument of amateur astronomers who value its combination of large aperture, all-around optical qualities, compact size, and reasonable price. Yet, some purists who insist on the optical near-perfection of modern apochromatic refractor tend to malign the SCT for its supposedly “soft” images and lack of contrast on lunar and planetary targets. Personally, I’ve been fortunate to own many examples of both telescopic types over the years, and thought it might be interesting and instructive ( and perhaps patently unfair) to test Celestron’s latest 8” SCT against some pretty formidable APO refractor competition.

Though Celestron offers its 8” SCT in several configurations, I opted for the C8-SGT with its aluminum tube, CG-5 computerized mount and Starbright XLT coatings to maximize its light throughput. While it is not within the goal of this review to evaluate mounting options, suffice it to say that the CG-5 mount with its rigid adjustable stainless steel tripod legs, smooth motion, and almost instantaneous vibration dampening, handles the 12 pound optical tube beautifully. The Go To feature works nicely as well, putting the desired object within the field-of-view of an 80x eyepiece 90 to 95 percent of the time.

For competition in this comparison, my Celestron 9.25” SCT without the XLT coatings was put on the line, along with a superb Takahashi FSQ-106N APO, a friend’s AP Traveler(when available), and (mercilessly) a Takahashi FS-152 fluorite APO.

Observations were completed over several nights varying from steady and turbid to brilliantly transparent with indifferent seeing conditions. Not all of the telescopes could be used for direct comparison on every night. Top-of-the-line star diagonals were employed on all of the telescopes, but selecting eyepieces of comparable characteristics was difficult because of the huge disparity in the scopes’ focal lengths and ratios. Nevertheless, all eyepieces employed were of top optical quality to largely eliminate ocular shortcomings able to degrade telescope performance.


The C–8 Optical Tube

Finished in an attractive glossy dark gray metallic, the C-8 optical tube surprised me with its compact dimensions. With the 1.25” visual back removed, the scope measures a mere 17.5 inches in length with a tube diameter of 9 inches. The standard 6x30 finderscope and full-length, golden orange anodized CG dovetail bracket add a couple of inches to its depth. If it were not for the need of some sort of protective case, this telescope would meet the requirements of airline “carry-on” luggage. Weighing a svelte 12 pounds, it can also be mounted on a reasonably light mount for portability. I found that a Universal Astronomics Unistar could handle this OTA nicely, creating a rather large-aperture “grab and go” with minimal weight.

Focusing is accomplished by the ubiquitous SCT back-of-the-scope knob that moves the primary mirror. Image shift was almost unnoticeable in my example of the C-8, a small displacement of about 5 arc seconds being visible in one small area of the focus range. With the twist-on plastic dustcap removed, the scope showed an interesting characteristic of its optional StarBright XLT coatings: the thin front correcting plate was almost invisible! The water white glass of the corrector and state-of-the-art coatings created close to 100 percent light throughput, a real plus when coupled with the high reflectivity of the primary and secondary mirrors. These special coatings do, indeed, appear to work as advertised. Though I didn’t have an older C-8 on hand for direct comparison, my personal recollections and observing notes seem to confirm the added light grasp and better contrast afforded by the XLT-equipped scope.


The Moon and Planets

With Saturn sinking into the western sky and Jupiter still high in the southwest, I put the Celestron into direct competition with the APO refractors. Frankly, I was surprised by the results. At 155x, using a TeleVue Nagler type 6, 13mm eyepiece, Jupiter was awash with detail in the C-8. Delicate scalloping was easily seen in the equatorial belts, and bluish festoons decorated the equatorial zone. The normally bland polar areas displayed significant low-contrast detail, and the four Galilean satellites were clearly defined discs of varying size and color. Moreover, the image was very bright at this magnification, allowing the faint details to be glimpsed without difficulty. The 4-inch APO’s also provided splendid views of the giant planet. If anything, the overall contrast was better in both the AP and Takahashi scopes. But at 150- 165 x, the image brightness began to suffer and engendered the creation of “floaters” that interfered with the image quality. This might not be a factor with younger observers, but the seasoned could find it moderately annoying. The fabulous Tak FS-152 , however, simply blew away all of the other telescopes. Details in the equatorial zone at 140- 185x had the sharpness and intensity of a pen-and-ink line drawing. An awesome display of telescopic perfection, but to be expected in light of the big Tak’s pedigree and price.

Saturn provided a similar display, the C-8 again providing a superb view despite mediocre seeing conditions. At 155x, the Cassini division in the rings circled the entire globe, and some very subtle belt and zone detail could be glimpsed on the planet’s disc. Only when the air became turbulent did the image degrade significantly. The 4-inchers again won in absolute image contrast and resistance to poor seeing conditions, but, overall, they showed less detail than the larger Celestron. This was the classic trade-off between aesthetics and capability. The 6-inch refractor once again combined the best of both worlds and easily provided the most satisfying image.

The first quarter moon was a marvelous sight in all of the scopes, and it was difficult to assess differences in performances. With the Tak FS-152 as the benchmark, I found that the C-8 showed as much as the big refractor, albeit with somewhat less contrast and sharpness. Intricate details within the Hyginus rill at 225x elicited a “wow” with both instruments as did the rugged cratering in the lunar south. A few days later, the Celestron flaunted its superior aperture in easily revealing the minute rill Rima Birt, the entire double chain of coalesced craterlets near Copernicus, and a wealth of fine detail within Gassendi at 155 and 226x (13mm and 9mm Naglers). The Tak FS-106 essentially displayed the same level of detail with marvelous contrast, but the limited aperture at high magnifications (212x) resulted in a dimmer image and more effort to perceive the most difficult features. The 6-inch Tak did, however, outperform both of the other scopes, almost creating the impression that the observer was viewing the lunar surface from an orbiting spacecraft. I was severely tempted at this point to banish it from the competition for attempting to violate the laws of physics.


Double Stars

As expected, the APO refractors excelled in tests of double star resolution, but the SCT was far from disgracing itself. A quick out-of-focus star test of the C-8 displayed a well-corrected optical system with nearly identical intra and extrafocal patterns. Without being an expert on the matter, I would estimate overall correction to be in the 1/6 to 1/7 wave range – certainly very good.

A classic test object, Epsilon Lyrae, was resolved into four razor-sharp, perfectly defined airy discs at 120-135x with all of the 4 and 6-inch refractors. Under fairly steady seeing conditions, there was no discernible flaring of the image, no intervening haze, and wonderfully delicate first diffraction rings. The Celestron also accomplished the split with ease, but the tightness of the airy discs wavered with the atmospheric turbulence, and the first diffraction rings were significantly thicker – a result of the SCT’s central obstruction. The much more challenging unequal double Delta Cygni provided a bit of a surprise. Aside from the ludicrously easy split provided by the 6-inch Tak, the 8-inch Celestron managed an easier split than the 4-inch APO’s. At 226x, the light grasp advantage of the SCT made the tiny secondary stand out clearly, while the smaller scopes’ first diffraction ring tended to mask the elusive companion. However, the tight, high-contrast images of the refractors made for a textbook display of optical perfection guaranteed to please the most finicky observer.

Finally, I tried to resolve the extremely difficult and close Gamma Virginis, currently a duo separated by less than one second-of-arc. With a gap of less than the classic Dawes’ limit for a four inch, the refractors could do no more than elongate the pair at over 300x, but the C-8 managed a close but definite split at 406x using an Orion 5mm Ultrascopic eyepiece. Very impressive! (The 6-inch Takahashi was not used on that night.)


The Deep Sky

It came as no surprise that the C-8s superior aperture gave it a decided advantage in deep-sky observing. With its XLT coatings, the SCT bumped its effective aperture to that of an 8.6” with the standard StarBright configuration. On a dark, transparent night with a naked -eye visual limit of mag 5.9, the Great Hercules Cluster M-13 was resolved to the core at 155x using a 13mm Nagler type 6. Hundreds of stars sparked throughout the cluster giving it an almost 3-D appearance as magnitude penetration went deeper than 14. The Takahashi FSQ-106 also served up a nice view at 140x with an Orion 3.8mm Lanthanum eyepiece, but only a hundred or so of the big globular’s stars were visible, averted vision being a necessity to bring out the fainter members. The 6-inch Tak fell somewhere between, making up for its limited aperture with an incredibly black sky background and stars that shone like tiny diamonds at 134x with a 9mm Nagler type 6 in the drawtube. Best of all, however, was the Celestron 9.25”. Even without the XLT coatings, the big CAT went deeper than the 8-inch and put handfuls of previously invisible stars within the range of averted vision. Here, as expected, aperture rules; and the long f/ratio of the Celestrons almost always excels in providing a memorable view of these objects.

In similar fashion, the Ring Nebula (M-57) was a more pleasing and detailed sight with the larger scopes . The 4-inchers clearly displayed the planetary’s annular shape at 100x, but it took the 8-inch to define irregularities in the ring and hint at the haze on its inside, a power of 155x still providing enough surface brightness to discern the finer details. In addition, the faint greenish cast of the ring could be perceived in the Celestron while the smaller scopes revealed a colorless gray.

Rich-field observing, not surprisingly, was clearly the domain of the refractors, particularly the short-focus Tak FSQ-106. With an effective focal length of just 530mm, a dedicated 2-inch Takahashi star diagonal, and 19mm TeleVue Panoptic yielding 28x, the little APO provided magnificent vistas of the Cygnus concentration of the Milky Way and framed almost the full extent of M-31. The long 2032mm focal length of the C-8 seemed to work best when coupled with a Celestron Ultima 30mm yielding 68x. With the 50+ degree apparent field-of-view, the resulting swath of sky was large enough to nicely frame the Double Cluster in Perseus, M-27, and the rich open clusters and diffuse nebulae of Sagittarius, except for the full extent of the Lagoon (M-8). With the optional Celestron f/6.3 focal reducer attached, the magnification dropped to 43x, with the actual field-of-view expanding to over one degree. This is not true RFT performance, to be sure, but it does put all but the very largest objects within the Celestron’s grasp.

Conclusions

The Celestron 8 has, in its many iterations, survived for 35 years for some very good reasons, but there are some caveats in acquiring and owning one:

A: The scope MUST be properly collimated to achieve its designed performance. Not particularly difficult, but of the dozen or so SCT’s I’ve purchased over the years, only ONE has kept its perfect factory alignment after its customary abuse in shipment.

B: The telescope MUST be at thermal equilibrium. Even if stored in an unheated garage, the C-8 may take a while to cool down. When the “plume” wafting off of the secondary obstruction silhouetted against the background of a bright out-of-focus star disappears, the scope is ready to use.

C: Poor seeing conditions affect the C-8 more than they do a smaller refractor: larger aperture, a secondary obstruction, and multiple light passes within the tube all conspiring against the SCT.


With these basic precepts in mind, there is little question that the Celestron 8 can be a very fine performer. While smaller APO refractors have superior image sharpness and ultra-wide-field capability, they do cost a good deal more. A large Dobsonian reflector in the 10 to 12-inch range costs less and provides greater light grasp, but the sheer weight and bulk can be a problem for many observers. So the C-8 is a compromise – but a very good one at that. It has a multitude of capabilities ranging from high-power lunar, planetary, and double-star observing, semi-wide field viewing, and fine characteristics for astrophotography and ccd imaging to amazing portability for its aperture. There are dozens of aftermarket accessories to customize the scope to the user’s needs. It can be attached to a simple alt-az mount for “grab and go” or a sophisticated Go To equatorial. That’s versatility. And very possibly the reason that this CAT has enjoyed at least 9 lives, this most recent generation being the best yet.

Clear skies!
Larry Carlino

Click here for more about the Celestron 8" Schmidt Cassegrain f/10 OTA with XLT coatings
. -Ed.


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