Celestron C11 XLT Review: Recommended OTA

Optics

The C11 XLT is a standard Schmidt-Cassegrain with a spherical 11” (280mm) f/2 primary mirror, a Schmidt corrector, and a 5x amplifying secondary mirror, making for a system focal length of 2800mm and a focal ratio of f/10. Most C11s have very good optics. The 2800mm focal length, however, limits you to a field of view of only about 1 degree, and the very outer edges of this will have some coma and other undesirable aberrations, so 0.9 degrees or so is a more realistic limit even with a 2” eyepiece. An f/6.3 reducer used in conjunction with a 2” eyepiece will vignette with the C11. 

It should be noted that while the C11 is technically capable of up to 600x or so, conditions will often limit you to half or even less than half of that magnification, and you should always make sure the scope is adequately collimated and cooled down before you blame the optics for providing less-than-sharp images.

Mechanics

Like almost every SCT, focusing the C11 XLT is accomplished by moving the primary mirror. This configuration can induce image shift while focusing, which is annoying when observing at high power or conducting planetary imaging, and mirror flops can ruin long exposures. If you’re planning on imaging of any sort with your C11, it’s best to find an aftermarket solution to lock the primary mirror in place if possible and at least fine-tune your focus with a Crayford focuser screwed onto the back to avoid both image shift and mirror flop. The C11 is collimated by adjusting a set of Philips head screws at the front of the telescope to tip/tilt the secondary mirror. Collimation is not a particularly difficult process; our collimation guide goes into further detail. Replacing the Philips head screws with thumb screws is not recommended, as it can lead to more frequent shifts in collimation, including while you are trying to use the telescope.

Unlike 9.25” and smaller SCTs, the C11 uses a large 3.25” accessory thread on the back, which can be used to mount various heavy-duty accessories, but an adapter to use the standard 2” SCT accessory thread is included with the scope.

Newer C11s have a carry handle attached to the back. While I would probably not recommend carrying the scope with one hand, this handle is a nice convenience. Additionally, the front corrector cover rotates and clicks into place so it doesn’t easily fall off while moving the scope.

The newer C11 XLT uses StarBright XLT multicoatings and water white corrector glass. Compared to an older orange tubed scope, there is a slight but detectable improvement in image brightness.

Accessories

The C11 XLT is pretty much always supplied with a straight-through 50mm finderscope of decent quality. While you probably will be using this scope on a GoTo mount and thus won’t need a large-aperture finderscope for anything, the inclusion is a nice bonus.

Additionally, the scope includes a 25mm E-Lux Plossl, 1.25” prism star diagonal, and a 1.25” visual back. To get the most out of your C11, you really need a 2” diagonal and wide-field 2” eyepieces, due to the scope’s ultra-long focal length.

Lastly, the C11 is available with either a Vixen (CG-5) style or CGE-style dovetail plate. In almost all circumstances, you will probably want the latter, since pretty much all the mounts you’d want to use with a C11 take Losmandy and CGE-style dovetails.

Mount Recommendations

For deep-sky astrophotography, you want as heavy-duty a mount as possible for the C11. Something with a 60-pound or greater weight capacity is preferable. Mounts of this size are generally expensive and not particularly portable, so if you plan on using your C11 for deep-sky imaging, you should plan on either storing the setup on a lockable dolly and rolling it out of the garage, or in some kind of observatory. Autoguiding is also absolutely required, and you should probably use an 80mm guide scope for optimum performance. These can be attached by obtaining and installing a second dovetail on the top of the C11 and piggybacking the guide scope assembly on top.

For visual use, planetary imaging, and perhaps use with the HyperStar system, something in the EQ6 class size range like the Orion Atlas, Celestron CGEM, or Sky-Watcher EQ6 is recommended. Any of these mounts can be set up with the C11 by one person in about ten minutes and are quite stable. The hardest part is hoisting the tube onto the mount, which is mainly due to the scope’s large size and the height it must be lifted to and not the weight. However, all of this can be done by a single, average strength user without too much physical strain, and the process is no more physically demanding than setting up a solid-tubes 12” Dobsonian.

Celestron sells the C11 bundled with their Advanced VX mount, and this combination will indeed function, but it is putting some strain on the gears and can be frightening to use in windy conditions. Other EQ5-class mounts usually have smaller (1.75” vs. 2”) tripod legs than the AVX and are completely out of the question for use with the C11.

Should I buy a Used Celestron C11 XLT?

A used C11 can be a great scope, be it an older orange- or black-tubed model or a newer XLT, though keep in mind that only the latest XLT versions have HyperStar compatibility. As always, check to make sure that the corrector is undamaged and the coatings are not subject to any fungus or other corrosion, as replacement of the optics is extremely costly and often nearly as much as just buying a new telescope. The oldest C11s may also lack any coating at all on the corrector plate, which can reduce light-gathering ability quite a bit below that of a typical telescope of this size.

Aftermarket Accessory Recommendations

Investing in a dielectric 2” mirror star diagonal like the Apertura carbon fiber twist lock model to screw onto the back of the C11 XLT is a must, as your maximum achievable field of view is severely limited with the stock 1.25” diagonal and visual back. While almost any eyepiece design works at f/10, at the minimum we’d recommend 3 or 4 oculars, starting with a 2” wide-angle like the Apertura 38mm SWA (74x) which maxes out the C11 XLT’s field of view, and additional eyepieces such as “goldlines”, Explore Scientific 82-degrees, Baader Hyperions, or another set of eyepieces in the 16-20mm, 11-14mm, and 6-10mm ranges of focal lengths. You may also want a Barlow lens, which can obviate the need for a short focal length eyepiece as well as be used for planetary imaging purposes with a suitable video camera. A UHC nebula filter such as the Orion UltraBlock is also a must-have for improving contrast on emission nebulae like Orion or the Horsehead Nebula with any telescope, including the C11 XLT.

Lastly, a dew shield for the C11 XLT will protect the corrector from fogging up due to condensation, which can also cause damage to the StarBright XLT coatings if it contains trace acids from pollen, sap or industrial runoff. The dew shield also acts like a lens hood and keeps glare from sources of direct stray light like the Moon, equipment or street lights from bouncing around the inside of the optical tube.

Astrophotography Capabilities

With a heavy-duty equatorial mount and f/6.3 focal reducer, the C11 XLT is usable for deep-sky imaging, and the C11’s HyperStar compatibility allows you to shoot at an ultra-fast f/2 focal ratio and 560mm focal length. The EdgeHD version is superior for deep-sky imaging to the regular C11 thanks to its flatter field and mirror locks, however.

The C11 XLT is ideal for planetary imagers thanks to its large aperture and long focal length. Coupled with a good 2x Barlow lens and a high-speed planetary camera like the ZWO ZWO ASI224MC, you can take awe-inspiring images of the Moon and planets. Many of the best planetary imagers use C11 XLT optical tubes to take award-winning shots.

What can you see?

he Celestron C11 XLT is an ideal telescope for deep-sky viewing, providing comparable views to those of a 10-12” Dobsonian but with a much narrower field of view due to its long focal length

Even in light-polluted areas, you can make out remarkable details in open star clusters such as M35, M11, M46, and M38. With the C11 XLT’s impressive light-gathering power, globular star clusters like M3 and M22 appear with lots of detail and resolve into individual stars, while features like the dust lanes in M13 and the tight core of M15 are easy to see. Planetary nebulae such as the Cat’s Eye, Blue Snowball, Blinking Planetary Nebulae show blue, turquoise, and emerald-green colors, and the larger ones like the Helix and Dumbbell (M27) look huge when viewed through this telescope, practically filling the field of view. When observing from light-polluted areas, emission nebulae like the Orion Nebula (M42) and Lagoon (M8) can still be seen; using a UHC filter yields even better results, but the best views are found with a filter and dark skies. Galaxies will require dark skies to reveal more than fuzzy smudges, but under dark skies they reveal tons of detail, with clusters like the Virgo Cluster containing hundreds of individual objects and features such as spiral arms and dust lanes visible in many of the brightest galaxies such as those from the Messier Catalog or Herschel 400. You can also split thousands of double stars regardless of your viewing conditions, many of which are made more colorful by the C11 XLT’s large aperture.

If you’re looking to get stunning views or take gorgeous images of the Moon and planets, then the C11 XLT is an ideal telescope for you. Its 2800mm focal length makes it easy to achieve very high magnifications without needing to use a Barlow lens or choose from a limited selection of specialty short focal length eyepieces. You’ll be able to see all kinds of features on the Moon, such as craters, mountains, and ridges. The phases of Mercury and Venus are also, of course, visible. When Mars is closest to Earth, you can see its various dark surface markings and polar ice caps. Jupiter’s cloud belts and Great Red Spot are also visible through the C11 XLT, as are its four Galilean moons, which show slight variations in color and brightness that correspond to broad geological features. Saturn’s Cassini Division and possibly even the Encke gap in its rings can be seen on steady nights at high magnification. Uranus is resolved on a night of good seeing with the C11 XLT as a tiny teal-blue dot with hints of slight detail in its atmosphere if conditions are favorable, as well as up to four of its moons if you’re lucky! Neptune appears as a tiny blue disk, with Triton showing up next to it. Even Pluto can be made out with this telescope by discerning viewers as a star-like point of light.