The C11 XLT Optical Performance
For over three decades, Celestron’s C11 Schmidt-Cassegrain (SCT) optical tube has been a mainstay in the brand’s line-up of telescopes, originally sitting between the C8 and C14, though now accompanied by the slightly smaller C9.25 as well. The C11 XLT is its latest version, which has improved StarBright XLT coatings for superior performance and compatibility with the Starizona HyperStar system.
The C11’s 11″ (280mm) diameter primary mirror, with an f/2 focal ratio, has its focal length amplified by a convex secondary mirror. This results in the whole telescope having a focal length of 2800mm and a focal ratio of f/10.
The optics in the C11 XLT, as I’ve observed, are excellent. For high-resolution viewing or planetary imaging, it is as good as, if not better than, most 10-12” mass-market Dobsonians that I’ve used.
Collimating the C11 XLT requires adjusting the screws at the front of the secondary mirror while pointed at a bright star, which is only needed occasionally but can prove to be quite complicated; our collimation guide explains the process in more detail.
The C11 is so huge, heavy, and fragile that I guess most of you would be worried about transporting it without damage. The easiest solution I know is to either wrap it in a towel and place it upright, buckled into a passenger seat with the corrector facing down, or by putting the telescope in a plastic bin lined with blankets or cushions.
The Focusing System and Its Issues
The focusing system of the C11 XLT, as with most catadioptric telescopes such as the SCTs offered by Celestron, uses a knob to move the primary mirror along a threaded rod inside the telescope, adjusting the focal plane’s position without moving any of the hardware attached to the telescope.
The primary mirror can wobble somewhat while focusing, causing “image shift.” What I’ve realized is that the image shift issue is kept at bay for the most part in the C11 XLT, and what’s more prominent are the wobbles of the Advanced VX mount.
This focusing system can also suffer from “mirror flop” during long exposure astrophotography due to the weight of the C11’s primary mirror, which causes blurring of the images. But it was hardly a concern for me since I wasn’t going to be doing deep-sky astrophotography with the C11 XLT on the Advanced VX mount. If you’re, there is a C11 EdgeHD model, which is designed for imaging and equipped with mirror locks to solve the issue of mirror flop.
Fittings To and On the Tube
The C11 XLT has a 3.25” thread on the rear of the optical tube, which can be utilized for certain add-on focusers and accessories, though it comes with a step-down adapter for standard 2” SCT threads to attach accessories such as a visual back, star diagonal, focal reducer, or camera adapter.
The C11 XLT optical tube has a standard CGE-style dovetail plate attached to the bottom, which will fit any mount that accepts Losmandy-style or CGE-style plates, such as the Advanced VX (which only actually fits the latter). There are holes drilled on top of the tube to attach a second dovetail bar should the need arise, and a carry handle is built into the back of the C11 to make it easier to carry.
My Take On The Accessories Set
The Celestron Advanced VX C11 SCT package comes with a 9×50 finder scope for aligning the mount on the night sky, which provides an upside-down field of view of approximately 5 degrees with crosshairs for exact pointing.
I think the finder’s optics are decent enough. The finder bracket is solid and holds its alignment with the C11 XLT well even when I transport it, which I believe is a crucial feature with a super long 2800mm focal length instrument.
A standard 1.25” visual back, 1.25” prism star diagonal, and 40mm E-Lux Plossl eyepiece are provided.
The 40mm E-Lux eyepiece yields 70x magnification with a 43-degree apparent field of view, translating to a 0.6-degree true field with the C11 XLT, slightly larger than the full Moon in the sky. Although I take this to be okay for basic use, it is not ideal due to its narrow field of view. To achieve a wider field of view, a 2-inch star diagonal and 2″ eyepieces will be needed, along with additional eyepieces for higher magnifications.
The Advanced VX Equatorial Mount
The Celestron Advanced VX mount is an equatorial GoTo mount that is solid and reasonably priced. But when equipped with the heavy C11 XLT optical tube, it is at its absolute limit even for visual astronomy.
The Advanced VX isn’t the best choice for a dedicated astrophotography mount, even if equipped with a smaller optical tube more suited for astrophotography. I’d prefer other mount options, like the Sky-Watcher HEQ5 Pro, for imaging purposes.
The Advanced VX’s weight capacity limit for visual observation purposes is 30 pounds, though I’ve seen older versions claiming a 35-pound capacity. The C11 XLT optical tube alone weighs 27.5 lbs without the addition of the finder, star diagonal, or eyepieces, which easily adds a few extra pounds.
The biggest concern with the C11 on the Advanced VX is not stability (though it’s certainly less than ideal) but damage to the mount. The cheap servo motors in the Advanced VX are not only responsible for its mediocre astrophotography performance but also make overloading the mount a bad idea. If too much strain is put on the motors, which can easily happen with the C11 XLT, I run the risk of over-torquing the motors and causing them to burn out. It seems to be that balancing the C11 perfectly on the mount is key to avoiding damage to the motors or a shaky setup.
The Advanced VX has a dual Vixen/CGE-style dovetail saddle to hold the C11 XLT’s CGE-style dovetail rail. However, it is not compatible with Losmandy-style dovetail plates without the addition of an aftermarket dual dovetail saddle from a third party.
Setting up the Advanced VX Mount
Setting up the Advanced VX for a night of observing with the C11 XLT is fairly straightforward. It takes some planning to safely install all three counterweights for balance, position them as best I can, and then attach the optical tube.
I then need to polar-align the mount. To polar align the Advanced VX, we can use either a polar scope or the All-Star Polar Alignment method.
After this, I align the GoTo mechanism. With the NexStar+ hand controller or a compatible WiFi adapter/smart device, alignment on the night sky is fast and accurate, and the mount prompts me to center several alignment stars in my eyepiece and confirm their location as reference points.
After alignment is finished, the mount will slew to and track any objects in its database.
Should I buy a Used Celestron Advanced VX 11″ SCT?
A used Celestron Advanced VX 11” SCT could be troublesome if the previous owner didn’t take care of it. Provided the mount and telescope are in working order, however, there’s no reason not to buy one.
You could always pair the C11 XLT with a heavier duty mount and the Advanced VX with a smaller telescope if you want, too.
Check to make sure that the C11’s optics are free of corrosion or damage, which would require replacing the whole optical set, as well as making sure that the Advanced VX runs smoothly. Thankfully, parts such as the finder, visual back/diagonal, and hand controller are easily replaced.
Alternative Recommendations
The undermounted nature of the Advanced VX 11” SCT means that I would probably recommend another telescope instead, such as a Dobsonian of similar size and capabilities, or a different mount packaged with the C11 XLT:
Under $2000
- The Apertura AD10 is cheaper, lighter, and less complex than the C11 XLT/Advanced VX as well as more stable, with a wide variety of accessories included by default. The Apertura AD12 offers slightly more light gathering ability.
- The Sky-Watcher 10” GoTo Collapsible Dobsonian is a bit more portable than a solid-tubed scope, thanks to its collapsible tube, while its smartphone/tablet control and FreedomFind technology offer the convenience of automated GoTo tracking without sacrificing manual aiming capability.
- The Sky-Watcher 12” Collapsible Dobsonian is available in both manual and GoTo variants, providing great flexibility in operation with the same collapsible tube design Sky-Watcher is known for and the FreedomFind encoders of the 10” model in its GoTo configuration.
$2000+
- The Sky-Watcher 14” GoTo Collapsible Dobsonian offers more aperture than the C11 XLT but isn’t significantly more bulky or hard to set up. It also allows for both automated and manual operation via your smartphone or tablet, with the FreedomFind encoders allowing for manual adjustment of the telescope’s position without disrupting the automated tracking functions.
- The Celestron Advanced VX 9.25” SCT is far more stable and lightweight than the C11 XLT but with only slightly worse performance on most targets.
- The Celestron CGEM II 1100 is significantly steadier than the Advanced VX 11” SCT but is identical in form and function apart from the slightly heavier CGEM mount head.
- The Celestron CPC 1100 GPS is based on the same C11 XLT optical tube as the CGEM II and Advanced VX C11 combinations, but is a bit simpler than an equatorial mount to assemble and set up. However, the combined tube/fork mount combination is heavy and awkward, as well as unable to be broken up for transport.
What can you see?
Despite the limitations of the Advanced VX mount, the C11 XLT is an ideal choice for anyone looking to do deep-sky viewing with comparable views to a 10-12” Dobsonian, albeit with a much narrower field of view.
Even in light-polluted environments, I’m able to make out open star clusters such as M35, M11, M46, and M38 with the C11’s impressive light-gathering power, though the largest ones do not fit in the C11’s narrow field of view.
Globular star clusters like M3 and M22 appear with plenty of detail in the eyepiece and are easily resolved into individual stars; features such as M13’s dust lanes and M15’s tight core are easily visible. The Cat’s Eye, Blue Snowball, and Blinking Planetary Nebulae also show off their vibrant colors.
The larger planetary nebulae, such as the Helix, Ring (M57), and Dumbbell (M27), look gigantic compared to the aforementioned objects in the C11 XLT. The Orion Nebula (M42) and Lagoon (M8) are also visible in brightly light-polluted areas, although using a UHC filter yields even better results.
Galaxies are practically invisible, or at least devoid of detail, under severely light-polluted skies. However, they explode with detail in the C11 XLT under darker viewing conditions. Hundreds of galaxies can be viewed in Virgo and Coma Berenices clusters, and the dust lanes in many of them, such as M82 or M104, are obvious; I can even resolve the spiral arms of M51 under good conditions. Thousands of double stars can also be split with this powerful telescope—something that’s made easy thanks to the GoTo system included in its Advanced VX mount.
The C11 XLT is a fantastic telescope for anyone looking to get fabulous views or images of the Moon and planets. The 2800mm focal length of the C11 makes it easy to achieve high magnifications without having to choose from a limited selection of short focal length eyepieces, and at f/10, you don’t need fancy, well-corrected optical designs for your oculars either.
- Through the C11 XLT, I can see plenty of tiny details on the Moon such as craters, mountains, and ridges, as well as Venus and Mercury’s phases.
- Mars exhibits many dark markings on its surface, as well as polar ice caps, though the best views are obtained when the planet is closest to Earth for a handful of weeks out of every biannual opposition.
- Jupiter’s cloud belts and the Great Red Spot can be easily seen with the C11 XLT, along with its four Galilean moons. During transits, their small disks and shadows can be identified, with slight variations in color and brightness visible on the disks of the moons, which correspond to broad geological features.
- Saturn’s rings easily reveal the Cassini Division along with possibly the Encke gap in the rings on an exceptionally steady night at very high magnification. Saturn itself shows atmospheric cloud bands like those of Jupiter, and a few moons are visible around it.
- Uranus is clearly resolved as a tiny turquoise orb with hints of atmospheric cloud detail visible under extraordinary conditions, and up to four of its moons are revealed by the C11 if I can manage to see them through Uranus’ glare.
- Neptune is a featureless, tiny blue disk, but its moon, Triton, is obviously apparent next to it.
- The C11 XLT is also capable of collecting enough light to reveal Pluto to the discerning observer, though it appears as a star-like point and will soon be out of reach entirely without a larger instrument as it fades and grows more distant from the Sun.
Astrophotography Capabilities
The Advanced VX mount is far from my favorite for deep-sky astrophotography, even with the smallest optical tubes, and the C11 XLT of course exceeds its weight capabilities even if autoguided and used at f/2 with a HyperStar for short exposures.
You’d need a CGX or even larger mount to even contemplate deep-sky astrophotography with the C11, and we’d recommend the EdgeHD version of the C11 for the job as well.
However, the Advanced VX is, for the most part, stable enough to carry the C11 XLT for planetary imaging tasks, and the telescope is certainly ideal for it.
When paired with a 2x to 3x Barlow lens, I can boost the C11’s focal length to between 5600mm and 8400mm for optimal sampling and image scale. With the Barlow lens, the ZWO ASI224MC or a similar high-quality planetary video camera connected to a laptop is all I need to take some breathtaking images.
