The C11 XLT Optical Tube
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 the latest version, with improved StarBright XLT coatings for superior performance and compatibility with the Starizona HyperStar system. The C11’s 11″ (280mm) f/2 primary mirror is amplified in focal length by the convex secondary mirror, and the telescope as a whole has a resulting focal length of 2800mm and a focal ratio of f/10. The optics in the C11 XLT are excellent, and it is as good as, if not better than, most 10-12” mass-market Dobsonians for high-resolution viewing or planetary imaging.
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,” but it’s kept at bay for the most part in the C11 XLT, and the wobbles of the Advanced VX will be more prominent anyway. This system can suffer from “mirror flop” during long exposures due to the weight of the C11’s primary mirror, blurring images, but it’s hardly a concern since you won’t be doing deep-sky astrophotography with the C11 XLT on the Advanced VX mount, and it’s solved on the C11 EdgeHD model, which is designed for imaging and equipped with mirror locks. 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 XLT has a 3.25” rear thread, 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 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. 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. The C11 is so huge, heavy, and fragile that some may be worried about transporting it without damage; the easiest solution 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 C11’s provided CGE-style dovetail plate allows for easy attachment to any mount that accepts Losmandy-style or CGE-style plates, such as the Advanced VX (which only actually fits the latter). Additionally, the telescope is equipped with top-mounted holes to attach a second dovetail bar, as well as a convenient carry handle on the back.
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. The finder’s optics are decent enough; the bracket is solid and holds its alignment with the C11 XLT well when transported, 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 this is 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 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, though it is at its absolute limit when equipped with the heavy C11 XLT optical tube. The Advanced VX may not be the best choice for a dedicated astrophotography mount, even when equipped with a smaller telescope more suited for the task, and other options like the Sky-Watcher HEQ5 Pro may be preferable.
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. To balance the heavy C11 XLT, three 11 lb counterweights are provided. The Advanced VX’s weight capacity limit is 30 pounds, though older versions claimed a 35-pound capacity. The C11 XLT weighs 27.5 lbs without the addition of the finder, star diagonal, or eyepieces, which will easily add 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, you run the risk of over-torquing the motors and causing them to burn out. Balancing the C11 perfectly on the mount is key to avoiding damage to the motors or a shaky setup.
Setting up the Advanced VX for a night of observing with the C11 XLT is fairly straightforward. t takes some planning to safely install all three counterweights, position them as best you can, and then attach the optical tube. You’ll then need to polar-align the mount. To polar align the Advanced VX, you can use either a polar scope or the All-Star Polar Alignment method. After this, you can 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 will prompt you to center several alignment stars in your 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.
The undermounted nature of the Advanced VX 11” SCT means that we would probably recommend another telescope instead, such as a Dobsonian of similar size and capabilities, or a different mount packaged with the C11 XLT:
- The Apertura AD10/Zhumell Z10/Orion SkyLine 10 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/Zhumell Z12/Orion SkyLine 12 offers slightly more light gathering ability than the C11 as well as a much wider field of view for visual use
- 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.
- 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.
Aftermarket Accessory Recommendations
When using the C11 XLT or pretty much any Schmidt-Cassegrain telescope, a 2” star diagonal and eyepieces are ideal for achieving a wider field of view. To get the most from this telescope, we recommend Apertura’s 38mm SWA (74x) and 2″ dielectric screw-on diagonal. The 38mm SWA will allow you to get a true field of just under 1 degree at around the same magnification as the stock 40mm E-Lux Plossl. A UHC nebula filter like the Orion UltraBlock is also ideal for improving contrast when viewing nebulae such as the Orion Nebula or the Swan Nebula, be it from the suburbs or under dark skies.
For planetary viewing with the C11 XLT, as well as smaller deep-sky objects like globular clusters and planetary nebulae, magnifications of 100x or higher give the best results. Your primary limitations are atmospheric seeing and the stability of the Advanced VX mount, which can limit you to no more than 200-300x on many nights, though the C11 is nominally capable of supporting 500x magnification. A pair of eyepieces in the 16–22mm range and 11–14mm range such as Baader 21mm Hyperion (133x) or Explore Scientific 18mm 82-degree, along with the Explore Scientific 14mm 82-degree (200x) or Baader 13mm Hyperion (215x) would be ideal at a minimum, and if your conditions support it, a higher magnification eyepiece or two such as the Explore Scientific 8.5mm 82-degree (329x) or even the 6.5mm 82-degree (430x) are great choices as well. A wide variety of different eyepiece lines and optical designs can provide sharp images thanks to the C11 XLT’s forgiving f/10 focal ratio, so don’t just limit yourself to our suggestions. You could also get a Barlow lens to use with your eyepieces and/or a planetary camera for imaging.
We strongly recommend investing in a dew shield for the C11 XLT, like with any other Schmidt-Cassegrain telescope. This simple accessory will help protect the StarBright XLT coatings on the Schmidt corrector plate from moisture damage, as well as prevent a premature end to your observing session, while also improving contrast by blocking out any stray light.
To get the most out of your Advanced VX, you’ll also need to accurately polar align it with either a polar scope or PoleMaster unless you want to use the All-Star Polar Align function. Power from either a rechargeable battery such as the Celestron PowerTank Lithium Pro or an AC adapter is also required to run the mount. And while normally considered a waste of money, vibration suppression pads can make a big difference with the extremely top-heavy C11/Advanced VX setup, as long as you can make sure not to lose them when you pack up for the night.
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, you’ll be 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 will 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 will be easily visible. The Cat’s Eye, Blue Snowball, and Blinking Planetary Nebulae will 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; you 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, you 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 you 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.
The Advanced VX mount is far from our favorite for deep-sky astrophotography, even with the smallest telescopes, 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 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, you 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 you need to take some breathtaking images.