The C11 XLT Optical Tube
The C11 Schmidt-Cassegrain optical tube has been a staple of Celestron’s Schmidt-Cassegrain telescope lineup since the 1980s, having been originally introduced to fill the gap between the portable C8 and the massive observatory-class C14 and updated into its present-day XLT configuration with improved StarBright XLT coatings and compatibility with the Starizona HyperStar system. Like most other SCTs, the C11 features a spherical 11” (280mm) f/2 primary mirror, a Schmidt corrector plate, and a 5x amplifying convex secondary mirror, resulting in a system focal length of 2800mm and a focal ratio of f/10. The optics in most C11s are very good, and these scopes are ideal for planetary observers and imagers who don’t want to deal with a Dobsonian or a monster C14 optical tube. The back of the C11 XLT has massive 3.25” rear threads for use with certain add-on focusers and accessories, though you get a step-down adapter for standard 2” SCT threads provided with the telescope to attach accessories such as a visual back, star diagonal, focal reducer, or camera adapter.
Collimating the Celestron C11 XLT is an easy process, requiring only the adjustment of three screws to tip and tilt the secondary mirror. Fortunately, this is an infrequently required procedure, and our collimation guide provides more information. The focusing system used in the C11 XLT is the standard moving-mirror internal system of most Schmidt-Cassegrains, with the primary mirror moving along a threaded rod. The mirror can rock on its support either during focusing or over time spans – the former causing “image shift” which can wreak havoc on planetary imaging, and the latter, mirror flop, affecting long-exposure images, which you won’t be doing with the CGEM II mount and an issue which is solved by the C11 EdgeHD model anyway. If you are concerned about image shift affecting your planetary astrophotography, it’s easy to add an aftermarket Crayford focuser to the back of the C11 to adjust focus without moving the primary mirror at all.
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 Celestron CGEM II supplied with the CGEM 1100 package. 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 to put the telescope in a plastic bin lined with blankets or cushions.
Accessories
The CGEM II 1100 package includes a 9×50 finder scope for aiming, which provides an upside-down field of view around 5 degrees across and with non-illuminated crosshairs. It is acceptable, if a little uncomfortable, to use it for extended periods. Since all you are using it for is to align the GoTo mount anyway, this is not a concern, and the bracket is solid enough to maintain precise alignment with the C11 XLT when transported.
You also get a 1.25” visual back and 1.25” prism star diagonal with the CGEM II 1100 and a 40mm E-Lux Plossl yielding 70x magnification, with its 43-degree apparent field of view translating to a 0.6-degree true field with the C11 XLT, or slightly larger than the full Moon in the sky. It is okay, but the narrow field of view is claustrophobic, and 70x is a bit low for planetary viewing, while the narrow true field is sub-par for deep-sky objects. Additional eyepieces are necessary to achieve a wider field of view with the C11 as well as higher magnifications, and you’ll certainly want a 2” star diagonal too.
The CGEM II Equatorial Mount
The Celestron CGEM II is a mid-range equatorial mount, falling between the Advanced VX and CGX in terms of price and performance. While it shares many of the same mechanics and shortcomings as the Advanced VX, such as its tripod, counterweight shaft, and servo motors, the CGEM II has a higher weight capacity of 40 lbs vs. the VX’s 30 lbs, with about half of that payload being suitable for imaging. The C11 XLT at 28 lbs bare is well in excess of the maximum ~20lb payload the CGEM II can handle for deep-sky imaging, and even with a Starizona HyperStar at 660mm focal length, that would be simply too much for the mount to bear.
Although it is an acceptable option for deep-sky astrophotography with smaller and lighter-weight telescopes, we do not recommend the CGEM II for dedicated imaging tasks compared to alternatives due to the CGEM II’s use of low-quality servo motors and its complicated PC interface, which requires connecting the hand controller to a USB adapter rather than the mount directly to your computer. In lieu of the CGEM II, we suggest the Sky-Watcher EQ6Ri Pro, the Celestron CGX, or the Losmandy G11 for deep-sky imaging, any of which could also hold the C11 XLT if purchased separately. Our review of the Celestron CGEM II mount elaborates on these details.
The CGEM II’s dual dovetail saddle can accept Vixen, Losmandy, and Celestron CGE-style dovetail plates. The CGE-style and Losmandy-style plates are essentially identical in size; the C11 attaches to the CGEM II with a CGE plate. The scope comes with two 17 lb counterweights for balance, which are more than sufficient with even the heaviest accessories attached to the C11, though other counterweights will fit the CGEM II’s standard-sized ¾” counterweight shaft.
The CGEM II is easy to set up for visual use with the C11 if you are able to safely install both counterweights and hoist the optical tube to above shoulder height. After balancing the optical tube by sliding it in the mount’s dovetail saddle along the declination axis, adjusting the counterweights, and leveling the tripod, you can align the mount using either a polar scope or the All-Star Polar Alignment method. With the NexStar+ hand controller or a compatible WiFi adapter/smart device, you can quickly and accurately align the mount on the night sky. Simply center several alignment stars in the eyepiece and confirm them to complete the alignment process. Once this is done, you can use the 40,000 object catalog to automatically slew to and track any celestial object of your choice.
Should I buy a Used Celestron CGEM II 1100 SCT?
If you’re looking for a bargain, consider purchasing a used CGEM II or CGEM 1100 package. While older CGEM models are similar to the present-day CGEM II, they don’t include the dual dovetail and some software improvements included in the CGEM II, though neither matters much if you just want to use the C11. Before making a purchase, make sure the mount powers on and operates smoothly and that the C11 XLT telescope is free of corrosion or damage to the mirrors and corrector plate. If any of these components are damaged, it will require replacing the whole telescope or optical set. If any accessories are missing, such as the hand controller or counterweights, they can be replaced but should result in a discounted price.
Alternative Recommendations
The Celestron CGEM II 1100 SCT is expensive for the capabilities it offers, and you may want to consider an equal or larger Dobsonian telescope or another SCT/mount configuration instead. If you are planning on doing deep-sky astrophotography, a separate, smaller optical tube and mount (preferably not the CGEM II) will be needed, and there are many options to choose from. Here are a few picks for visual astronomy and planetary imaging that can duplicate or exceed the CGEM II 1100’s functions:
Under $2000
- The Apertura AD10/Zhumell Z10/Orion SkyLine 10 is much cheaper and less complicated to set up than the CGEM II 1100 and features a wide field of view along with a nice included accessory bundle.
- The Sky-Watcher 10” GoTo Collapsible Dobsonian is perfect for those looking for something more portable, as the collapsible tube makes it easier to transport. Plus, it can be controlled via your smartphone/tablet and also features Sky-Watcher’s FreedomFind technology to allow for manual aiming if the telescope’s electronics are in use, without any interference with its GoTo functionality.
- The Sky-Watcher 12” Collapsible Dobsonian is fairly compact when disassembled thanks to its collapsible tube and is available in both manual and GoTo versions, with the latter offering the same smartphone operability and FreedomFind encoders as the 10” Collapsible model.
$2000+
- The Sky-Watcher 14” GoTo Collapsible Dobsonian offers more aperture than the CGEM II 1100 but is similar in overall weight and complexity to set up. As with the smaller GoTo Collapsible Dobsonians offered by Sky-Watcher, you can easily use the telescope manually or with its GoTo/tracking controlled by your smartphone or tablet.
- The Celestron Advanced VX 9.25” SCT sacrifices relatively little aperture compared to the C11 XLT, but is considerably cheaper and far less heavy, making it a much smaller commitment to purchase or use compared to the CGEM 1100.
- The Celestron CPC 1100 GPS uses the same C11 XLT optical tube as the CGEM II 1100, but is a bit cheaper and less complex to assemble thanks to its alt-azimuth GoTo mount. However, the fork mount is not removable from the C11 optical tube and the combined package is heavy and awkward to carry around or set atop its tripod.
Aftermarket Accessory Recommendations
A 2” star diagonal and eyepieces are usually the preferred choice for achieving a wider field of view with a large Schmidt-Cassegrain like the C11 XLT. Apertura’s 38mm SWA (74x) and 2″ dielectric screw-on diagonal are an ideal combination for achieving the maximum possible field of view with this telescope. Additionally, a good UHC nebula filter, such as the Orion UltraBlock, will crank up the contrast of nebulae at the eyepiece with the C11 XLT under almost any conditions.
The C11 XLT can handle up to 500x magnification on a night of extremely steady seeing, though typical conditions will limit you to around 200-300x even on a good night. You’ll want eyepieces to achieve higher magnifications regardless, however, as the best planetary views are almost always found at above 100x. The Baader Hyperion 21mm (133x) or Explore Scientific 82-degree 18mm (155x) provides a good medium power, while an Explore Scientific 14mm 82-degree (200x) or Baader 13mm Hyperion (215x) provides the next step up with the C11 XLT. For the highest typical magnifications you could possibly use, an Explore Scientific 8.5mm 82-degree (329x) and/or 6.5mm 82-degree (430x) would be ideal. These are just suggestions, however; the f/10 focal ratio of the C11 is very forgiving of eyepiece designs, and you could use a Barlow lens in place of higher power oculars, which would also come in handy for planetary imaging with the C11 XLT.
As with any Schmidt-Cassegrain telescope, we highly recommend a dew shield for the C11 XLT. This will help to prevent condensation from forming on the Schmidt corrector plate, which will not only ruin your view but can potentially damage the StarBright XLT coatings or the glass itself. A dew shield also helps to reduce stray light entering the telescope, resulting in improved contrast and better protection of the corrector plate from fingerprints or other careless touching.
A polar scope or PoleMaster is essential for accurately achieving polar alignment with the CGEM II in lieu of the All-Star Polar Align function in the mount. You’ll also need either an AC adapter or a rechargeable battery like the Celestron PowerTank Lithium Pro to run the mount.
What can you see?
The C11 XLT optical tube is a powerful telescope for deep-sky viewing. Even in light-polluted skies, you can still make out open star clusters like M35, M11, M46, and M38 with the C11’s impressive light-gathering power. Globular star clusters like M3 and M22 can be seen in incredible detail, revealing features like M13’s dust lanes and M15’s tight core. Planetary nebulae are also a delight to observe with the C11, with the Cat’s Eye, Blue Snowball, and Blinking Planetary Nebula showing off their vibrant colors. The Ring (M57) and the Dumbbell (M27) appear gigantic at the high powers the C11 XLT is boxed into.
Even in brightly light-polluted skies, the bright Orion Nebula (M42) and Lagoon (M8) are visible with the C11 XLT, though the best views are always going to be with dark skies and a UHC filter. Galaxies, while virtually invisible under light-polluted conditions, explode with detail in the C11 XLT; hundreds are visible in clusters like those found in Virgo and Coma Berenices, dust lanes in galaxies like M31, M64, and M82 are obvious, and you can even resolve the spiral arms of M51 under good conditions. The C11 XLT can also split thousands of double stars, which the CGEM II can easily find for you.
The C11 XLT is an excellent telescope for anyone wanting to explore the Moon and planets. At f/10, you don’t need fancy eyepieces or extremely short focal lengths to get sharp, high-power views. The Moon’s craters, mountains, and ridges can be seen in all of their glory. Mercury and Venus’ phases are easily resolved, along with Mars’ polar ice caps and numerous dark markings on its surface. Jupiter’s cloud belts and the Great Red Spot can be easily seen, as well as its four Galilean moons. During transits, the small disks and shadows of the moons can be observed, along with hints of detail on Io, Ganymede, and Callisto showing up as slight variations in brightness across the surfaces of these moons.
You can resolve the Cassini Division in Saturn’s rings with the C11 XLT, along with possibly the Encke gap on a very good night. Saturn’s cloud bands and a few moons are also easy to see. Uranus’ tiny disk can show hints of detail with the C11 XLT under very good conditions, and up to 4 of its moons are visible alongside it if you can pick them out from the planet’s glare. Distant Neptune is difficult to resolve clearly on a bad night but its blue disk can be seen on a steady one, while Triton is fairly obvious next to the ice giant and Pluto can be seen with the C11 under dark skies as a star-like point, albeit barely.
Astrophotography Capabilities
The CGEM II is nowhere near sufficient in tracking accuracy or weight capacity to be used with the C11 XLT with or without a Starizona HyperStar f/2 conversion or f/6.3 reducer. The C11 XLT is also inferior to the EdgeHD 1100 for such tasks. If you were to do deep-sky astrophotography with a C11, you’d want the EdgeHD version and a mount like the Celestron CGX-L, the Sky-Watcher CQ350, or the EQ8R.
The C11 XLT is excellent for planetary imaging atop almost any mount. By pairing it with a 2x to 3x Barlow lens, its focal length can be boosted to between 5600mm and 8400mm, allowing for optimal image scale. A high-speed planetary video camera such as the ZWO ASI224MC, coupled to a laptop, and the aforementioned Barlow lens are all you need. With this setup and good viewing conditions, you can achieve spectacular results. Many world-class lunar and planetary imagers use C11 XLT or C14 XLT telescopes to take their images.
