Celestron C6 XLT Review: Recommended OTA

The C6 is a 6” (150mm) f/10 Schmidt-Cassegrain with a focal length of 1500mm. Like all Celestron Schmidt-Cassegrains, the C6 XLT uses a spherical primary mirror, an aspherized convex secondary mirror, and a Schmidt corrector plate to focus sharp images and provide a long focal length in a compact package due to the magnifying nature of the convex secondary mirror. The C6 XLT has always been sold with Celestron’s StarBright XLT coatings, giving it slightly higher transmission than scopes made with older, non-XLT StarBright coatings or uncoated Schmidt corrector plates. A new C6 XLT has only slightly less light-gathering ability-at least on paper-than an uncoated, barebones C8 from the 1970s (though the old C8 will win in resolving power).

Compared to the older, smaller C5, the C6 XLT has more than 50% more light gathering ability and about 25% more resolving power. The C5’s large central obstruction of around 40%, compared to the C6 at around 35%, leads to lower contrast and performance than specs would indicate, and tends to cost about the same as the C6. The C6 XLT is marginally heavier and bulkier than a C5, is more widely available, and is much more capable—so there’s really no reason to get a C5.

Optically, the C6 XLT tends to be quite good—much better than a typical C5 or C8, and on par with the C9.25. This is probably a combination of the relative newness of the tooling used to produce the Schmidt corrector plates and the care that goes into their manufacture. Thanks to the relatively thin corrector, the C6 tends to cool down to ambient temperatures fairly rapidly compared to a larger SCT or similar-sized Maksutov-Cassegrain, albeit more slowly than most 6” Newtonian reflectors.

The C6’s baffle tube diameter limits the normal field of view possible with eyepieces, focal reducers, and/or cameras to a ~35mm image circle, which corresponds to about 1.3 degrees across. Using a reducer doesn’t change this-you’re still limited to the same usable field of view size before vignetting becomes a problem.

Mechanics

Like most Schmidt-Cassegrains, the C6 XLT focuses by moving the primary mirror back and forth along a sliding rod inside the tube. Large SCTs tend to suffer from “image shift” or “mirror flop” as the mirror wobbles during focusing or slowly slides during a long photographic exposure. The C6 XLT is, thankfully, largely immune to this problem due to the small size and consequently low mass of its primary mirror and internal parts.

The C6 XLT can be collimated by adjusting a set of three screws on the secondary mirror. You will likely seldom need to do this – check out our collimation guide to learn how. Don’t be tempted to put thumb screws on the secondary mirror; this will just lead to it going out of collimation more frequently, or even while the telescope is in use.

Like Celestron’s larger SCTs, the C6 XLT is compatible with Starizona’s HyperStar system. By removing the secondary mirror and installing the HyperStar, the C6 can be used as an f/2 photographic system with a mere 300mm focal length. The C6 XLT Hyperstar lens system costs nearly as much as the scope itself, however, and you’re limited to smaller astronomical-only cameras, but this combination can still produce spectacular results.

Attaching accessories to the back of the C6 XLT is done with standard threaded accessories which fit on the back of the C6’s rear port. You can attach focal reducers, camera adapters, or, of course, a visual back and star diagonal like the ones provided. The C6 XLT can fit a 2” diagonal, but the internal baffle tube causes vignetting with anything with more than a ~35mm field stop, so not all 2” wide-angle eyepieces will actually work. Something like a 34/35mm “SuperView” or Panoptic will vignette, and is arguably overkill. An f/6.3 focal reducer will allow you to achieve a similar visual field to the maximum possible with 2” accessories, but using only 1.25” low-power eyepieces. It is also quite useful for imaging.

To attach to a mount, the C6 XLT uses a standard Vixen-style dovetail rail bolted to the bottom of the optical tube.

Accessories

The C6 XLT is provided with a 1.25” visual back, a 1.25” prism star diagonal, a 1.25″, a 25mm Plossl eyepiece providing 60x magnification, and a 6×30 magnifying finderscope. The included prism star diagonal is well-made, and there’s little point in upgrading from it. 

The included 25mm Plossl ocular is Celestron’s standard “E-Lux” Plossl with an apparent field of view of around 55 degrees, and makes for a great low-power eyepiece for the C6 XLT with or without an f/6.3 focal reducer. As with any telescope, you’ll want to expand your collection to at least a few eyepieces for a variety of different magnifications for viewing different objects under different conditions.

The included 6×30 finder scope is not much to get excited about, showing stars moderately fainter than what’s visible with the naked eye and providing an upside-down field of view of about 7 degrees, which is equivalent to that provided by a pair of typical 7×35 or 7×50 binoculars, albeit much dimmer. You could replace it with a red dot or 9×50 if you wish, but given the typical things you’re likely to be looking at with the C6 XLT-or the tendency to use it on a GoTo mount-there’s little point in worrying too much as you’ll seldom use the finder anyway. 

Mount Recommendations

The C6 XLT is only about 8 lbs, so a lot of smaller mounts can hold it. The C6 is sold bundled with Celestron’s NexStar SE mount and Evolution mount, both of which are great if you want a GoTo alt-azimuth system. Alternatively, the Sky-Watcher AZ-GTi is a good fit. A GoTo alt-azimuth mount will allow you to do planetary imaging with the C6 XLT, and short exposures with HyperStar and a suitable camera are also possible. 

If you want an all-manual mount, the Explore Scientific Twilight I or Celestron CG-4 would be our pick. 

For imaging, a mount like Celestron’s Advanced VX or the Sky-Watcher HEQ5 is appropriate for holding the C6 XLT, your camera, and an autoguiding setup. The C6 is not our first choice for a beginner deep-sky rig, but it can still do a great job and is more flexible than a 6” Ritchey-Chretien or Newtonian.

Should I buy a Used Celestron C6 XLT?

A used C6 XLT is a great scope. If the corrector plate is cracked, however, there is no repairing the telescope without replacing the entirety of the optics due to the “matched” optics of every individual Schmidt-Cassegrain Celestron produces. Otherwise, there’s little to go wrong provided the optical coatings are in good shape—both on the mirrors and corrector lens. Fungus can pop up on the correctors and may or may not be able to be removed without harm. The mirrors can be recoated with fresh aluminum if they have damage, but removing them from their holders and re-installing them is next to impossible without professional help, and Celestron charges too much for recoating themselves to make such an operation worthwhile.

Aftermarket Accessory Recommendations

The C6 XLT really needs a dew shield to work well. Some people opt for a simple dew heater ring to keep condensation off the corrector plate, but a dew shield has the additional benefit of keeping curious hands and insects off the corrector, and shields the inside of the telescope from off-axis starlight, which can cause glare, helps reduce general sky glow to increase contrast, and helps mitigate contrast loss from severe light pollution, moonlight, or nearby direct lighting. Think of it as sort of a lens shade for the telescope. Making a dew shield is fairly easy to do yourself, but there are a variety of commercial options, ranging from Celestron’s basic plastic dew shield to AstroZap’s heated dew shield and Celestron’s bespoke heated metal dew shield.

Eyepiece selection for the C6 is a matter of personal preference, but at the very least you’ll want eyepieces of around 15mm (100x), 9-10mm (166x/150x respectively), and 6mm (250x) for a good range of magnifications—the C6 tops out at around 300x in usable magnification on a good night when viewing bright targets. The “Goldline” eyepieces are ideal for this if you’re on a budget. Or you could get a zoom eyepiece, which works well at the C6’s slow f/10 focal ratio. For very low power, a 32mm Plossl or 20-28mm wide-angle eyepiece is a good idea if you don’t plan on using a focal reducer; the 25mm Plossl provides a pretty wide field with an f/6.3 reducer.

As previously mentioned, whether you’re viewing, imaging, or both, Celestron’s f/6.3 focal reducer is a great idea to purchase with the C6. You screw it on ahead of your eyepiece/camera and it reduces the scope’s focal length while preserving a sharp, flat field. 

Lunar and planetary imaging with the C6 XLT is best with a high-speed, high frame rate, and high resolution astronomical CMOS video camera like the ZWO ASI224MC or ASI120MC, which you use to shoot a few minutes of video with tools like Sharpcap or Firecapture and then stack, process, and sharpen with software like Registax or AutoStakkert. These cameras are also great for autoguiding a deep-sky astrophotography rig. However, for optimal image scale, you really need a massive focal length of at least 2000mm; the usual recommendation is to aim for a focal ratio of f/20 to f/30, which requires a focal extender or Barlow lens to achieve. The Apertura 2.5x Barlow lens or Tele-Vue 2.5x Powermate brings the C6 XLT up to 3750mm in focal length and a focal ratio of f/25, perfect for these types of cameras. They can also be used visually, in conjunction with your eyepieces, of course.

Astrophotography Capabilities

The C6 XLT is an extremely versatile telescope—it’s great for visual astronomy, planetary imaging, and deep-sky imaging at three different focal lengths and f/ratios. For planetary imaging, the C6 XLT is best when coupled with a 2x or 3x Barlow lens and a high-speed video camera meant for the purpose, such as the ZWO ASI224MC. The C6’s small aperture means it’s less affected by mediocre seeing conditions than a larger instrument, but it can still resolve a lot of detail. 

As with any telescope, what you can image with the C6 XLT when it comes to deep-sky objects is influenced by your light pollution conditions, the quality of your astrophotography mount, and the quality of your camera. You can image with the C6 at f/2 and 300mm with the Starizona HyperStar and take wide-field shots of nebulae and star clusters, or switch it to a general-purpose instrument with Celestron’s f/6.3 reducer to get a focal length of 950mm. 950mm is enough focal length for galaxies and globular star clusters, but short enough that guiding and tracking don’t have to be absolutely perfect either. We wouldn’t particularly recommend shooting deep-sky images with the C6 at its native f/10 focal ratio, as you’ll need long sub-frames to get good results, and 1500mm is enough to be difficult for a mount such as an HEQ5 to keep up with, but it can be done. 

What can you see with the Celestron C6 XLT?

The C6 XLT has enough aperture to resolve globular clusters like M13 or M22 into individual stars under all but the worst city skies. You’ll be able to see pretty much all of the galaxies in the Messier catalog with the C6 XLT, and under dark skies, some begin to show detail. On a clear and dark night, the spiral arms in M51 and M33 begin to reveal themselves, along with the dust lanes in galaxies like Andromeda (M31), the Sombrero (M104), and the Cigar Galaxy (M82). With severe light pollution, you’re unlikely to see much of them at all, however. Under dark skies, you should have no trouble picking out thousands of elliptical galaxies accompanying their more familiar and aesthetically pleasing spiral neighbors in groups like the Virgo Cluster or the various galaxy groupings in Draco, Leo, Pegasus, Ursa Major, and Canes Venetaci. 

Open star clusters, provided they still fit in the C6’s field of view, look dazzling no matter your light pollution levels, and the Orion Nebula always proves to be spectacular. A few planetary nebulae are also big and bright enough to be seen with the C6 XLT, though resolving fine details or gathering enough light to show colors is beyond the capabilities of any 6” telescope.

The C6 XLT is, of course, great for viewing the Moon and planets too. Not only can you see the ice caps on Mars, but a few dark markings on the red planet are visible when it’s close to Earth at high magnification with the C6. You can see the phases of Mercury and Venus, and the Moon shows tens if not hundreds of thousands of interesting details, ranging from nation-sized craters to tiny ridges mere hundreds of meters wide, mountain ranges, and craterlets stacked atop one another. Jupiter’s moons are dazzlingly bright and reveal their disks when they transit in front of the gas giant, casting inky black shadows in their wake. Jupiter shows vivid cloud bands with all sorts of constantly-changing hues and structures, and you should be able to make out the Great Red Spot. Saturn’s rings are easy to see even at low power; with high magnification, the C6 XLT shows the Cassini Division within the rings, Saturn’s own cloud bands, and a handful of moons. Uranus is little more than a greenish dot and its moons are too dim to see with a 6” telescope, while Neptune appears smaller still, but the C6 XLT should be able to faintly reveal its only large moon, Triton.