Schmidt-Cassegrain telescopes (SCT) were all the rage in the 1970s and 1980s when they were introduced, rivaled only by the Dobsonian telescope in popularity for both newcomers and experienced users. Today, Schmidt-Cassegrain telescopes, or SCTs, still dominate midsize amateur and research observatories and are commonly seen at astronomy clubs and star parties.
In today’s world, however, the SCT faces stiff competition and less variety than in years past. Gone is the rivalry between Meade and Celestron. Meade Instruments is no longer a reliable or consistent producer of Schmidt-Cassegrains and has been bought out by a competitor, Orion. Due to the high cost of setting up production and the availability of other optical designs that are arguably superior in many aspects, none of the other manufacturers besides Celestron and Meade offer SCTs.
Schmidt-Cassegrains use two mirrors—one concave and one convex—and a thin, unusually curved Schmidt corrector to achieve sharp images. This corrector may look like a flat window pane but is actually a precise, delicate, and hard-to-replace optical surface. SCT optics are made in “matched” sets. This means that the corrector is made on what is basically a die, and the shapes of the mirrors are slightly tweaked to make up for any flaws in the way the corrector was manufactured. As such, breaking one of the three optical surfaces means both of the others have to be replaced.
Most SCTs come in aluminum optical tubes and focus by moving the primary mirror back and forth to adjust the spacing between the mirrors, thus moving the focal plane by huge amounts while not actually moving the mirror very much physically. Almost all SCTs have standard threads at the back, to which accessories such as star diagonals, focal reducers, or camera adapters can be attached. Some users also attach an external Crayford focuser to an SCT to provide a slightly stiffer focusing mechanism than the internal one.
Before discussing the best Schmidt-Cassegrain telescopes, let’s list a number of key advantages of SCTs over other types of telescopes:
- Compactness: SCTs have very short and stubby tubes but very long focal lengths (typically the focal ratio is f/10, with a few f/8 and f/11 models). This makes them more portable than basically any non-collapsible reflector of the same aperture. However, the accompanying heavy-duty mount and tripod for an SCT may offset this difference in portability.
- Long focal ratio: The long focal ratio of SCTs also means they are very easy on cheap eyepieces compared to a fast refractor or reflector, with far less coma, edge-of-field astigmatism, and other problems.
- Sealed tubes: Because of their internal focusing mechanism and front corrector plate, SCTs are basically sealed internally if caps are kept on the scope. If the telescope is kept covered, the mirror coatings are at little risk of degradation, provided the instrument is not in an extremely polluted, damp, or dirty environment. The mirror coatings on the oldest Celestron SCTs are over 50 years old, and most will probably last another 50. Most reflectors seldom have their mirror coatings last this long.
- (Slightly) lower maintenance: Most SCTs hold collimation better than mass-manufactured reflectors, particularly if the stock collimation screws are kept installed and well-tightened. Collimation only requires adjusting the secondary mirror, as opposed to both the primary and secondary mirrors.
- No chromatic aberration: SCTs have basically no chromatic aberration because of their front corrector plate.
- No diffraction spikes: SCTs don’t have a spider to hold their secondary mirror, and thus don’t produce spikes on bright stars like a Newtonian reflector or some of the other catadioptric designs out there.
- Cooldown time: The Schmidt corrector is thin, so an SCT will acclimate to ambient temperatures faster than a Maksutov-Cassegrain will.
However, Schimdt-Cassegrain telescopes have a wide variety of disadvantages, including:
- Long focal length: Being f/10, even a moderately sized SCT has a huge focal length. Celestron’s C8 8” optical tube has a focal length of 2 meters! A typical 8” Dobsonian has a focal length of only 1200 mm. At larger sizes, the long focal length becomes a huge issue; a 14” or 16” SCT has a tiny field of view compared to a typical Dobsonian of that aperture. The long focal length of SCTs also means they need more accurate tracking and guiding for astrophotography. An f/6.3 reducer helps, but has its own set of issues.
- Long focal ratio: The long focal ratio of f/10 also requires long exposure times, magnifying issues in tracking and guiding when doing deep sky astrophotography. An f/6.3 focal reducer reduces this issue at the expense of possible vignetting with some cameras as well as cost.
- Sealed tubes: If debris or bugs enter your SCT or something breaks internally, solving the problem requires removing the corrector plate, a delicate process that takes steady hands, precision, and all sorts of precautions to re-seat the corrector correctly, or sending the scope back to the manufacturer for loads of money to have them fix it.
- Collimation inconveniences: Collimating an SCT isn’t insanely difficult but can only be done at night and requires tools and keeping the scope pointed at a bright star. Replacing the collimation screws with thumb screws merely exacerbates the need to collimate more often, and dedicated laser collimation systems for SCTs are expensive and complicated to use.
- Large central obstructions: SCTs have large central obstructions because of their big secondary mirrors. This makes the contrast lower compared to Newtonians or Maksutov-Cassegrains, which have lower central obstructions.
- Cooldown time: Compared to a Newtonian reflector, a Schmidt-Cassegrain telescope still requires more time to cool down due to the large secondary mirror also needing to cool down and the corrector plate trapping warm air inside the tube.
- Focus issues: SCTs actually require a lot of precision to focus due to the extreme curvature of their primary mirrors, which can be a pain when using high magnification or trying to do astrophotography. To make matters worse, the primary mirror can shift on its rod during focusing, making the target appear to bounce around the field of view or flop over a long period of time, ruining a long-exposure image. This can be alleviated by locking the mirror in place during photographic exposures and/or attaching an external Crayford focuser to the back of the telescope, but either requires a significant amount of monetary expense and/or DIY work.
- Mounting woes: Older SCTs come on fork mounts or manual equatorial mounts, which require precise polar alignment in order to be used correctly and can be troublesome to aim manually—especially with the narrow field of view of the telescope itself. Newer computerized mounts may be less complicated, but they still frustrate the user and have questionable longevity due to their cheaply made components.
Ranking The Best Schmidt-Cassegrain Telescopes by Price
Recommended Schmidt Cassegrain Telescopes, Individually Reviewed
1. Celestron NexStar 5SE – Choice Under $1000
Celestron’s NexStar 5SE is the only package offering their C5 optical tube. The C5 is one of Celestron’s oldest models in their lineup, dating back to 1971. The C5 is very compact, with an aperture of 125 mm and a focal length of 1250 mm. On the surface, it should be comparable in terms of views to a 6” Dobsonian. However, the C5’s huge secondary mirror and baffle assembly obstruct about 40% of the aperture by diameter, lowering the resolving power and contrast of the telescope significantly and also reducing its light-gathering ability to about the equivalent of a 100 mm (4”) aperture telescope. So in practice, you don’t have a lot of effective aperture to work with.
The 5SE uses a smaller version of the NexStar SE mount, which attaches to the tube with a tool-free Vixen-style dovetail clamp and a matching rail fixed to the body of the telescope. Interchanging the tube and mount is possible, but few other telescope optical tubes will clear the bottom of the mount, and there’s not much of a point in putting the C5 optical tube on a different mount or tripod anyway.
The NexStar SE mount requires a fair amount of setup, but once working, will slew to any target of your choosing from its database of 40,000 objects and track them pretty accurately too. A built-in equatorial wedge to use the scope in an equatorial configuration for long exposures is provided, but is of little use due to a complete lack of fine adjustments and the C5 optical tube’s inability to properly illuminate a camera sensor with an f/6.3 reducer, arguably necessary for long exposures. The NexStar SE mount is controlled with a provided hand paddle, but you can easily purchase and install a WiFi dongle to control the NexStar SE mount remotely with your phone or tablet.
The NexStar 5SE includes a single 25mm Plossl eyepiece providing 50x magnification, a 1.25” prism star diagonal and threaded visual back, and a red dot finder for aligning the mount. You will almost certainly want additional eyepieces, though the 5SE is limited to 1.25″ accessories only, despite the ability to physically attach a 2” diagonal.
2. Celestron NexStar 6SE – Choice Between $1000-$1500
A 1” increase in aperture may not seem like much to bother with, but in the case of the Celestron NexStar 6SE, it’s a much bigger deal than the specs would suggest. The C6 optical tube has significantly more light gathering ability, resolution, and contrast than the C5. The optics tend to be made to a slightly higher standard of quality, and the whole tube is negligibly larger in physical size. You can also use the telescope with an f/6.3 focal reducer or 2” star diagonal without suffering from vignetting problems for viewing or photography, which provides a huge boost in value too. The 6SE also uses a beefed-up version of the NexStar SE mount with a taller fork arm.
The 6SE can show you a fair amount of detail on the Moon and planets, and performs wonderfully on deep-sky objects, especially under dark skies. With 6” of aperture, you can properly resolve globular clusters and view thousands of galaxies, a few dozen of which have some kind of detail visible in them under dark skies. The scope is still small enough to carry on a plane with the tube attached to the mount!
As with the 5SE, only a single 25mm Plossl (60x) and a basic 1.25” visual back, diagonal prism, and red dot finder are included, so you’ll want to add more accessories later on to get the most out o f the scope.
3. Celestron NexStar Evolution 6 – Choice Between $1500-$2000
The NexStar Evolution 6 has identical optics to the NexStar 6SE, but with greatly upgraded mechanical features. The Evolution mount has clutches, which can be unlocked to point the scope manually if power is lost, a built-in rechargeable lithium-phosphorous battery, and can be controlled with either the included hand paddle or your smartphone/tablet. The scope also has an eyepiece rack and is slightly easier to assemble and transport; like the 6SE, you should be able to bring the tube and mount head assembly as carry-on luggage in a case.
The Evolution 6 also comes supplied with two eyepieces; a 40mm 1.25” Plossl providing 38x and a 13mm 1.25” Plossl ocular providing 115x. You’ll still probably want additional eyepieces and a focal reducer or 2” star diagonal for the maximum range of useful magnifications, but these are enough to get you started. As with the 6SE, a 1.25” visual back, prism star diagonal, and red dot finder are also included.
4. Celestron NexStar Evolution 8 – Choice A Between $2000-$2500
- Larger aperture shows you more deep-sky objects and resolves more detail on the Moon and planets
- Built-in battery, WiFi, manual mode clutches
- Still fairly easy to transport
The 8” Evolution model has the same mount and accessories as the 6” Evolution, but the included C8 optical tube provides significantly more light gathering and resolving power without significantly adding bulk. The Evolution mount provides a rock-solid support for the 8” optical tube; the same cannot be said of the cheaper alt-azimuth GoTo mounts provided with some other models. As with the 6” Evolution, the C8 can be detached and mounted on a variety of different alt-azimuth or equatorial mounts as long as they use a Vixen-style dovetail saddle.
The same accessories as the 6” Evolution are provided with the 8” model – a 1.25” 40mm Plossl (51x), a 1.25” 13mm Plossl (156x), a 1.25” prism star diagonal, a 1.25” visual back adapter, and a red dot finder. The C8 really benefits from a 2” diagonal and accessories to get the widest field possible at low magnifications, however, so we’d definitely recommend adding one if you can.
The 8” Evolution can still be transported on a plane as with the 6” model, but you may need to put the mount head in luggage and only bring the tube as a carry-on.
5. Celestron Advanced VX 8” Schmidt-Cassegrain – Choice B Between $2000-$2500
The Advanced VX 8” is simply Celestron’s C8 optical tube bundled with their Advanced VX German equatorial mount. The AVX is a computerized GoTo mount that tracks on an equatorial axis instead of an alt-azimuth, so it is vastly superior for long-exposure astrophotography. Setting it up does require more assembly and effort than an alt-azimuth mount like the NexStar, however, and the whole package is a bit more cumbersome to transport. Once set up, however, the Advanced VX 8” works great for lunar and planetary astrophotography. For deep-sky astrophotography with a DSLR or cooled camera, a focal reducer and autoguider are necessary for good results, so make sure to factor those into your budget as well.
The Advanced VX isn’t limited to working with just the C8; you can attach almost any optical tube with either a Vixen or Losmandy-style dovetail thanks to the Advanced VX’s dual saddle plate. The mount works best with a scope a bit smaller and shorter in focal length than the C8 such as a 4” refractor or 6” Newtonian reflector.
For accessories, the Advanced VX 8” includes a standard 1.25” visual back, 1.25” prism star diagonal, a 25mm 1.25” Plossl ocular providing 60x, and a 6×30 finderscope attached to the telescope tube. Of course, you’ll want to purchase at least a few additional accessories to get the most out of this instrument, whether it’s for visual astronomy or astrophotography.
6. Celestron Advanced VX 9.25” – Choice A Between $2500-$3000
- Slightly better views, easier focusing and collimation compared to 8” models
- Disassembles into a lot of small and manageable pieces
- Some astrophotography capabilities
The 9.25” Advanced VX is a bit of an upgrade over the 8” models, with just a smidge more light gathering and resolution. The C9.25’s slightly different optical design makes it a bit easier to focus and some claim it has better optics too, though this is partly due to the slightly more lax collimation tolerances of the scope. As with the 8” Advanced VX kit, not a lot is included with regards to accessories. The C9.25 is about at the limit of what the VX can hold, and as such it isn’t really usable for deep-sky astrophotography, but shots of the Moon and planets are perfectly doable.
7. Celestron NexStar Evolution 9.25” – Choice B Between $2500-$3000
The NexStar Evolution 9.25” has all of the same features and accessories as the Evolution 6” and 8” models, but with the addition of the C9.25 optical tube. However, the C9.25 is a bit big for the Evolution mount, and the mount has been accordingly beefed up by including a heavy-duty tripod. The Advanced VX setup is a little less cumbersome as a result – but if you prefer the simplicity and convenience of the Evolution mount, the NexStar Evolution 9.25” is for you.
8. Celestron Advanced VX 8” EdgeHD – Choice C Between $2500-$3000, Best for Astrophotography
- EdgeHD optics make for sharper and flatter astrophotos
- Advanced VX mount is enough to get you started in astrophotography
- Great views of the Moon, planets, and deep-sky objects
The EdgeHD version of the 8” Advanced VX Schmidt-Cassegrain adds Celestron’s EdgeHD optics which provide a significantly flatter field and sharper images than the standard C8 8” Schmidt-Cassegrain. A 50mm instead of a 30mm finderscope is included. For visual astronomy, you probably won’t notice a huge difference from the regular 8” SCT model besides a slightly improved cool-down time for the optics, but the EdgeHD’s flattened field is helpful for long-exposure astrophotography. As with the regular version, however, for best results an astrophotography setup really requires a focal reducer, autoguiding, and a polar scope for easy and accurate polar alignment.
9. Celestron NexStar Evolution 8” EdgeHD w/StarSense – Choice D Between $2500-$3000
- EdgeHD optics make for sharper and flatter field visually, albeit marginally
- StarSense auto-align technology slightly reduces setup time
- Built-in battery, WiFi, manual mode clutches
The 8” EdgeHD Evolution package adds Celestron’s EdgeHD optics to the regular 8” Evolution, though since the Evolution mount isn’t designed for astrophotography you’re unlikely to benefit from this improvement. More significant is the addition of Celestron’s StarSense auto-aligning system, which aligns the telescope with stars without any input from the user – just set up, level the tripod, and turn it on. The StarSense system does require a completely clear night sky free of clouds or obstructions such as trees or buildings, however. Additionally, with the StarSense technology active you are limited to using Celestron’s standard SkyPortal app to control the telescope, which lacks crucial features like a red-tinted display option and far fewer catalogued objects. Otherwise, the performance, operation, and accessories are the same as the regular NexStar Evolution 8”.
10. Celestron CPC 1100 – Choice Over $3000
- Big aperture gives stunning lunar, planetary, and deep-sky views
- Some astrophotography capabilities for both Solar System and long exposures
- Relatively simple to set up, albeit heavy
The CPC 1100 is the largest telescope Celestron offers in an alt-azimuth mounted fork configuration, and for good reason. The 11” Schmidt-Cassegrain optical tube is already huge and bulky, and combined with the CPC fork mount it makes for a downright intimidating telescope. However, the C11 optical tube offers unmatched views of the Moon, planets, and deep-sky objects compared to smaller instruments, provided you can fit them into its relatively narrow field of view.