The Optical Tube
The CPC 800 GPS uses Celestron’s legendary C8 optical tube, which has been around since 1970. The C8 is an 8” (203mm) f/10 Schmidt-Cassegrain telescope (SCT) with a focal length of about 2032mm. It uses a spherical f/2 primary mirror and a convex aspheric secondary mirror to focus light and make a very long focal length system inside a short tube. It also has a Schmidt corrector at the front to get rid of the severe spherical aberration caused by the spherical primary mirror.
The newest version of the C8 optical tube, and the one offered as part of the CPC 800 GPS, is the “XLT” version, featuring Celestron’s StarBright XLT coatings for slightly higher light transmission, a BK7 instead of a plate glass corrector plate, and the ability to use Starizona’s HyperStar system to remove the secondary mirror and convert the scope to an f/2 Schmidt camera with the HyperStar corrector lenses.
Most C8s in general are good performers, and the C8 XLT is no exception. These scopes (provided they are collimated, which is a fairly easy process) are great for viewing the Moon, planets, and smaller deep-sky objects that fit in their limited field of view. The C8 is also a great choice for planetary imaging and some deep-sky imaging, though getting one to work well for deep-sky imaging can be a bit tough for beginners.
The C8 focuses, as with most SCTs, by moving the primary mirror along a rod inside the optical tube, changing the spacing of the primary and secondary mirrors to shift the location of the focal plane. This also changes the actual focal length of the telescope slightly, by up to about 100 mm or so. The focusing mechanism of the C8 also naturally leads to the mirror wobbling a bit as it travels along the rod, and this manifests as “image shift” when focusing; the object you’re looking at or trying to image will appear to bounce around the field of view. Over long periods of time, the C8’s primary mirror can also sag on the focusing rod and produce a similar effect called “mirror flop”, which ruins long-exposure astrophotos. The EdgeHD version of the C8 sold by Celestron fixes some of these problems and produces a sharper and flatter field for astrophotography; it’s a must-have if you plan on doing lots of deep-sky imaging and is sold on the CPC fork mount as well as a variety of others.
As with almost all SCTs, accessories attach to the back of the C8 optical tube using standard SCT threads and adapters. You can attach an f/6.3 focal reducer, a visual back, a star diagonal, camera adapters, a stand-alone Crayford focuser, and all sorts of other accessories in seconds thanks to these standardized threads.
The CPC 800 GPS comes with only a basic set of accessories. Celestron provides you with a 9×50 finderscope, a 1.25” visual back, a 1.25” prism star diagonal, and a 1.25”, 40mm Plossl eyepiece (which produces 52x magnification with the CPC 800). The included visual back and star diagonal are just fine, but a 2” diagonal or an f/6.3 reducer will allow you to get a much wider field of view than is possible with only 1.25” eyepieces at f/10.
The included E-Lux 1.25″ 40mm Plossl eyepiece is fine, but because it has a 1.25″ barrel diameter, it only provides a 40° apparent field of view. This makes you feel like you’re looking down a drinking straw and gives you a true field of view of only 0.75°, which is just a little bigger than the full moon. A 2” eyepiece or using the f/6.3 reducer would allow you to get a field of up to 1.2 degrees, which is quite a bit more useful in viewing larger deep-sky objects.
For a finder, the CPC 800 GPS comes with a 9×50 straight-through unit that slides onto the optical tube on a non-standardized bracket. This finder provides a roughly 6° field of view (albeit upside down) with crosshairs and shows stars or even bright deep-sky objects a few magnitudes fainter than what you can see with your eyes alone—arguably overkill for a GoTo telescope, but a nice bonus.
The CPC 800’s fork mount is a GoTo alt-azimuth fork mount. Though it has adjustable clutches and can technically be aimed manually when powered off, this is not a great idea and you cannot use the scope manually when powered on without ruining your alignment. The C8 optical tube is semi-permanently attached; removing it is not a quick nor easy process and should only be done if you plan to bolt on a dovetail and switch the tube to a different mount.
Setting the CPC 800 GPS up for a night of observing is pretty easy. You extend and level the tripod, attach the spreader tray, and then put the scope on top. After tightening three screws to secure the fork mount to the tripod, you align the scope on two or three stars in the sky and you’re ready to go. The GPS unit in the CPC 800 is somewhat of a gimmick; it saves you a few seconds by updating your location, date, and time information but otherwise does not do much to help with aligning or actually using the telescope.
One of the biggest downsides of the CPC 800 GPS, and indeed the entire CPC line, is the sheer bulk of the thing. The tripod is rock-solid but takes up a lot of space, as does its huge accessory tray. The scope/fork combination on the CPC 800 isn’t too heavy, but is quite wide and may be hard to fit in a closet, suitable case, or a jam-packed trunk in a smaller vehicle.
The CPC fork mount uses all-metal gears and can be autoguided. As such, if you put it on an equatorial wedge and add guiding, it is pretty decent for deep-sky astrophotography, though a German equatorial mount (GEM) is more versatile, easier to polar align, and should probably be what you learn to use for astrophotography first.
Should I buy a Used Celestron CPC 800 GPS?
A used CPC 800 GPS will make a fine scope. However, given the complex SCT optics and the fork mount’s electronics, there are a few things to watch out for. A cracked or missing corrector plate is a no-go; replacing it costs more than a new optical tube. GPS units in some older units can fail, which doesn’t affect operation much but certainly reduces the value of the scope. You should also check to make sure the entire mount aligns, slews, and tracks properly, and that the coatings on the corrector lens and optics are in good condition.
The CPC 800 GPS would probably not be our first choice for a scope in its price range, or even for one of the C8 telescope bundles Celestron sells. Here are a few of our alternative picks:
- The Celestron NexStar Evolution 8” is similar in functionality to the CPC 800 but with a more compact form factor, built-in lithium battery, and the ability to be controlled either with the included hand controller or your smartphone. Celestron throws in a bonus 13mm Plossl eyepiece, too. A 6” model and 9.25” model are also available.
- The Celestron Advanced VX 9.25” offers more aperture and better performance than the C8 optical tube along with the versatile (and easily compacted) Advanced VX mount, which is not ideal for deep-sky imaging with the C9.25 but can be used with smaller optical tubes for deep-sky imaging or the C9.25 for planetary imaging as well as observing.
- Celestron offers the C8 atop the Advanced VX mount as well, though we’d recommend you go for the C9.25 if you can afford it and want such as setup.
- The Celestron Advanced VX 7” Maksutov offers better planetary performance than the C8 (similar to the C9.25) but inferior views of deep-sky objects, essentially no deep-sky astrophotography capability and a rather claustrophobically long focal length for a 7”.
- The Sky-Watcher 12” GoTo FlexTube is a lot more capable when it comes to viewing all objects and imaging the planets than any of the smaller SCTs on our list, and surprisingly portable too. A lighter, cheaper all-manual version is also available.
- The Sky-Watcher 10” GoTo FlexTube is more compact than the 12” model by far, and offers superior views of all targets and slightly better planetary imaging capabilities than the Celestron 8” and smaller optical tubes. As with the 12” model, a manual version is available which shaves quite a bit off the price and bulk.
- For those looking for the ultimate in portability, Explore Scientific’s 12” and 10” truss tube models fold up into a space smaller than the volume occupied by the CPC 800 and its fork mount alone, though either model entirely lacks GoTo or even functional accessories bundled in.
- A plethora of 12”, 10”, and smaller manual Dobsonians from various vendors such as Apertura, Zhumell, Sky-Watcher, Orion, and Celestron are also available – check out our rankings page for more information.
Aftermarket Accessory Recommendations
Upgrading the CPC 800’s accessories is going to take quite a bit of money, and the more you spend, the more options you have. You certainly need a dew shield, for starters – it keeps moisture, bugs, and curious hands off your corrector plate, and it also reduces glare and stray light inside the CPC 800, much like a lens hood does for a camera.
As for optical accessories, we’d recommend you purchase a 2” dielectric star diagonal, which screws directly onto the back of the CPC 800 using its standardized threaded connections. A 38mm Apertura SWA will give you 53x magnification with the CPC 800 and a 1.3-degree true field of view, almost double that of the stock 40mm E-Lux Plossl eyepiece. A medium-power eyepiece like the SVBONY 26mm SWA is a good idea—it’ll give you 78x magnification and a 0.9-degree true field. If you’re insistent on sticking with 1.25” only eyepieces, a 20mm “redline” eyepiece (102x) also works for medium power to complement the stock 40mm E-Lux Plossl.
For even higher magnification with the CPC 800, a 9mm “redline” eyepiece will give you 226x magnification, which is about as much as you can expect to use on all but the best nights.
A UHC (ultra-high-contrast) nebula filter doesn’t make the effects of light pollution go away, but will increase contrast on nebulae with the CPC 800 GPS or any telescope regardless of sky conditions, which can make a huge difference under light-polluted skies. It will make nebulae like Orion (M42), the Dumbbell (M27), the Lagoon (M8), and others stand out more. Under moderately dark skies, you can see things like the Veil Nebula in Cygnus or the Helix Nebula in Aquarius that you couldn’t see before. We would recommend you purchase the Orion UltraBlock UHC filter for this task. If you are using a 2” star diagonal, the 2” model will screw onto both your 2” eyepieces and your 1.25” adapter for use with your 1.25” eyepieces and accessories.
What can you see with CPC 800 GPS?
While you’re limited in what you can see outside the solar system if you’re stuck under light-polluted skies, the CPC 800 GPS can still show you a lot, even from the city or suburbs. Not all large open star clusters are going to fit in the scope’s limited field of view, especially with the stock 1.25” diagonal and eyepiece, but those that do are jaw-dropping and show hundreds of colorful stars, such as the Double Cluster in Cassiopeia, the Wild Ducks (M11) in Scutum, the Pleiades (M45) in Taurus, and many more. The outer members of bright globular star clusters, like M13, can be resolved under all but the worst sky conditions with the CPC 800, and you can split close double stars too. Bright nebulae like Orion (M42) or the Lagoon (M8) can be seen from light-polluted skies, but are washed-out shadows of their glorious appearance under dark skies.
Under dark skies, the CPC 800 is a powerhouse for deep-sky viewing. Galaxies like M31, M64, M65, and M82 show their dramatic dust lanes. M51’s spiral arms begin to become visible on the best nights. The Virgo Cluster shows dozens, or even hundreds, of faint fuzzy galactic members, and you can see the two companion galaxies orbiting M31 (Andromeda) – M32 and M110 – as big fuzzy spots next to its magnificent dust lanes, which stretch well outside the CPC 800’s maximum field of view. The Orion Nebula (M42), the Lagoon (M8), and the Swan (M17) transform under dark skies – especially with a good UHC nebula filter – and show jaw-dropping details. The Dumbbell (M27) and Helix look great, as do smaller and more colorful planetary nebulae like the Cat’s Eye, the Blinking Planetary, and the Blue Snowball Nebula.
The CPC 800 is great for viewing Solar System objects, too. Expect to have no trouble resolving the phases of Mercury and Venus, dust storms and ice caps on Mars, and thousands of interesting surface features, even just a few miles across, on the Moon. When Mars is close to Earth, you can see a few dark markings on it too. Jupiter shows its cloud belts, festoons, storms and of course the Great Red Spot, along with its 4 large Galilean moons, which appear as disks – along with their jet-black shadows – as they transit across Jupiter’s face, which happens at least a few times per week. Saturn’s rings look magnificent along with the Cassini Division within them, and the CPC 800 can reveal some cloud belts on Saturn itself and a smattering of moons, albeit all only as star-like pinpoints of light. Uranus’ disk can be resolved, and its 4 largest moons are technically visible under dark skies but are unlikely to be spotted. Neptune is barely distinguishable from a star, but its moon Triton appears next to it – albeit dimly. Pluto, as with Uranus’ moons, is technically within reach of an 8” telescope like the CPC 800, but a larger telescope would be best to spot those distant worlds.
Thanks to its motorized tracking, in the stock configuration, the CPC 800 GPS is more than capable of planetary and lunar astrophotography (or solar with an appropriate front-aperture solar filter). Just pop in a Barlow lens to boost the system to between f/20 and f/30, a suitable high-speed guide/video camera like the ZWO ASI224MC, plug in your laptop, and you’re all set. Mars, Jupiter, and Saturn will look fabulous provided you have decent seeing conditions.
For deep-sky astrophotography, you can get away with short exposures of up to 30 seconds with no guiding and the use of either an f/6.3 reducer or HyperStar and a suitable camera, but anything longer than that will be blurred by field rotation thanks to the CPC 800’s alt-azimuth mount. Plop the scope on an equatorial wedge and add autoguiding, and you’ve got a pretty good astrophotography rig. Keep in mind, however, that the CPC fork mount, even when appropriately set up on a wedge, is not going to be as versatile, accurate, or convenient to polar align as a regular German equatorial mount, and an 8” SCT is really not the best choice for beginners looking to get started in deep-sky imaging.