Review: Celestron CPC 800 Telescope

Author: Gregory Brannon

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Celestron has marketed a fork-mounted 8” Schmidt-Cassegrain Telescope, or a C8, since the 1970s. Since then, they have gone through many changes, and the CPC800 is one of the more recent variants. The CPC 800 is a high-resolution 8” telescope with a computerized “Go-To” mount. It can be used in altitude-azimuth (up, down, and all around) mode, or in polar-aligned equatorial mode, and it will track the rotation of the sky in both configurations. The CPC 800 is a very good planetary telescope, capable of easily reaching high magnifications, and it has enough aperture to be useful for deep-sky-object observing as well. The computerized functionality allows it to easily slew to any position in the sky, and it has a built-in catalogue of many objects, both deep-sky-objects and solar system objects. It can probably point at more things than it can usefully show. Because of its long focal length, it struggles to fit in especially large Deep-Sky-Objects--you won’t be able to even view the entire Pleiades unless you use very wide-field eyepieces and an upgraded 2” Star Diagonal.

The CPC 800 unit I have evaluated is used for public outreach at GTCC’s Cline Observatory in Jamestown, North Carolina, where the ability to find arbitrary objects very quickly comes in handy. However, whether a Go-To telescope really makes sense for you depends upon your observing style. In my opinion, finding objects manually using a Dobsonian reflector is more fun and engaging, especially when it comes to dim objects. There is a certain ‘thrill of the hunt’ that Go-To telescopes lack, and though it can be frustrating on occasion when you just can’t figure out a manual star-hop, the CPC800 replaces that with the occasional frustration of computer misalignment and other electronics issues. Reading star charts, or a book on star-hopping, is generally more fun and engaging than staring at a hand controller, or reading a manual or troubleshooting guides.

Total Score

8/10: Very good, but not the best

Score Breakdown (out of 5)

Read our scoring methodology here.

FOR

  • Very high quality diffraction-limited optics.
  • Sturdy, stable mount and tripod. Shaking is virtually a non-issue.

AGAINST

  • Somewhat sparse accessories.
  • At least after a few years of use, the computer can behave unpredictably.

The Competition

The CPC800 has a few notable competitors in the field of 8 inch Schmidt-Cassegrains, even from Celestron itself. Celestron’s NexStar 8SE is considerably cheaper, however its mount is grossly undersized and shaky. Celestron’s Evolution 8 is a cheaper telescope on a lighter but still plenty stable single-fork-arm mount.

Meade’s 8” LX200 and LX90 lines are the equivalent series of fork-mounted Schmidt-Cassegrains and have always been direct competitors. They generally have feature parity--high quality optics and go-to electronics.

If you’re not sure if you’d really want Go-To computerization and you are looking for a visual-only telescope, seriously consider an 8” Dobsonian. Or even a 10”, 12”, or 14”--Dobsonians are so much cheaper than SCTs that for the same or similar price you can get some pretty enormous telescopes, and the CPC800 is bulky enough that the weight of a Big Dob shouldn’t be any more intimidating. And consider--the optical tube of the CPC800 might be very small, but the actual floor space it takes up is larger than a big dob, due to the tripod.

Our Verdict

The CPC800 is a usable telescope. Excellent optics, and the occasional frustrations of computer glitches are rarely mission critical. It’s not especially easy to learn how to use--you can be star-hopping with a Dobsonian in minutes, but most of my fellow Observatory volunteers seem to be too intimidated or frustrated by the CPC800 to use it. I prefer to use a Dobsonian, but in a public outreach context, the instant go-to is very useful.

An 8” SCT can be a very powerful planetary imaging tool, and it’s technically possible to do deep-sky-imaging too (though it’s far more technical due to the long exposures necessary).

It’s not the best at what it does, but it is good at what it does. I can’t quite wholeheartedly recommend it, I like Dobsonians too much, but I can’t condemn it either. The computer glitches seem to be something which accumulates with age, and apparently there are ways of resetting the controllers to fix some of the glitches. But having the chance of dealing with bugs in the observing field is always a frustrating prospect. That’s more of a condemnation of go-to telescopes in general than the CPC800 in particular.

Celestron CPC 800 SCT

Celestron CPC 800

Rating: 4

Optical Tube Assembly

The CPC800’s OTA is permanently attached to the mount. It is technically possible to defork it, remove its built-in counterweights, and attach a dovetail for use in other mounts, but this is a lot of difficult, high-stakes DIY work. Celestron sells the OTA on its own as the Celestron C8 with two choices of dovetail.

The CPC800 is a Schmidt-Cassegrain Telescope (SCT). There are two curved mirrors and a corrector plate. A Cassegrain Telescope uses a parabolic primary mirror and a hyperbolic secondary mirror. Since short-focal-length parabolas are difficult to manufacture, an SCT uses a spherical mirror, which would produce a ton of Spherical Aberration, blurring and distorting the image. To prevent Spherical Aberration, the light first passes through a Schmidt-Plate, a thin lens that introduces the exact opposite aberration. Then it hits the concave spherical primary mirror and bounces forward, where it hits the convex hyperbolic secondary mirror, and then the light is folded backward through a hole in the primary mirror, and out through the back. The image can then be inspected with a camera or eyepiece. The convex secondary mirror not only folds the light path, but acts as a magnifier, which increases the overall system focal length so that it becomes much longer than the length of the tube. An SCT is very compact but easily capable of high magnifications and large image scales.

The CPC800 is an 8” (200mm) aperture, 2000mm focal length (f/10) telescope. This is an ideal telescope for imaging and observing planets and the Moon. Optical theory suggests a maximum useful magnification of about 400x, which would be provided with a 5mm eyepiece. Atmospheric perturbations will typically limit the seeing to perhaps 200x, but 400x is attainable on very good nights.

The star-test is really fantastic. The star images look virtually identical on either side of focus, there’s no turned down edge or astigmatism. This is a genuinely very sharp telescope. Views of Jupiter, Saturn, and the Moon are fantastic. Despite the large secondary obstruction (31%), the contrast is not noticeably worse than an 8” Newtonian, and since SCTs are rarely out of collimation (optical alignment is spot on even after being used for several years, even with a few bumps), the CPC800 will usually outperform the 8” Dobsonians we use it with. The inside of the tube is blackened and baffled. However, the black paint used on the inside of the tube is more of a matte dark gray, which might slightly reduce contrast.

That secondary obstruction, though it is not especially noticeable, is still degrading the image. When in good collimation, a Newtonian with a smaller obstruction might have a higher resolution, but it’s doubtful that you’d really be able to tell the difference in a focused image.

The primary mirror is focused by shifting the mirror forward and backward, rather than by moving the eyepiece or camera back and forth using a focuser drawtube. Therefore, focus is achieved by slightly changing the focal length of the telescope. The mirror will also tilt very slightly as it is pushed and pulled by the focuser knob screw, leading to a small amount of image shift. The image shift is noticeable starting at about 167x, but even at 250x, the highest power we regularly use, it is not objectionable. I would not worry about image shift, even at 400x.

The CPC800 has XLT coatings, which increase the image brightness by a small amount by increasing transmission through the corrector plate and reflecting off of the mirrors.

The CPC800 is Fastar compatible, allowing the secondary mirror to be removed and replaced with a corrector lens and an astro-imaging camera or DSLR. In deep-sky-imaging, it is much much easier to produce high quality images using short exposure times at the blazingly fast focal ratio of f/2 than at the SCT’s native f/10. If you’re interested in a dedicated f/2 reflector, check out Celestron’s RASA, an astrograph that cannot be used visually.

Deluxe/EdgeHD Version

For astrophotographers interested in wide-field deep-sky-imaging at f/10, the EdgeHD variant includes additional corrector optics to correct for off-axis astigmatism, increasing the quality of the images. This is practically useless for visual use unless you’re using extremely high quality, wide-field eyepieces with a two-inch diagonal. The default version can also employ a reducer-corrector, which reduces the focal ratio to f/6.3 and also corrects off-axis astigmatism.

CPC GPS Mount & Heavy-Duty Tripod

The CPC800 uses a NexStar hand controller, which is programmed with a 40,000 object database. Very, very many of these objects will be impossible to view with the eye. The database includes double stars, named stars, named deep-sky-objects, objects from the NGC and IC catalogs, Messier objects, and solar system planets and the Moon. It is also possible to put in R.A. and Dec. coordinates to locate an object not in the catalog.

The CPC800 must be plugged in to be used. Unlike smaller Go-To telescopes, it does not use batteries of its own. We keep it plugged in using an outdoor power outlet installed at our observing pad, and we keep a cart in front of the power line so no one trips on it. You do not want to trip over your power cable in the dark and ruin your alignment!

The mount must be aligned before it can be used. Those using the telescope in altitude-azimuth mode will likely be using Solar System Align, which requires just the date and time and manual-alignment with a planet or the Moon, or they will use Two-Star alignment, where you aim at two stars to build up a pointing model.

We exclusively use the CPC800 with an Equatorial Wedge, a structure that goes between the tripod and the telescope to point the azimuth axis toward the North Star and allow it to track the sky with more or less one motor. Tracking with this method is generally more accurate and, in imaging contexts, there will be no image rotation. I cannot comment on personal experience about the accuracy or ease of use of the altitude-azimuth mount alignment modes, but I haven’t heard any horror stories. The alignment process we usually use is EQ North Align->AutoAlign.

My experience using the mount is sort of mixed. When it works, it’s really good, and you can very quickly find and confirm objects one after another. But there are occasional glitches and problems that end up requiring restarting the alignment. In particular, at some point in the last couple years, the GPS functionality seems to have stopped working properly–the telescope no longer acquires the correct time and date, which can mess up the alignment if you don’t notice. Sometimes the NexStar controller just turns off. The telescope continues to track, but there’s no way to turn the controller back on and point the telescope unless you turn the mount off and back on, again requiring a restart.

Alignment is definitely a case of what you put in vs. what you get out. If you take care at every step to be as precise as possible–most crucially in the physical alignment, pointing the wedge north, and resetting the RA/Dec axes of the mount–you’ll get better results. Still, though, most of the time you’ll end up somewhat off center even in a wide field eyepiece, and I don’t expect to find the object when using a high power eyepiece. This puts some limits on the ease of finding some dim deep-sky-objects, especially planetary nebulae, which require high power to differentiate from stars. If the object doesn’t end up centered in the field of view, confirming it will still be pretty difficult.

The mount has two clutches which allow the telescope to be aimed manually if, for some reason, you so desire. Without any kind of manual slow motion, this would probably be an exercise in frustration–it’s mostly there for the initial alignment setup and so the telescope can be folded down with the power off. The telescope cannot be aimed manually while the telescope is aligned, as alignment would be completely lost.

The mount has two carry handles: one big, easy-to-use silver one, and one subtler, more difficult handle moulded into the other fork arm. This is crucial for transport and for getting the telescope up onto its wedge. The telescope and mount weigh 42 pounds, which is almost as much as an entire, fully assembled 8” Dobsonian.

There’s not much to say about the tripod. It’s the right height for comfortable viewing while sitting down, and it’s acceptable when standing up. It’s extremely sturdy, there are virtually no vibrations from people stomping around, even on concrete, and it’s really heavy when the wedge is on it. Actually, the wedge weighs much more than the tripod itself–the tripod is a reasonable 19 lbs on its own. The wedge adds an additional 38 lbs.

The wedge is a big, heavy chunk of metal. It is a hinged platform that can elevate the telescope between 0 and 90 degrees for polar alignment anywhere in the world. The wedge will add roughly an additional $400 to the price of the telescope. A wedge is necessary for any kind of serious imaging, but for visual observing, it probably isn’t that crucial.

Accessories

The CPC800 is a serious telescope and is probably intended to be a second telescope rather than a kit for beginners. As a result, its accessories are pretty bare, counting on you to either have an existing collection of accessories, or be willing to spend more money to build up a collection.

The CPC800 includes one single eyepiece, a 40mm Plossl. This is an interesting choice. The maximum field of view usable in a 1.25” format eyepiece would be provided by a 32mm Plossl. The 40mm Plossl provides a lower magnification, but the true field of view is the same. As a result, the 40 has a small apparent field of about 40 degrees, whereas Plossls are usually good to 52 degrees. The 40mm Plossl provides 50x magnification. For most applications, the eyepiece with the same true field of view but a wider apparent field is better, so why did they use a 40mm instead of a 32mm? In long-focus telescopes with focal ratios of perhaps f/6 or longer, a 40mm eyepiece will show brighter images than a 32mm eyepiece. At f/10, images of some larger, low-surface-brightness Deep-Sky-Objects might be somewhat difficult to see. Messier 27, the Dumbbell Nebula, and Messier 31, the Great Galaxy in Andromeda both reward low magnifications, so the 40mm Plossl might make sense.

The CPC800 comes with a 1.25” visual back and a 90-degree star diagonal. By default, this telescope is restricted to 1.25” accessories, which limits the usable field of view. The star diagonal is perfectly acceptable. I don’t think it’s anything special. To my knowledge, it’s a typical mirror star diagonal with regular coatings, not special XLT coatings. The diagonal is needed for a comfortable viewing orientation. It would be impossible to fit your head under the mount most of the time if you had to look straight through. The diagonal shows a mirror-reversed image. A typical erecting amici prism diagonal would degrade the image.

The CPC800 comes with an 8×50 straight-through finderscope. A straight-thru (as opposed to right-angle-correct-image) finderscope is more useful for aiming the telescope against the real sky during the alignment process (by keeping both eyes open, you can align the crosshair with the star in the sky and it will also be aligned in the finder). These finderscopes are very common and generally good quality, with bright, sharp images. The finder is also powerful enough that you can use it to center an object if it ends up outside the field of view after the computerized slew, as long as the object is bright enough. Unfortunately, there’s no way to adjust the focus position of the crosshair of the finder, and the crosshair is out of focus on ours, which makes it slightly harder to align a star, especially when the sky gets dark. For some reason, the finderscope mount bracket is a proprietary quick-release bracket, instead of the usual Synta-style finder shoe. So you may have to buy a new finder shoe if you want to mount a different finder or a guidescope.

That’s it! That’s all the accessories the thing comes with!

Aftermarket Accessories

You’re definitely going to want some more eyepieces. There’s a whole range of possible magnifications, but for our observing sessions we’ve settled on a 25mm, 15mm, or 18mm, 12mm, and 8mm. The included 40mm is alright for wide field viewing. We don’t usually go to very high magnifications during public nights, but if you want to take full advantage of the telescope’s optics and reach 400x on nights with a spectacularly steady atmosphere, a 5mm eyepiece would be necessary–or maybe conservatively a 6mm for 333x. A 10mm eyepiece with a 2x Barlow, or a 15mm eyepiece with a 3x Barlow, would also provide maximum power. Barlows aren’t as useful with long focus telescopes. Consider using a dedicated long-eye-relief planetary eyepiece design instead of a short-focus, tight-eye-relief Plossl.

To take full advantage of the telescope’s field of view, a 2” diagonal to replace the 1.25” visual back would be needed, and 2” eyepieces with very wide fields of view can be used. The gold standard is TeleVue, but at f/10, cheaper wide-field eyepieces may perform acceptably.

Celestron sells a powertank, which is a chunky battery to keep the telescope running in the field. Any 12v power supply ought to work.

Imaging is a whole other can of worms, which is outside of the scope of this review. The CPC800 can be used for simple planetary snaps using a smartphone or a DSLR, but when used with dedicated planetary cameras it can get stunning results.

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