Celestron CPC 8 Schmidt Cassegrain Telescope Review

(published in 2005 in the magazine LE STELLE)

Before I comment on the new GPS-assisted automatic aiming, I feel it is at least necessary to describe the performance and innovations brought to the classic eight-inch optical tube.
Aesthetically speaking, it stands out from previous models for its new, more rounded optical body in the primary mirror area, a new focusing knob and a cap, which protects the secondary mirror support, with a more modern design.

When removing it, you'll notice the classic three adjustment screws, which allow you to adjust the instrument's collimation using a Phillips screwdriver. The cover protecting the corrector plate has also been modified: two small prongs at the edges provide an optimal grip compared to previous models, although when closing it, you must pay attention to the two guides located inside the cover.

It was also a pleasure to note how the designers have perfected the cell that contains the finder: it uses a system, consisting of two screws plus a spring, which allows for quick installation and quick alignment with the main instrument.

The optical surfaces were found to be perfect and free from streaks, marks or inhomogeneities, the new treatment  XLT starbright, gives a pleasant reddish color; the correcting plate is made of glass Crown with a diameter of 203mm, while the primary consists of a mirror in Pyrex. Its movement during focusing was found to be much smaller than that observed using other commercial telescopes with the same optical configuration; this, in part, is likely due to the triangular structure that acts as a guide for the primary mirror, which is adjustable by means of three screws located near the focusing knob. Previous models, on the other hand, rested solely on the internal dewshield, resulting in inadequate component adjustment over time.

In an instrument of this class I certainly would not have considered a rapid primary locking system assisted by an electric focuser superfluous, which would have given an extra touch of class to this new telescope from Celestron.
The star test revealed only a slight spherical aberration, while astigmatism and chromatic aberration of the corrector plate were absent.

The corrector plate did not show any tension, while the sensitivity detected in the snap test It confirmed good optical workmanship.
If you observe on a high-humidity evening, the humidity will settle on the corrector plate, ruining your observations. I solved this problem in August by making a lens hood from a rubber mat used to store wet boots or umbrellas in the trunk of your car. Total cost? Less than 15 euros.

 

FORK AND TRIPOD

The fork is significantly oversized compared to the optical tube, which seems to "suffocate" between the large arms that contain it. It goes without saying that vibrations are well contained, even when using heavy accessories. The distance between the optical tube and the computerized base of the fork also allows for the mounting of a focuser. Crayford in concert with a two-inch mirror diagonal: however, we are just a few millimeters from the base of the mount. The built-in control panel is equipped with two ports “Aux”, of the input of a possible autoguide, of the remote control interface via personal computer, of the input of the keypad and of the 12 V power socket which, observing from the garden at home, we often powered with a very convenient scanner transformer.
The kit includes a cable for connecting to a car's cigarette lighter socket. The power button also seemed well-sized.

The new ball bearing system, which moves the axis of the’azimuthh, it is very smooth and precise, as well as being a compromise between rigidity and compactness, and it has worked accurately for months. The handle for unlocking the axes was also very comfortable, also oversized and easy to maneuver, even when wearing heavy winter gloves. The declination adjustment has not been updated, except for the twin locking handle to the one just mentioned. However, there are no knobs for micrometric adjustment of the instrument without the use of electronics, nor are there graduated circles.
The CPC I had was in the simple altazimuth configuration. An equatorial plate is obviously available for photography.

The tripod should be able to support even instruments with a larger diameter, compact, and includes a very easy mounting system.
In just four steps you can unfold it, widen the folding levers and fit a plastic spacer triangle  which also serves as an eyepiece holder: in this case, we would have preferred greater "load capacity" since the three holes for simple 31.8mm eyepieces seemed reductive. Furthermore, the central circular section is compromised by the tripod fixing bar.

The only element that I consider truly inappropriate is the position of the bubble which, instead of being inserted near the fork, has been placed on the upper base of the tripod, compromising its use with the fork mounted.

This makes leveling the tripod difficult. Whoever will be mounting the telescope will have to pay attention to the ground where the CPC will be located during observations, or else they will need to equip themselves with a small additional bubble and attach it near the base of the mount.

Assembling the telescope's upper part is fairly quick: a system featuring knob-type screws allows for quick and easy attachment of the telescope. First, however, you'll need to center the thread that houses the central pivot and rotate the base until you can insert the screws into the threads at the base of the fork. Especially the first few times, you'll need to be careful how to insert the mount's central seat into the large screw protruding from the tripod base. Perhaps a double handle on the sides, rather than a single one, would have been helpful.

The mounted telescope, due to its size, might discourage smaller people. However, the use of lightweight materials such as plastic and aluminum allows the instrument to be lifted easily without too much effort.

SKY ALIGN SYSTEM

I now move on to describe the heart of the CPC which represents the real novelty from Celestron.
The Sky Align, direct successor of the system Nexstar, is able to detect the coordinates of the observation location through the system GPS. By connecting to three satellites, it acquires the information necessary to calculate, with a precision of just a few meters, the position from which it operates, while at the same time quantifying universal time. By giving preference to this first alignment system, it will appear, on the display of the control panel, a scrolling text that will advise the user to center three celestial objects.

After framing the first one, the button inside the finder will be pressed ENTER: The CPC will prompt you to use the arrows again to center the object in the eyepiece; in this case, the motor speed will automatically decrease. Obviously, the greater the precision you choose, the better your aiming ability will be. For the second star, you should choose one that is quite far from the first, using the same methods. After repeating the process for the third star, the system will align very precisely.

Through the system “auto-two star alignment” It will be possible to align the telescope using a single reference star, which will be available from a menu on the keypad and scrollable using the up and down keys (keys 6 and 9 on the keypad, respectively). After centering it within the finderscope, the telescope will automatically select the second star: the operator will simply have to center it precisely within the eyepiece field.

Using an 8mm eyepiece, I have always obtained a good alignment even with the simple non-automatic two-star method, which differs from the first ones because the amateur astronomer will have to worry about directing the two known stars and completing the various pointing phases.

I'd like to point out that the CPC has been shown to "find" celestial objects even using two very close stars, such as the Pole Star and Mizar in the nearby constellation Ursa Major. It's obviously preferable to choose two "opponent" stars in the sky, as I used to do in July, using Arcturus and Deneb.
There is also single-star alignment, which is undoubtedly not as precise as the other two systems.
I find the option very useful too “Solar System Align” which allows you to align the CPC through a planet, the moon or the sun.

For safety reasons, the sun is not present in the standard display mode. You must therefore exit the solar alignment menu by pressing the "UNDO" key, which functions similarly to the "ESC" key or the "CTRL+Z" system on personal computers.

Pressing the “MENU” button will select the “UTILITIES MENU”, and pressing “ENTER” will then activate the “SUN MENU” option. Finally, it will also be possible to use the sun as an alignment system.
The CPC, pointing at our star, often without paying attention, as mentioned, to the leveling of the tripod, has always shown Venus and Saturn not only in the finder but also in the field of an eyepiece. wide angle  of 30mm that we often use in this kind of daytime research.

The last alignment system is specific for the equatorial plate and is divided into “EQ NORTH ALIGNMENT” and “EQ SOUTH ALIGNMENT” to allow observations in both hemispheres.
First of all, the tube must be positioned so that the “ALT” altitude references match each other. The telescope must then be rotated towards the meridian to begin the actual alignment phase.

The database installed at the heart of the CPC includes over 40,000 objects, excluding 99 customizable free spaces, useful for archiving comets or asteroids. It includes: the NGC catalog, the Messier catalog, the IC catalog, the Caldwell catalog, the Abell catalog, planets, asterisms, constellations, 25 objects for CCD imaging, and the SAO catalog.
If you want to point the telescope at a specific point, you can manually enter the coordinators.
The available speeds are sufficient but not as comprehensive as other motorized systems: they start at 3° per second, up to a speed of 64X after a few options. Perhaps we're being picky, but we would have preferred a few more options, especially during the aiming phase, where 8°-10° per second would be very welcome to speed up the alignment phase.

 

FIELD TEST

Using the CPC for over three months has made me reevaluate our proverbial “indifference” towards telescopes equipped with automatic pointing. While they do not allow the novice to learn to navigate the constellations on his own, they do allow him to observe dozens and dozens of objects in a short time, even under not particularly dark skies.

Searching through the finder, even a 50mm one, is often fruitless due to light pollution. With the CPC, I was able to observe dozens of faint galaxies that, although detectable through star hopping, would take significantly longer to admire. Furthermore, among the various functions provided by the new American telescope, there is the possibility of planning a tour of the available objects; by selecting a specific constellation, you can admire all the objects associated with it in the database.
Even our fellow amateur astronomers have benefited from this technology, returning home satisfied after having observed a greater number of objects than we are used to showing them.

I have explained the alignment methodology several times to people who are unfamiliar with practical astronomy, and they have, in most cases, managed to quickly align the instrument without our assistance.

The keypad is well-sized and easy to use, even with winter gloves. The (adjustable) lighting is good, and the display's letters are large and easy to read. The automatic scrolling system is perhaps a bit slow, but that's a very subjective opinion.
The focusing system knob, which shows the rotation for infinity focus, is more ergonomic than the previous ones, the movements are smooth and precise: even at high magnifications, during the star test we did not notice any movement of the primary lens sufficient to affect the photographic shots.

There is little to say about the optics of the eight-inch Celestron: known for many years by enthusiasts of catadioptric systems, it has proved, in the version with coating Starbright Slightly brighter than the 1996 C8 used as a comparison. I estimated this improvement at about half a magnitude, although I am aware that the Starbright coating improves the sensitivity of CCD cameras more than that of our eyes; the reduction of reflections when observing the moon or the brightest planets and stars also seemed optimal.

The contrast is obviously not as good as that of a refractor, but if properly collimated, it can provide very respectable images. Lunar observation is phenomenal with a binocular splitter from the same company, which you'll find a review of on the pages of Astrotest.it.

The accessories included in the standard package are of reasonable quality, although such an instrument would deserve to be used with more high-performance eyepieces. The use of a dielectric mirror diagonal and various eyepieces is particularly noteworthy. Wide Angle enhanced the observation of deep sky objects, while a series of orthoscopic eyepieces allowed us to observe the most intricate lunar rime, proving once again that an eight-inch Schmidt Cassegrain is capable of perfectly showing the rime within the Vallis Alpes when ideal lighting conditions exist.
Compared to the previous instrument, the images seemed sharper and even during daytime observations we noticed a better representation of the colours. This could be due both to the new treatment and to the intense use of the classic C8 in our possession, so we cannot fully confirm it.

The motors are quite quiet and there are no significant vibrations during object tracking, even at 300x magnification.

If I had to summarize in a few lines I could confirm that the CPC seemed to me to be an intelligently designed instrument, with an excellent automatic aiming system, efficient optical performance and an ideal compromise between robustness and portability.
I would have preferred at least a quality eyepiece, an electric focusing system (also optional), the presence of graduated circles, the positioning of the bubble on the base of the fork, and the adjustment of the secondary mirror via comfortable knobs; despite trying hard, I am unable to find any other defects.

Optical scheme
Schmidt-Cassegrain
Diameter: 203mm
Focal length: 2000mm
Obstruction:0.34
Seeker: 8x50mm
Eyepieces supplied: PL40mm
Other accessories
31.75mm diagonal, visual back, 12V power supply
Weight: 18kg fork + optical tube, 8.5kg tripod

Disclaimer. This review was published on April 16, 2005. Please note that the impressions of use are entirely personal and given by Astrotest testers in complete freedom, without any commercial obligation or relationship and based on proven experience in using these optical instruments. Astrotest does not sell telescopes. For this reason, pFor any information, updates and/or price changes, please contact the official distributor, Auriga Srl, directly.