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Celestron C8 XLT Review

Celestron C8 XLT Review

An original orange-tube Celestron C8 was my second ‘serious’ scope, bought new in 1980. I had previously owned an excellent Astro Systems 8” Newtonian on a massive equatorial mount, but I rarely used it because it was so time consuming to take out and set up. Unfortunately the C8 didn’t get much use either, because compared to that Newtonian it frankly wasn’t much good.

When I got back into astronomy a dozen years back, I dusted off the C8, used it for a while, then ‘upgraded’ to various refractors. Ten years ago, as part of that process, I wrote my first ever review – a comparison between the C8 and a TMB 100/8 refractor. I spent a lot of time carefully comparing both scopes, but the upshot was that the refractor was streets ahead of the C8 in almost every way.

Since then many people have told me my original C8 must have been a dud, that a typical modern C8 is a much more competitive instrument. So for the tenth anniversary of that first review I decided to re-visit the C8 with an open mind. I obtained a factory-fresh C8 XLT OTA in order both to write this review and to re-run the comparison with a 4” apochromatic refractor.

At A Glance

Telescope

Celestron C8 XLT OTA

Aperture

203.2 mm

Focal Length

2032 mm

Focal Ratio

F 10

Secondary mirror

64mm

Central Obstruction (incl. holder/baffle)

80mm = 35%

Length

432 mm (17”)

Weight

5.67 Kg

 

C8 XLT OTA as delivered.

Design and Build

Optics

As I’m sure you know, the C8 is a Schmidt-Cassegrain Telescope (SCT), a type that belongs to a class of telescopes that uses both refractive and reflective elements in combination – a catadioptric. In this case, the main mirror focuses onto a secondary which in turn reflects the light back through a hole in the primary, so it is indeed a  variant on the Cassegrain;  a corrector plate at the front supports the secondary mirror and corrects aberrations in the primary optics.

SCT optical configuration, from celestron.com

The SCT design has many advantages in terms of compactness and light weight (the corrector is thinner and lighter than that in a Maksutov, for example). However, that compactness requires steeply curved optical surfaces – the primary mirror is F2 - which can be hard to make well and close tolerances are required if performance is to be good.

C8 Primary and baffle tube seen through the almost-transparent XLT-coated corrector.

Another disadvantage of the design is the large central obstruction. In this case the secondary mirror and housing are 80mm across, equating to 35% of the width. Note: Celestron claim 31%, which reflects the diameter of the secondary mirror without its housing and baffle, but it’s the overall 35% obstruction that counts.

A 35% obstruction is at the upper end of what is acceptable for visual use. A large obstruction does reduce contrast, but the main effect in my experience is to worsen performance in poor seeing.

XLT coated corrector plate and 35% central obstruction.

Like most commercial SCTs, the C8 is an F10 design. That means it has a ~2000mm focal length, which is relatively long and means the maximum field of view with a 1.25” eyepiece like a 32mm Plossl is 0.77°, but in fact the usable field, due to off-axis aberrations, will be more like 0.6°.

The baffle tube is just 38mm clear aperture, so fitting a 2” visual back wouldn’t allow maximum benefit from 2” eyepieces with the widest field stops. Theoretically you could get about 1.1°, but again off-axis aberrations will limit the usable field to less than 1°.

So the C8 is never going to give you much more than a 1° field of view.

One big advantage of a long focal length is that you can get high powers without using complex, expensive eyepiece designs – Plossls and Orthoscopics are adequate.

StarBright XLT

Early C8s had an un-coated corrector, but this one has proper multi-coatings (as you can see in the photos). Celestron call this premium coating technology ‘StarBright XLT’, as distinguished from the earlier ‘StarBright’ coatings.

 

StarBright XLT employs multi-layer mirror coatings and multiple layers of magnesium and hafnium fluoride on the corrector plate. Celestron also use a better, more transmissive glass for the corrector than in previous models. Overall, Celestron claim 83.5% transmission for the system, up from 72% for the original StarBright coatings.

Fastar

Fastar compatible secondary. Those screws are your collimation adjustment.

The secondary mirror housing is removable, so you can plug a camera and doublet corrector in at prime focus to image with the focal ratio of the remarkably fast F2 primary mirror. Because of the square function relating f-ratio to exposure time, that means 25x shorter exposures!

Celestron no longer make Fastar hardware, but oddly still support the architecture which is nowadays implemented by third parties like Starizona.

Tube

The C8 is very compact and light weight for an 8” telescope. In fact at just 17” long and weighing 5.6 Kg it is carry-on portable, but those diminutive specs make for easy mounting too.

The tube and castings are completely different in detail from the original C8, but the overall design  remains much the same with solid metal front and rear castings and an aluminium tube. Build quality looks more mass-produced than the original C8, but is actually very good – typical recent Synta in fact.

This version has a grab handle at the back which is very convenient.

A plastic dew-cap is provided, but unlike the original push-fit C8 cap, this one twists and locks in place – a good compromise between push and screw-on types.

Focuser

C8 Focuser, standard 1.25” visual back and grab handle.

Like many Cassegrains, the C8 focuses internally by moving the main mirror. This has advantages over an external focuser – it keeps the tube sealed, the eyepiece in one place and allows lots of focus travel. In many cases, though, it also comes with issues – notably the tendency to cause the image to move when changing focus direction (‘image-shift’) and mirror-tilt that can move the sweet spot when you back out and degrades the image as well. As we will see, this C8 (unlike some earlier versions) suffers from neither of these issues: the focuser is smooth and precise.

Mounting

The C8 on test has a Vixen dovetail bar running the length of the OTA, but you can also get a version with a CGE bar. The bottom of the bar has a number of ¼-20 threads that can be used to attach a dovetail clamp; I used them to fix the OTA to a pair of small Losmandy clamps which then slotted onto the upside-down dovetail atop the rings of my permanently mounted large refractor.

C8 Vixen-compatible dovetail with handy ¼-20 threads.

The C8 is light enough to go on most small mounts. My Vixen GP coped fine, but for the test I mainly used it piggy-backed on my AP1200 for maximum stability.

The C8 piggy-backed atop my TMB175/AP1200.

Accessories

The accessories delivered with the C8.

The C8 OTA comes with a (very gold!) mounting bar, 1.25” visual back, star diagonal, 6x30 finder and a 25mm eyepiece as standard.

The finder is baffled and multi-coated and has good eye relief and a bright field; it’s identical to other Synta 6x30 finders. The eyepiece is a nice multi-coated Celestron E-Lux Plossl and the mirror diagonal looks decent quality too.

There are a huge range of aftermarket accessories for the C8 and I won’t attempt to catalogue them here.

In Use – Astrophotography

I was able to get a some good DSLR snaps of the last-quarter Moon with the C8, but a comparison with images taken in the same session with my 7” APO is revealing. Copernicus is near the optical axis and though it’s sharper and more detailed in the image taken with the refractor, the real difference is seen nearer the field edge: the Clavius/Plato region is incomparably better in the refractor because it controls aberrations far better off-axis.

Moon through C8 XLT – unprocessed with insets of Copernicus and Clavius.

Moon through 7” refractor taken minutes later – unprocessed with insets of Copernicus and Clavius.

The following snap of M37 with an inset enlargement of the stars in the left corner reveals slight miscollimation and substantial off-axis coma, even on the smallish (APS-C 23.6mm x 15.7mm) chip of my Nikon 5100.

M37 through C8 with enlargement of comatic stars in top left corner.

In Use – The Night Sky

General Observing Notes

Cool-down is slower than a typical doublet refractor, but not as lengthy as a Maksutov which has a much thicker corrector plate.

The focuser is very smooth and precise. It has minimal image shift and what there is only manifests itself at high powers. Focus is the same in both directions, unlike some moving-mirror systems where the sweet-spot has moved when you back out. I found the focuser very easy to use when imaging with a DSLR.

Focus is a an absolutely precise point, a real ‘snap’, both visually and when imaging – a sure sign of a quality optic. This is in marked contrast to my own original orange-tube C8 and a Meade LX200 that I tried since, both of which had very mushy, imperfect focus over a wide range of settings. Poor scopes often have imprecise focus because parts of the optic focus light to a slightly different point than others.  

Coma increases progressively off axis, so that by 70% or so stellar images have taken on a distinctly comatic form, even in the flat field of a 13mm Ethos. In this respect the C8 underperforms a good APO (especially a well-corrected triplet).

One problem that everyone knows about with SCTs is dew and indeed on one night the C8 dewed up after just fifteen minutes and meant I had to beg my daughter’s hair-drier. A dew-shield would be your first C8 accessory.

Star Test

The star test was simply excellent – one of the best I have seen in a catadioptric with evenly-illuminated, identical diffraction patterns either side of focus. This explains the snappy focus and much of what follows in this review. The C8 was however slightly out of collimation from the factory.

The high optical quality of this C8 really surprised me having seen a  number of very imperfect SCTs over the years.

The Moon

A 25 day old Moon – a waning crescent -  was a beautiful sight in a 32mm Plossl giving 63x. The whole Moon easily fitted in the field and revealed both the crescent and Earthshine. I did notice a shadow effect from the central obstruction that caused illumination to drop off centre field, but it didn’t detract from the view.

Upping the magnification to 182x with an 11mm Plossl showed numerous shadow-filled craters scattered across bright Oceanus Procellarum and lots of detail in the larger craters like Kepler and Marius. Wrinkle ridges and bright rays showed up to great effect and contrast and sharpness were first rate.

Although the Moon was low in the pre-dawn, seeing was good and I lingered over the Aristarchus Plateau which was just off the terminator. Aristarchus was filled with shadow,  but Schroter’s Valley revealed its internal profile and floor, whilst a lot of detail – domes and ridges – could be made out on the plateau.

In good seeing the C8 delivers wonderful views of the Moon.

The Planets

Venus

Venus low in the late afternoon sky of Christmas Day showed the thinnest sliver of a bright crescent. The C8 showed it to perfection with none of the nasty flare or chromatic aberration you sometimes see.

Mars

Mars was still low and small (perhaps 10” diameter). Even so, at 182x the gibbous disk was sharply defined and both the polar cap and Mare Acidalium were easily seen (I always seem to catch Mars with Mare Acidalium in view for some reason).

Jupiter

Near the zenith and in good seeing, Jupiter revealed a wealth of detail at 181x with an 11mm TV Plossl. Numerous belts and partial belts in the polar regions were seen, as were variations in the width and tone of the NEB and SEB and small dark storms in the polar regions. Though the view was broadly similar, there was more detail on offer than in the 100mm APO set up nearby; in particular the C8 delivered more contrast than the smaller aperture, making detail easier to pick out.

On another occasion, even when low in the early morning pre-dawn sky, Jupiter showed a lot of detail in steady seeing. The Great Red Spot was right on the limb and the thickened region of vortices down-wind from it was easily seen. Nearby, Ganymede was just coming off transit as a perfect disk.

Saturn

Saturn was still very low in the dawn sky at the time I was doing this test, even so in good seeing the view was excellent. The Cassini division, ring shadow and darkened polar hood were easily visible and there was a hint of banding in the hood as well. Again, the view was crisp and sharp with no aberrations and snappy focus at 182x.

Uranus

Herschel’s planet shows a good clean disk at 181x with its typical grey-buff colour.

Overall, the C8 makes a very fine planetary scope in good seeing.

Deep Sky

Clusters

The Pleiades don’t all fit in the field of a 32mm Plossl – the widest field you can get with a 1.25” eyepiece. Whilst viewing them, I noticed some stray-light reflected from bright stars when right on the field stop.

M36, M37 and M38 in Auriga all fitted well in the field, but the stars didn’t have that sparkling appearance they do in a refractor. However, M37 revealed a lot more stars than in a smaller aperture and its characteristic central flower-like shape.

Off-axis aberrations do mar the view of extended objects, such as clusters, compared to a better corrected design.

Globular clusters – like M56 and M15 - look good at medium magnifications in the C8 and their outer parts resolve.

Doubles

Castor was an easy split, but in mediocre seeing stars bloat more than they do in an unobstructed aperture making tight doubles harder to split.

Star colours seem less strong than they do in good refractors, perhaps because the light is distributed into a broader point-spread function. So Albireo looked very bright, but lacked the contrasting blue and orange you get in an APO (even a larger one like the TMB 175).

Nebulae

Planetary nebulae generally look good in the C8, where aperture counts and off-axis aberrations are less important. So the Ring and Dumbbell nebulae (and to some extent the Crab) look brighter and more defined with a clearer shape than they do they do in say a typical small refractor. However, the narrow field and off axis coma mean that the wide-angle ‘space-walk’ view of M57 floating amid the stars that you get with an APO doesn’t quite happen in the C8.

Summary

The people who told me that a modern C8 is a very different (i.e. better) telescope than the classic orange-tube model were right. Whilst the two are very similar in design, none of the components are the same and indeed the current model is made in China by Synta, not in the U.S.. But functionally the main difference is not down to the design of the tube and castings, or even the multi-coated corrector, but is simply a matter of optical and mechanical quality. My original C8 had a pretty crummy star test; the new one’s is virtually perfect. My orange-tube C8 had image shift and mirror tilt issues when focusing; the new version’s focuser is excellent. And in a sense that’s really all you need to know, because the rest just follows on from it. Indeed the Chinese don’t make SCTs like the Americans did … if this sample is anything to go by, they make them much, much better!

So the new C8 gives excellent views of many types of object, but particularly the Moon and planets, which reveal lots of detail through the C8 in good seeing. What’s more, the C8 is very compact, light-weight and easy to mount as well. Even image-shift – long a source of frustration for C8 owners – is very well controlled and the focuser is a pleasure to use, both visually and for imaging.

The C8 does still have some downsides:

·         It cools slowly and the exposed corrector means it dews-up easily.

·         The long focal length and narrow baffle tube limits maximum true field to about a degree.

·         Off-axis aberrations are worse than some other designs: stars are noticeably comatic beyond ~70% field width.

·         The C8 underperforms in poor seeing, noticeably so compared to an unobstructed aperture.

·         Star fields lack that brilliancy you get with a fine APO.

·         Collimation clearly has a tendency to shift in transport. Why can Maks and refractors be made with robust permanent collimation, but not SCTs?

Despite being far more portable and easy to mount and use, it is also true that the C8 still underperforms a good long focus Newtonian of the same aperture (though to a lesser degree now that it has such good optics).

Overall though, the C8 is a marvellous telescope – at what it does, which is to provide serious aperture and resolution in a package so light and portable it would fit in a check-in bag and will  mount on an EQ5. And for that reason the C8, despite its drawbacks, is more relevant today even than it was half a lifetime ago when I bought my first one.

I am surprised to find myself tempted to buy a C8 as a portable planetary and lunar scope. I’m a C8 fan at last! But then again of course it’s really a SkyWatcher now anyway …

The current C8 XLT is very highly recommended for the Moon, planets and small DSOs. The view is often more pleasing through a small refractor, but when the atmosphere is steady the C8 delivers outstanding solar system views for its compact size and modest cost. BUT, I don’t recommend it for star fields and extended nebulae, due to its small field of view, off-axis aberrations and lacklustre stars.