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 fifteen years
back, I dusted off the C8, used it for a while, then ‘upgraded’ to various
refractors. 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 telescope. So, I
obtained a factory-fresh C8 XLT OTA
for the tenth anniversary of that first review in order to re-visit the C8 with
an open mind.
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 |
Data from Celestron.
What’s
In the Box?
I love a factory-fresh unboxing!
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.
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.
A further negative point for all
commercial SCTs (yes, even Takahashi’s, apparently) is that the corrector is
made of float glass – same stuff as your window panes. That means the potential
for striae and inhomogeneities varying from sample to
sample.
C8 Primary and baffle tube seen through
the almost-transparent XLT-coated corrector.
XLT coated corrector plate and 35%
central obstruction. The corrector, like all SCTs’, is float-glass.
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, but still pretty low compared to the ~97% you might expect from a
modern refractor.
Fastar
compatible secondary. Those screws are your collimation adjustment.
Fastar
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.
C8 Focuser, standard 1.25” visual back
and grab handle.
Focuser
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.
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.
C8 Vixen-compatible dovetail with handy
¼-20 threads.
The C8 piggy-backed atop my
TMB175/AP1200.
The accessories delivered with the C8.
Accessories
The C8 OTA comes with a (very gold!) Vixen/CG5mounting
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 some DSLR snaps of
the last-quarter Moon with the C8 (albeit in crummy seeing), but a comparison
with images taken in the same session with my 7” APO is revealing, despite the
turbulence. 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.
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. You would need a flattener for deep sky imaging (or the
more expensive ‘Edge’ version).
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.
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 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 spherical
aberration makes parts of the optic focus light to a slightly different point
than others.
Aberrations increase progressively off
axis, so that by ~70% field width stars 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.
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
to be aware of:
1. It cools slowly and the
exposed corrector dews-up easily.
2. The long focal length and
narrow baffle tube limits maximum true field to about a degree.
3. Off-axis aberrations are
worse than some other designs: stars are noticeably comatic
beyond ~70% field width.
4. You’d need the ‘Edge’
version, or at least a flattener for deep sky imaging.
5. The C8 underperforms in
poor seeing, noticeably so compared to an unobstructed aperture.
6. Star fields lack that
brilliancy you get with a fine APO.
7. Collimation clearly has a
tendency to shift in transport.
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.
The current C8 XLT is very highly recommended
for the Moon, planets and small DSOs. 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.
Updated by Roger Vine 2018.
