Celestron C5 XLT Review
I really try to remain impartial when reviewing gear. But
it’s no secret that until recently I was pretty down on SCTs. The orange-tube
C8 I bought new in 1981 had never been much good, optically or mechanically. A
friend’s recent Meade 8” had been similarly poor, as had the 12” Meade I used
on a university course. To be honest I thought it was the basic design and
concept – something to do with a biggish central obstruction and fast primary
with moving-mirror focusing. I was wrong.
The problem with the SCTs I had tried, I realised, wasn’t the
fundamental design, but that old quality thing again. The recent, Chinese-made
C8 XLT I reviewed in late 2013 had the same large obstruction, fast primary and
moving-mirror focus as those other SCTs; it was even very slightly out of
collimation. But nonetheless it was an excellent telescope, one I would seriously
consider owning myself: it gave fantastic lunar and planetary views for its
size and cost.
Over the last few years I have been trying out as many
portable lunar and planetary scopes as I can. Given my positive experience with
a modern C8, I was keen to try the smallest SCT Celestron
make, the C5. The C5 has been around since the late Seventies in various forms
and on various fork and equatorial mounts, but here I’m testing the basic OTA
with the latest XLT coatings.
At
A Glance
|
Telescope |
Celestron C5XLT OTA |
|
Aperture |
127mm |
|
Focal
Length |
1250mm |
|
Focal
Ratio |
F 9.8 |
|
Central
Obstruction (incl. holder/baffle) |
48mm
(37.8%) |
|
Length |
280mm w/o
visual back and obj. cap (330 with). |
|
Weight |
~2.7 Kg |
What’s
in the Box?
The C5 OTA was delivered packed in its
(standard) carry case.
Design
and Build
Optics
As I’m sure you know, the C5 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.
The SCT design has advantages in terms
of compactness and light weight. The corrector is thinner than that in a Maksutov, so the C5 is lighter and more back-heavy than the
Skywatcher Skymax-127, for example. It is shorter
too. However, that compactness requires steeply curved optical surfaces – the
primary mirror is F2; compare that with the F3 primary on a Takahashi Mewlon Cassegrain.
Such a fast mirror can be hard to make well and close tolerances are
required if performance is to be good. That thin corrector plate is made of
float glass too, same as your windows, which can have inhomogeneities.
Another disadvantage of the design is
the large central obstruction. In this case the secondary mirror and housing
are 48mm across, equating to almost 38% of the width. A 38% obstruction is at
or beyond the very top end of what is acceptable for visual use and is the
largest I’ve ever reviewed; 33% is generally accepted as the maximum for a
high-performance system (excepting the large obstructions found in some
astrographs, where it matters less than for visual use). A large obstruction
does reduce contrast, but the main effect in my experience is to worsen
performance in poor seeing.
Like most commercial SCTs, the C5 is an
F10 design. That means it has a 1250mm focal length, which is relatively long
and means the maximum field of view with a 1.25” eyepiece like a 32mm Plossl is 1.2°, but in fact the usable field, due to
off-axis aberrations, will be more like 1.0°.
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 1.5°, but again
off-axis aberrations will limit the usable field to perhaps 1.25°.
So
the C5 is never going to give you much more than a 1.25° field of view and this
is a disadvantage of this (or any other SCT) compared to a refractor.
One big advantage of a long focal length, though, is that you can get high
powers without using complex, expensive eyepiece designs – Plossls
and Orthoscopics are adequate.
SCT optical configuration, from
celestron.com
C5 Primary and baffle tube seen through
the almost-invisible XLT-coated corrector.
XLT coated corrector plate and 38%
central obstruction.
StarBright XLT
The first (70s and early 80s) C5s 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.
The C5 is very compact and light weight
for a 5” telescope.
Tube
The tube and castings are completely
different in detail from the original C5, 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 C5 and isn’t up to the
standard of the C8: the OTA is fixed together by screws, some of which aren’t
seated all that well. Overall external build-quality is adequate but no more.
A plastic dew-cap is provided, but
unlike the C8 cap this one is a simple push-fit.
Focuser
C5 Focuser, standard 1.25” and visual
back.
Like many Cassegrains,
the C5 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 C5 (unlike some earlier
versions) suffers from neither of these issues: the focuser is smooth and
precise.
Mounting
The C5 on test has a Vixen dovetail bar
running the length of the OTA. Mounted on my Vixen GP it needed just the
smallest counterweight to balance it. The dovetail would easily allow the C5 to
mount up on any small equatorial or altaz mount
accepting the Vixen dovetail (an EQ5, Porta etc).
The bottom of the bar has two ¼-20
threads that can be used to mount the C5 on a photo tripod (it would have to be
a hefty one); I used them to fix the OTA to a small Losmandy
clamp which then slotted onto the upside-down dovetail atop the rings of my
permanently mounted large refractor.
C5 Vixen-compatible dovetail with handy
¼-20 threads.
The C5 piggy-backed atop my
TMB175/AP1200.
Accessories
The C5 OTA comes with a
permanently-attached vixen dovetail 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.
The C5 comes with a very useful soft
case that is very well padded with dense ethafoam to
provide lots of bump-protection. The case is light and properly carry-on
portable at 39x29x20 cm. In reality this soft case is going to be a more useful
travel case than most hard cases and really adds to the C5s travel-scope
credentials.
Celestron
C5 carry case – ready to go with you.
In
Use – Daytime
The C5 is sold as a spotter and it does
give excellent daytime views up to 100x and beyond. There is no chromatic
aberration worth noting and sharpness is very good centre field (though not at
the edge). I was able to watch birds in the copse 200m away as if through binos in my own garden.
In daytime, best focus is an absolutely
crisp point and I note that the focuser is very precise and has almost no image
shift; what tiny amount there is remains well damped at all times. Best focus
is also exactly the same focusing out and then back, suggesting mirror tilt is
well controlled.
The C5 makes a great budget long-telephoto
lens and I got some crisp photos of a roosting pigeon about 50m away; a
paparazzo could doubtless make good use of the C5 for getting snaps of A-lister
indiscretions.
All
in all, the C5 makes a very fine high-power spotting scope during the daytime.
Snap of a roosting pigeon through the
C5.
In
Use – Astrophotography
For astrophotography, the C5 suffers
from two problems:
1) Off
axis aberrations: field curvature and coma
2) Arcing
reflections from bright stars just out of view
It might do better with a flattener and
some judicious flocking of the baffle or focuser tube.
Below are an
unprocessed image of the Moon (the best I could manage from about sixty shots)
and a misty single frame of M42 to show those arcs from bright stars outside
the field and the comatic stars towards the field
edge (even on an APS-C sensor).
Moon through C5 XLT – unprocessed image
is a bit soft and worsens off-axis, but overall not bad.
M42 through C5 to show the reflection
arcs caused by bright stars just out of view: 45s Fuji XM-1 APS-C at ISO 3200
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. Still, slow cool-down limits the C5’s
use as a quick-look telescope.
I noticed
that the C5 view through a lowish power (32mm plossl) was unpleasant due to secondary mirror shadow, especially
on the Moon.
Star
Test
The star test
was excellent: virtually identical either side of focus with good, even
illumination.
The
Moon
As with other
designs that have a large central obstruction, the C5 suffers when seeing is
poor. On a night with a lot of fine turbulence and the Moon just past 1st
quarter, I had the C5 set up next to Takahashi’s little FS-60Q 60mm
apochromatic refractor, with a 13mm Ethos in the C5 and a 6mm Ethos in the
FS-60Q giving roughly 100x magnification in both.
Not only did
the Tak’ cool completely within 15 minutes from a
warm room, it delivered a much steadier (though also much dimmer) view. I also
noted that under those conditions the little refractor actually resolved more
detail, with Rima Huygens and rima Ariadeus more obvious than through the C5. The FS-60Q also
gave a better view of the whole Moon,
due to a flatter field and less off-axis aberrations generally.
Poor seeing affected
the C5 more than my 175mm refractor, even though sensitivity to seeing
generally worsens with aperture. At 100x, the view through the big refractor
shimmered, but the image remained intact and detailed. Meanwhile the C5 was
mushy, the whole image blurred. Blame that big secondary mirror.
In the early
morning, when seeing is usually better here, I finally got some really stable
conditions. The C5 gave a good view at last at 114x with an 11mm Tele Vue Plossl. The Moon was just a
few days past full and I enjoyed picking out craterlets in Mare Criseum and exploring the deeply-shadowed mountains and embayments around its rim and the drowned craters Lick and
Yerkes. Even so, the view wasn’t as crisp and contrasty
as in a small APO and more unfocussed light was washing out from the bright
limb into what should have been black space.
Jupiter
Unfortunately, Jupiter was the only
planet around during the period of my test.
At a magnification of 114x with an 11mm
TV Plossl, Jupiter was just a mushy ball in mediocre
seeing, but in quite good seeing all the main features were easily visible: the
polar hoods with hints of fine banding, the NEB and SEB (obviously), with
darkened and thickened areas and hints of other cloud belts too. I could just
about pick out the GRS when it came into view.
Overall, Jupiter in good seeing looked
similar to a good 60-70mm APO, though perhaps not quite as crisp and contrasty and again with more stray light around the
planet’s limb.
Deep
Sky
Perhaps
surprisingly, visual deep sky is the C5’s forte. That theoretically narrow
field is really plenty wide enough for most things and the off
axis aberrations still leave a decently flat portion almost a degree in
width. Then again, the XLT coatings and big (for a tiny scope) aperture help to
deliver lots of bright stars and better picked out nebulosity than you get in a
small refractor. The high optical quality also helps out here: stars are nicely
pinpoint and brilliant.
Clusters
M35 looked good with pinpoint stars
off-axis until about 80% field width. At low powers on deep sky you don’t get
the unpleasant secondary-shadowing effect you do with the Moon and so 39x with
a 32mm Plossl gives great views of the deep sky
through the C5.
M36, M37 and M38 in Auriga
These clusters are favourites of mine.
The C5 filled them with plenty of stars, and off axis aberrations were confined
towards the field edge, so didn’t spoil them.
The Pleiades
Unfortunately, the C5 suffers from the
same problem as the Sky-Watcher Sykmax-127 Maksutov I
tested earlier this year: bright stars just outside the field of view form arcs
of reflection that intrude a long way into the field, spoiling the view. You
can clearly see this on the image of M42 below. Even so, The Pleiades looked
good through the C5, with bright stars.
M31
The Andromeda galaxy is big, so
shouldn’t work in something like the C5, but actually the whole central section
was nicely bright and picked out from the star fields in our own galaxy and I
could start to make out the big dark lane to one side.
Bode’s
Nebula
The two contrasting galaxies of Bode’s again looked good through the C5. No longer quite
the faint blobs they are in a small refractor, dark skies and averted vision
gave hints of their true shape and structure.
Globular clusters
The C5 is good for globular clusters.
They benefit from the extra light gathering power and don’t tax the narrowish field of view. M15 looked like a bright fuzzy star
at 39x and started to reveal its components at 114x, even under a murky,
pre-Moon-rise sky.
Doubles
Rigel was split, despite extended
plumes of seeing thrown off from the diffraction rings, but I had trouble with
Epsilon Lyrae: blame that sensitivity to seeing
again.
Nebulae
M42 was surprisingly nice, with detail
and even some colour, but those arcing reflections did spoil the view a bit.
Even so, the XLT coatings and the extra aperture gave the C5 an advantage over
a small refractor here.
The Ring Nebula again worked well
through the C5 – floating in a nicely populous star field, its shape was really
obvious at 39x.
Other brighter planetaries,
like The Crab and the Dumbbell, also looked good through the C5, especially if
you are used to the dim views through a small refractor.
Celestron C5 XLT vs Takahashi FS-60Q
In one way this isn’t fair: the
Takahashi is much more expensive. But it’s an interesting comparison of two
valid but different ways to create a super-compact travel scope with eclipse
potential. A list of main comparison points follows:
·
The C5 is actually much shorter and
more compact, but a kilo heavier
·
The FS-60Q will pack just as small
because it takes apart
·
The longer focal length of the C5 means
greater native image scale, but narrower field
·
The C5 gathers a lot more
light so is (a lot) better for most deep sky objects
·
The C5 should have much better
resolution, but this doesn’t work out in practice
·
The FS-60Q cools much more quickly
·
The FS-60Q deals with poor seeing much
better
·
The FS-60Q is much sharper on axis
·
The C5 suffers from off-axis aberrations,
too much so for serious imaging; the FS-60Q has a very well corrected field
across a full-frame sensor
Overall, the Takahashi is much the
better scope unless visual deep sky is your thing.
Summary
Given my good experience with the C8
XLT a year ago, I really wanted to like the C5; and I fully expected to – it’s
just a smaller C8, after all.
The C5 does have a lot of positives:
it’s very small and light and has excellent optical quality and a very good
focuser. It’s sold as a spotter and indeed gives very good views up to 100x in
the daytime; it makes a great terrestrial telephoto lens too.
Unexpectedly, I didn’t really like the
C5 as a Lunar/planetary scope. Its biggest problem is extreme sensitivity to
seeing due to the 38% central obstruction. On none of the evenings I tested it
could I get a really clean lunar or planetaryimage out of the C5, even though my
refractors were still delivering sharp, detailed views alongside.
The optics weren’t fundamentally faulty or substandard:
the C5 did finally manage clean views in really stable pre-dawn seeing.
The C5 suffers the usual SCT drawbacks
of off-axis coma and field curvature, so for astrophotography you would really
need a flattener. Then there are those arcing reflections. But that said, the C5 does have a real astronomy niche –
for visual deep sky. The combination of lots of aperture for its size, good coatings and enough
well-corrected field of view for all but the most extended objects, makes for
great views of clusters, planetary nebulae and smaller bright galaxies.
Meanwhile, if you do want an
ultra-compact catadioptric for the Moon and planets, I’d go for a Sky-Watcher
Skymax-127: it’s cheaper and gives much better views, though it is a bit bigger
and heavier.
I highly recommend Celestron’s
C5 XLT as a terrestrial spotter or telephoto lens, or for visual use as a
highly-portable deep sky scope. However, it doesn’t deliver the sharpest
planetary views and is sensitive to poor seeing. For the Moon and planets buy a
small APO or Maksutov instead.
Updated by Roger Vine Jan 2018
