Sky-Watcher
Skyliner-400P FlexTube SynScan Dobsonian Review
The
400P with FlexTubes retracted.
Ever
since John Dobson came up with the idea of a big Newtonian reflector on a
simple push-pull mount, Dobsonians have been steadily taking over. Once the domain
of ATMs (Amateur Telescope Makers) working in plywood and cardboard, the ‘Dob’
is now a mass-market item. These days you can buy off-the-shelf Dobs from 3” to
30” at every conceivable price point and sophistication level. The reason? Simply that Dobsonians give you
more telescope for less: less complexity, less setup hassle, less cost.
After
several years of modest apertures, following the sale of my 12” Dall-Kirkham, I
found myself craving a light-bucket: something to make the most of my darkish
home skies and let me explore the Deep Sky. I had dreams, I think, of seeing
galactic spirals with my own eyes, structure in emission nebulae, shape and
form in small planetary nebulae.
Personally,
I had only owned a single Dob – an 8” F8 optimised for planets. But this time a
larger instrument made sense, maybe much larger. But I needed something quick
to set-up: given the crummy climate up here anything else wouldn’t get used.
The only thing that made sense was a 12”+ that I could keep in my garage and pull
onto the drive and the only big Dob’ I know that fits (literally) is a
Sky-Watcher FlexTube.
Now
I’m dubious about GOTO on small telescopes – what’s the point of an electronic
system that starts by expecting you to find Capella and Vega, then
offers to show you the Moon? But where GOTO emphatically does make sense is for
big apertures, where it can find things it would take half the night to locate
with a star map and finder. So – sacrilege to traditionalists – I wanted GOTO
on my Dobsonian. The model I set my heart on was simply the biggest I could
afford: the Sky-Watcher Skyliner FlexTube SynScan 400P (more names than
Dumbledore).
This is a big review for a big scope – I
really want to try to get to the bottom of what a scope of this type and size
will show for those, like me, used to much smaller apertures.
Design
and Build
Sky-Watcher’s
Skyliner dobs from 10”-16” all share the same build and the company
black-and-white livery. Sky-Watcher’s largest and smallest Dobsonians have
different design features – a solid tube for the small ones up to 8”, a
truss-tube for the 18”+ models.
As
we’ll see, Skyliner build quality is generally very good by the standards of
mass-market Dobs.
Optics
Like
all Dobs, the heart of the 400P is a Newtonian reflector. The Newtonian is one
of the simplest designs, but also one of the best, if properly made. In this
case it’s a 16”, 406mm mirror operating at F4.4 (1800mm F.L.). By the standards
of other designs (refractors, Maks, SCTs) this sounds super-fast, but these
days F4.4 is pretty slow for Dobsonian optics. Such slowish optics make for a
long tube, but also make it easier to achieve a good optical figure (the
concavity to be ground into the glass is shallower). A slower f-ratio also
means less of the off-axis aberrations (particularly coma – the tendency for
stars to look comet-shaped) that is inherent to the parabolic optics of fast
Newtonians and virtually guarantee Tele Vue sale of another “Parracorr”
corrector under F4.
The
primary mirror of the 400P is parabolic, as it should be, but I’m not sure if
it’s made of Pyrex (the smaller ones claim they are). Does the mirror glass
matter? Well, Pyrex keeps its figure better with temperature changes and so is
preferable to the cheapo option of plate glass. The 400P’s mirror is probably
Pyrex; the mirror has certainly been cast specially for the purpose (not just
cut from a big plate), because it is a conical, centre-mounted design with
radial ribs for stiffness and reduced weight. The whole primary mirror assembly looks very well designed and made.
Collimation is by three easy-to-access knurled
knobs in the primary support. The system is spring-loaded and includes clamps
to lock it up.
The
secondary is supported on a thin-vane spider. This design makes diffraction
spikes longer, but less pronounced and is my preference for visual use as it
degrades the aperture less. Again, quality of the secondary assembly looks
good.
16”
(400mm) primary mirror
Secondary
and thin-vane spider.
Conical
primary mirror with centre-support cell and collimation knobs.
Tube
If
the optics of the 400P are conventional big-Dob fare, the tube design sets it
apart and warrants the ‘FlexTube’ branding. Where others have a solid tube, or
a lattice tube, or a tube with detachable poles (Meade’s big Dobs), the 400P
has a short main tube housing the primary and three sliding struts carrying the
secondary housing. So the secondary packs down onto the short main tube when
it’s not in use.
To
set up and observe, you simply undo three clamps and pull the secondary up to
its (considerable) full height. The struts are stiff, made of metal and don’t
in fact flex very much, contrary to that unfortunate name! But I do find that
to ensure collimation is held, I need to squeeze the last millimetre of travel
out before locking them.
Rubber
‘bumpers’ are attached around the base of the FlexTubes to cushion impact if
you lower the secondary assembly too fast, but they create a ~5cm gap when the
tube is packed – easy ingress for spiders and other undesirables. Some people
remove the bumpers so the upper and lower OTA sections fit tightly together.
Build
quality on the tube is very good. The tube and secondary housing are seamed
aluminium painted metallic black (“Black Diamond”) with heavy cast adapters to
hold the mirror cell and flex-tubes clamps. The tube interior is flat-black
painted, but there are a few exposed screws (attaching the side bearings).
FlexTube
clamp.
Flextube
bumper – mind the gap!
Portability
The
tube and base are certainly heavy, but the whole lot would go in the back of a big
estate car or 4x4 (it fitted in a Mercedes ML, for example), just by detaching
the tube from the base (it attaches via a dovetail incorporated into the
SynScan bearing). This makes the 400P potentially quite portable for star
parties. Putting it together really needs two or three people, however, because
the tube needs to be held horizontal and slotted into the dovetail, whilst
someone inserts the two handled bolts that hold it in place.
I
fixed rubber-tyred castors to the base-board so I can just pull the 400P onto
my drive; handles are thoughtfully attached to the forks to help. Despite its
weight, the 400P is easy to manoeuvre like this, no problem for one person
unless a significant hill is involved. With the FlexTubes retracted it’s an
easy fit through a garage door.
Warning: if you fit castors, choose locking
ones or the base will move when slewing and you won’t get (or retain)
alignment!
Focuser
The
focuser on the 400P is a gem. The smaller Sky-Watcher Dobs have the basic
single speed model, but on the 14” and 16” you get the dual-speed Crayford from
their imaging Newtonians. It has a rather short travel and many eyepieces need
an extension tube, but otherwise it’s exemplary – very smooth, stable and free
of image-shift. It comfortably supports a 13mm Ethos on a long extension tube
without any change in function.
Mount
The
mount is constructed from thick, dense chipboard (not MDF) covered in a white
laminate, edge-trimmed with thick black rubber. It’s a quality material, looks
good and is rugged and robust. Aside from the base-board, which is
pre-assembled, you build the mount yourself from a flat-pack, but unlike many
other commercial Dobs, this one isn’t to be glued or screwed. Instead, it all
fastens together by means of handled bolts that thread into nuts bonded into
the chipboard. This system allows the
mount to be easily and repeatedly disassembled for transport.
One
of the handled bolts that holds the base together.
Detail
of altitude bearings.
Accessories
The
400P comes with a couple of basic Plossls (25mm and 10mm), an extension tube
(which you will need) and a (nominally) 9x50 finder. These are all standard SW
items of decent, but not outstanding, quality.
It’s
not a great finder – a bit dim and narrow of field. A quick exit-pupil check
suggests it may really be stopped-down to something like a 7x42 and the optics
aren’t the brightest or sharpest.
The
scope would benefit from a few good eyepieces (all Tele Vue eyepieces work well
with fast focal ratios, some others don’t) and probably a better finder too.
Synscan
The
‘FlexTube’ system is one innovation in this range of Sky-Watcher Dobsonians,
whilst the Dual-encoder SynScan GOTO system is another. With most GOTO systems,
you have to slew the mount on its motors once you’ve aligned it, you can’t undo
the clamps and slew it by hand or you lose the alignment. The dual-encoders on
the 400P get around this limitation, so you can seamlessly switch between
pushing it around by hand, slewing it with the buttons on the SynScan handset,
or using GOTO.
If
you are familiar with Sky-Watcher’s SynScan GOTO system, then skip this
section. It was new to me, so I thought I’d go through it for those who haven’t
used one before. What follows is the theory; I discuss how it works in practice
in the ‘In Use’ section.
You
power-up the system with a small red switch on the unit and then control it
from the handset. Having entered the date and time (it doesn’t store these) and
location (which it does store), you then begin alignment. This starts with
setting the scope pointing roughly north and level.
The
two basic options, easy align and two-star align, are the same except one
chooses the stars for you whilst the other doesn’t. If you accept the easy
align default, it then prompts you to select the sky quadrant you want to start
in (which you’ll select based on what’s visible). You then have to centre the
first star it chooses. Once you’ve done so, you hit the button and the 400P
slews to the second alignment star. Finally, when you confirm the second star
is centre-field, you finish the alignment, but be aware that take too long over
it and the alignment will fail.
Once
aligned, the 400P can find things from GOTO menus and will also track by
nudging the axes minutely to keep the object centred - a real luxury for owners
of conventional Dobs … in theory.
Sky-Watcher
400P with FlexTubes extended, ready for use.
In
Use – The Night Sky
Once
manoeuvred onto the drive (surprisingly easy with the castors fitted), it’s a
matter of locking the castors (or it will shift when slewing and GOTO won’t
work properly) and raising the secondary assembly on the FlexTubes. At this
point you realise that the 400P is quite tall; most people will need to use a
step-ladder for objects at altitudes over 45°. It’s also obvious that I would
struggle to store such a big ‘scope without the FlexTube system.
Finally,
you remove the dust cover from the primary tube and the ‘shower cap’ from the
secondary and you can start observing.
Cool-down
There
is a lot of glass in that mirror and you know it’s going to take a long time to
cool. It does. On frosty nights it won’t have settled sufficiently to permit
high powers for several hours, even from an unheated garage. Fortunately, for
low-powers on Deep-Sky, it’s quite usable straight away.
Collimation
Being
used to refractors, this is not my favourite word. Fortunately, I never needed
to adjust the secondary and I collimated using the three chrome knobs for the
primary only. I got initial collimation by making a simple Cheshire eyepiece
from a 1.25” dust plug with a hole drilled dead-centre. Perfect alignment is
then done using a de-focused star at high power and just centring the secondary
shadow in the diffraction pattern. No laser required! This is tricky at first,
but easier once you get the hang of it.
If
you make sure you squeeze all the travel out, the FlexTubes don’t alter
collimation much. What’s more, the small changes they do make can equally well
degrade or improve collimation. On one occasion I had got decent but not
perfect alignment, then found that on the next use it was absolutely spot-on!
Collimation-system
close-up.
Star Test
How
much optical quality do you expect from a scope where the whole setup costs
less than a basic set of UK-manufactured 16” mirrors? Well actually the star
test is basically excellent: the same either side of focus, with little or no
residual spherical aberration. However, I found the diffraction rings
indistinct, probably down to thermal turbulence above the mirror, but perhaps
also down to some surface roughness –
not unexpected on a machine ground mirror at this price point.
In
focus things are good, with excellent focus snap and sharp stars. The thin
spider vanes make for long, faint spikes that are not intrusive.
Getting Aligned
The
first thing you are going to need is 12V power to the mount. Probably the best
way is to use some kind of battery unit sitting on the base board between the
forks– there’s plenty of room.
If you use an off-board power supply (mains
and a transformer, or a battery unit) you will run into problems because the
Synscan unit on the fork arm turns with the scope and an external cable soon
gets wrapped around the base and snags.
Alignment
begins with SynScan choosing a bright star and asking you to set the scope onto
it. However I found the range of stars to be much more limited than with say
Meade’s Autostar system: on several occasions it told me there were no bright
stars in my chosen quadrant, though the sky above indicated otherwise! In early
winter, choosing the ‘north western’ quadrant always got me Vega and Capella –
sensible choices.
After
that the fun starts. With a focal length of 1800mm, you need the widest-angle
2” eyepiece you can lay your hands on (and excellent finder alignment) in order
to easily locate and centre the alignment stars. Unfortunately, the included 25mm
Plossl gives a field of just 0.77° with which you will struggle. Even with the
1.5° field of a 55mm Plossl you will be surprised at how hard it is to get that
alignment star in the field of view and then centre it … whilst teetering on a
step-ladder, leaning over to try to look in the straight-through finder. In
theory the dual-encoders mean you could push the scope around to do this, but I
would recommend you use the handset to slew it with the motors, because hidden
in the small print is the fact that the
manual encoders have a lower resolution.
I found I couldn’t get good alignment if I
used manual slewing to get on to the alignment stars.
Various
slew speeds from very slow to remarkably fast can be selected from the ‘Rate’
button by a numbering system (‘fast’ and ‘slow’, or even 400x, 600x etc would
be better), but make sure you select a slow speed or it’ll zoom off and you’ll
be starting again! Incidentally, the speed with which it slews surprised me –
this is a big scope after all and the motors really shift it around.
Once
you’ve got the first alignment star centred and hit the button, it slews
roughly onto the second star. As I said, the motors drive that big OTA around
very rapidly – make sure nothing is in the way! Then you follow the same procedure
to centre the second star with the handset controls and finally push the button
again to confirm you got it centred.
If
you did everything right it should say ‘alignment successful’, if not start
again. But even if it worked, I found that GOTO was then far from reliable,
sometimes finding your chosen object accurately, sometimes not. My drive is
slightly sloping; probably a flatter location would have improved things. Manual slewing with the motors works very
well though, as does the automatic tracking.
Overall, I found the GOTO system awkward,
time-consuming and frustrating. Every observing session deteriorated into a
protracted alignment process. Sadly, I only started liking the 400P and
enjoying the views once I stopped using SynScan.
General Observing Notes
You
don’t have to use SynScan and for quick looks you won’t bother. So the first
thing I noticed is how jerky and sticky the mount is when you push it around
manually. The Dob guys call this ‘stiction’ and the 400P has loads of it, so much
it actually creaks and groans at times and using it at high-powers is
frustrating. One upside of the stiff action is that it doesn’t ‘sag’ under the
weight of a big eyepiece the way some Dobs do.
Another
source of frustration is the finder. Located where it is, with a
straight-through eyepiece, you often have to lean right over to look into it,
clutching a FlexTube for support – precarious stuff. A right-angle finder would
really help. The Sky-Watcher finder-mount system of an o-ring at the front and
two adjustment screws with a spring-loaded plunger at the back makes adjustment
easy. Unfortunately, it’s also easy to knock it out of alignment and you really
need good finder alignment on this scope.
Getting
at the straight-through finder often means a big, risky lean-over. Right-angle
please!
Where
I observe it’s quite dark, so the open tube didn’t give me many problems with
stray light, but I will still buy a shroud for the FlexTubes. The reason is
that, when swapping extension tubes, it would be all too easy to fumble and
drop something onto the mirror. I also found the secondary dewed up quickly,
which a shroud would help prevent.
As
I said, the focuser is excellent, but you do need to swap between extension
tubes to accommodate different eyepieces – quite a hassle perched on a
step-ladder.
The
first eyepiece I tried was a Pentax XW14 and field curvature was quite severe.
Initially, I thought this was the scope, but then remembered that some Pentaxes
are sensitive to short focal ratios. Sure enough when I swapped to Tele Vue
eyepieces (Plossls and Ethos) much of the field curvature disappeared: TV
eyepieces are guaranteed to F4, whereas many others are not.
With
the right eyepiece, field-curvature and off-axis coma are not bad enough to
mandate a corrector (Parracorr or similar), but you do notice them if you are
used to a flat-field refractor. One effect of the coma is that stellar
brightness drops off towards the field stop and this may impact absolute
performance on deep sky.
Deep Sky
Since
a big Dob’ is for most people (and certainly to me) mainly about Deep Sky, I’ll
tackle that first and in the most detail to try to answer the pressing question
for anyone coming from smaller apertures: “what
can I see with this scope?”
M42
The
Orion Nebula is spectacular: generally similar to the view in my 7” refractor,
but much brighter with a touch more colouration. Colour aside, it starts to
approach the detail you get in a basic one-minute DSLR exposure with a smaller
scope. The central area is a
well-defined “square” shape, with a lot of detail in the nebulosity. There is
much more detail in the extended nebulosity than you get with a smaller
aperture, especially in the “cloud bank” in one of the arms. With averted
vision you can detect structure far out in the extended lobe, which is
generally much more visible than with a smaller scope. Lanes in the dark areas
are very well defined, as is the structure in the smaller companion nebula.
M13
This
big globular cluster (an ancient collection of red stars) is very bright and
resolved almost to the core with averted vision.
M57
(the Ring Nebula)
The
Ring is very bright in a 13mm Ethos at 138x, even with direct vision and some
Moonlight. Nebulosity within the ring is easy to see and there is a hint that
the outer rim is redder than the rest. The wispy nebulosity in the corners of
the ‘eye’ is very obvious, as is the overall shape.
M31
(the Andromeda Galaxy)
At
low power with a 2” wide-field eyepiece, M31 shows a very bright core and some
hint of the upper dark lane. The companion galaxy is very obvious. Overall,
though, the view is similar to that through a smaller scope, just brighter.
M33
I
found M33 a touch disappointing – the usual fuzzy blob. The best view I ever
had of this galaxy was with A Tele Vue NP127 and for some reason the Dob’
doesn’t match it: on a fairly clear, dark night I still couldn’t see much
evidence of the spiral arms.
It’s possible you would need a
field-flattener, such as a TeleVue Parracorr, to get the best views of extended
objects.
M45
The
whole Pleiades just fit in the 1.5° field of a 55mm Plossl (the maximum field
available with a 2” eyepiece). The main stars are very bright (almost painfully
so with this aperture), but show no more nebulosity than with a big refractor.
But with tight stellar images and a minimal stray light, it’s a pretty view for
such a big scope (more pleasing than I recall with a big SCT, for example).
M27
The
Dumbbell Nebula is very bright and distinct through the 13mm Ethos and shows
its shape well with direct vision. You can see that the outer part of one lobe
is distinctly brighter than the rest, just like in photos.
M15
Another
globular cluster, but a smaller more compact one than M13. Showed a very bright
core, but not fully resolved at lower power; a mass of stars at higher powers.
M1
The
Crab Nebula is well defined and easy to locate. It shows its shape better than
a through smaller aperture, but has no more structure – it’s still just a fuzz.
Open
Clusters
The
Double Cluster is very bright with a myriad stars, but coma and field-curvature
spoil it a little. The same comments apply to other open clusters I tried,
including the Beehive and those in Auriga.
For
extended star fields generally, open clusters included, I prefer the view
through a flat-field refractor.
Double
stars
I
only tried a few of the usual suspects, but Rigel, Epsilon Lyrae and Castor
were all very easy to split, even without perfect collimation. The basic
physics says bigger apertures win on resolution.
Summary of Deep Sky Performance
Kriege
and Berry’s ‘The Dobsonian Telescope’ gives several size-performance categories
for Dobs:
·
8-10”
·
12-16”
·
17.5-22”
·
25-32”
·
36-40”
What
does this tell us? For one thing, I might think of a 16” as a big scope, but
it’s not by Dob standards. For another, 16” is in the same category as 12” and
14”, so don’t expect a radical upgrade from 12” to 16”.
Overall
you can certainly expect bigger, brighter, deeper views with this aperture than
with a more modest scope (say 7-10”), but whilst some DSOs look much brighter
and more detailed, others don’t. So set
your expectations. A 16” this may be, but don’t expect to look at M33 and see
an obvious spiral galaxy with direct vision, at least not with my skies and
eyes. The Deep Sky views are generally brighter and deeper than with my 7”
refractor, but are emphatically NOT in some completely different league (as I
naively hoped they would be).
The Moon
I
didn’t buy this scope for the Moon, but it delivers really spectacular views of
it (but only once collimated!) – very sharp and full of vivid contrast and
detail. The problem is that even a thin crescent is blindingly bright – you
would need a dense filter for extended Lunar use. The whole Moon just about
fits in the field of view too – a bonus compared to longer focal-length scopes
(like big SCTs) where you are limited to bits and pieces of Luna.
DSLR
snap of Luna with the 400P – it just about fits, is sharp and detailed.
Jupiter
My
initial views of Jupiter with the 400P, during a small-scope test undertaken
recently, were poor, the image mushy and un-sharp. I even said so in my Questar
review. But the problem was one (or a combination) of three factors, not poor optics:
·
Inadequate
cool-down.
·
Poor seeing.
·
Imperfect
collimation.
I
tried again late on an icy night when it had been freezing all day and the
atmosphere was still. This time collimation was spot on in the star test and
the 400P had been sitting on the drive cooling for many hours. With the 13mm
Ethos (again) giving 138x the view of Jupiter was very good indeed: the focus
crisp, the planet sharply defined with all the previous “mushiness” gone.
Lots
of cloud belts were visible, together with detail of a newly-emerged region of
white storms and vortices in the NEB. The belts in general showed more contrast
and colour than they do in a smaller scope. I had a Tele Vue NP-127 tracking
Jupiter on the same night and though the views were generally similar, I
thought the 400P delivered a brighter, more colourful and higher-contrast
image, marred just a little by diffraction spikes.
I
did try a 6mm Ethos (giving 300x), but although the view was acceptable, it was
a little soft. This might well be down to that surface roughness I think I
spotted in the star test.
However, it seems that if you give it plenty
of cool-down and choose the right night, the 400P will deliver impressive
planetary views up to medium powers.
Summary
More
than most reviews, this one comes in two parts: for me the Newtonian reflector
itself is almost all good. Once it’s properly cooled and collimated it offers
wonderful views – of Deep Sky (of course), but of planets too.
The
Sky-Watcher 400P gave views of Jupiter that really surprised me … when properly
cooled and in excellent seeing. Even the Moon is wonderful through the 400P –
detailed and contrasty (but blinding without a filter).
Optical
quality appears good, certainly plenty ‘good enough’ for purpose; it will take
up to 250-300x in good seeing and even splits doubles well. Focus snap is very
good for such a big optic. The focuser lacks travel, but is otherwise really
very good indeed. Yes, it needs a long cool-down, even from my unheated garage,
yes it needs proper collimation; but these comments apply to large Newt’s in
general.
The bottom line is that the optics are frankly
better than you have any right to expect for the price. Overall
performance/price of the OTA is amongst the very best.
Then
there is the mount: just as the OTA is for me virtually all good, so the mount
is virtually all bad. It fits together well with a clever system to make it
easy to tear down and transport, but in operation it’s very flawed. SynScan
takes ages to align well and on mine is temperamental in use; I only started to
like the 400P when I gave up on GOTO. Unfortunately, the push-pull bearings are
a lesson in how Dob bearings go wrong if you ignore conventions on Ebony Star
and PTFE: so full of creaky stiction that finding and tracking at high power
are a nightmare.
If I were to buy again, I would go for the
basic version and spend the SynScan money on a really good, large, wide-field,
illuminated right-angle finder and (re-)learn to find faint fuzzies the hard
way.
The
really intrepid might even consider building a ‘proper’ Dob mount for the OTA,
with big bearings employing ‘ebony star’ or equivalent and quality PTFE.
The
Sky-Watcher 400P OTA is highly recommended: quality optics, effective
’FlexTube’ folding system and superb focuser. Unfortunately, I was much less
impressed by the SynScan mount.
55mm Plossl for scale –
the Sky-Watcher 400P is a big scope!
Buy Sky-Watcher Skyliner-400P FlexTube SynScan GO-TO Dobsonian from Wex here:
