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

Optics

16” (400mm) primary mirror

Secondary and thin-vane spider.

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 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.

Conical primary mirror with centre-support cell and collimation knobs.

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.

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 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.

FlexTube clamp.

Flextube bumper – mind the gap!

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).

Portability

The tube and base are certainly heavy, but the whole lot would go in the back of a really 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.

Being a Synscan model means a departure from conventional Dob bearings, because both bearings have to incorporate encoders (the GOTO sensors that measure how far the mount moves). The azimuth bearing sandwiched into the baseboard looks conventional and reassuringly large. One altitude bearing is built-in to the SynScan unit, whilst the other is a plastic drum, bolted onto the OTA, supported by a central bolt, that sits on two small free-spinning wheels attached to the mount – no PTFE or laminate here. One thing immediately obvious is that the altitude bearings are a lot smaller than the minimum size conventionally recommended for smooth-running Dob’ bearings.

 

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. 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.

How does the SynScan work?

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.

In Use

Sky-Watcher 400P with FlexTubes extended, ready for use.

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 the SynScan.

Ease of use

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. What’s more, it’s not a great finder to start with – 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.

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.

Observing Notes

General

As I have 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 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 the NP127 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. It 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. The 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.