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Sky-Watcher Skymax-127 Maksutov Review

I’d been looking for a planetary/lunar travel scope for ages and had been trying out various models that didn’t quite work: poor optics, not enough aperture, too large, too heavy; always something not quite right. When the Sky-Watcher Skymax-127 came up for sale at a modest price from a friend of a friend I bought it solely with the intention of writing this review. Then, as I started using it, I realised that heremight just be the planetary travel scope I was looking for.

At A Glance

Telescope

Sky-Watcher Skymax-127 Maksutov

Aperture

127 mm

Focal Length

1500 mm

Focal Ratio

F 12

Secondary mirror

42 mm

Central Obstruction

42mm = 33%

Length

320 mm

Weight

3 Kg

 

Design and Build

What is a Mak’ ?

I am going to start with a quick review of the Maksutov optical system, because though the SW 127 is a conventional design, it has some slightly unusual parameters. Skip this section if you aren’t interested.

The ‘Mak’ (or Maksutov-Cassegrain in full) belongs to a class of telescope – the Catadioptric - that combines mirrors and lenses. There are many different types of Catadioptric; the type on test is called a Gregory Maksutov. The design is quite recent in telescope terms – the original design was published (by John Gregory) in 1957 and was rapidly adopted by Questar, but has only become widely available in the last decade or so.

The Gregory Maksutov is basically a Cassegrain (i.e. a folded optical design that focuses the light via a secondary mirror through a hole in the primary mirror) with a corrector plate at the front. In the Gregory design the secondary mirror is just an aluminised spot on the back of the corrector. Why does the Maksutov need a corrector when the Cassegrain gets along nicely with just mirrors? The answer is spherical aberration.

The simplest type of concave mirror, like a shaving mirror, has a curve that is a segment of a sphere: simple because if you just grind away at a piece of glass that’s what you get. Trouble is such a ‘spherical’ mirror doesn‘t focus all the light to a single point. To get around this defect, most Newtonian reflectors have a primary mirror that is a segment of a different type of curve – the parabola – that does focus all the light to a point.

A ‘classical’ Cassegrain also has a parabolic primary, but needs an even more complex curve, a hyperbola, for its secondary. That’s why classical Cassegrains are rare in amateur astronomy – they are hard to make. The Maksutov gets around those complex curves by having a spherical primary mirror corrected by a spherical negative meniscus lens to get all the light to focus at a point: so the Maksutov is a well-corrected telescope that’s relatively easy to make.

The disadvantage is that to keep the central obstruction reasonable, many Maks have a focal ratio of around F15. In recent years, some Maksutovs have appeared that push this down to a more general-purpose F12 – the Skymax-127 is one of those.

Optics

Look into the business end of the Skymax-127  and what you see looks like any Gregory Mak’: a steeply curved and nicely multi-coated corrector plate with a silver spot that is the secondary mirror.

Skymax-127 has excellent coatings, 33% obstruction despite being F12.

John Gregory’s original Maksutov design had a focal ratio of F23. More recent designs typically have focal ratios around F15. One of the reasons I hadn’t considered the little Sky-Watcher as a travel scope is its shortish focal ratio of just under F12 (1500mm focal length). Such a relatively fast focal ratio typically comes with compromises - such as a larger central obstruction – that are not ideal for visual use on the planets. However, the Skymax-127 has a 33% central obstruction, exactly the same as a typical F15 Maskutov. Why does this matter?

Purely from experience I reckon a 33% central obstruction is the maximum for a really good planetary telescope, but it’s a limit supported by theory. For an optically perfect telescope with a 33% obstruction has about the same theoretical performance as an unobstructed telescope (i.e. a refractor) with the ‘diffraction limited’ optics that most manufacturers guarantee as a minimum.

So with a 33% obstruction, the Skymax-127 has the potential to perform really well on planets … as long as the optics are well-made.

There is a ‘but’ though. With F12 optics and a 33% obstruction, there is the possibility of stray light problems (which is why most Maks of this focal length have bigger central obstructions).

Tube

One advantage of the Gregory Maksutov is that you get a short tube and this is where the travel-scope credentials of the little Sky-Watcher start to become apparent. In fact, the Skymax-127’s stubby OTA measures just 32cm (12.5”) long with the visual back removed – about the same length as a Takahashi FS60, one of the very smallest APOs. The tube width is also modest at 145mm. Very compact it certainly is, but the thick corrector plate and rugged build add weight, so the Skymax-127 is one of those telescopes that weighs more than it looks. Even so, it weighs around the 3Kg mark and so is still easy to carry and mount.

Build quality appears to be excellent, with none of the rough edges you got with earlier generations of Synta product. The back plate and lens ring are cast metal and thread on to the seamless tube. Finish is the now-ubiquitous ‘Black Diamond’, which means that the cast parts are in cream powder-coat, the tube in a metallic black. Make no mistake, there is nothing cheap or flimsy about the Skymax-127.

Focuser

The focuser is typical of a Maksutov – a knob protruding from the back-plate moves the primary mirror. This arrangement gets a lot of bad press because in some cases you get a lot of image-shift when changing focus direction. In this case, though, image-shift is minimal and the focuser is smooth and precise without being too light (so an accidental brush won’t change focus). There is plenty of travel to accommodate most eyepiece or imaging needs. The only negative point about the focuser is that when you focus through and then back up, the focus point isn’t quite where it was on the way out. This is not a big problem, but does make finding perfect focus harder than it would otherwise be.

The visual back is 1.25” only and attaches via a standard SCT thread, so you could easily swap on a different one. The standard visual back has set-screws rather than a compression-ring, but on the plus side it has threads for a T-mount, so attaching a DSLR is easy (another of my boxes ticked).

Three small recessed hex-screws in the back-plate, surrounding the visual back, allow for collimation of the primary mirror, but this one is perfectly collimated so I won’t touch them.

Mounting

The Skymax-127 has a Vixen-pattern dovetail attached to the OTA (no rings), so it goes straight on a Sky-Watcher mount; an EQ5 is more than adequate. The rail also has four ¼-20 threads at 10mm spacings, allowing attachment to a large photo tripod in various positions for balance. Strangely, this also means it attaches directly to TeleVue’s Gibraltar 5 – an expensive AltAz mount made for the TeleVue NP-127 (the forks on other TeleVue mounts are too narrow). The Skymax-127 also works well on SW’s own Altaz4 mount or Vixen’s Porta alt-azimuth mount via its standard Vixen dovetail bar.

Skymax-127 has Vixen type dovetail plus ¼-20 threads at 10mm spacing.

Accessories

Standard accessories include a decent finder that is nominally 6x30 and attaches to a standard finder dovetail (so replacement with an RDF would be easy). A basic diagonal and eyepieces – 10mm and 25mm – are also provided, giving sensible powers of 60x and 150x respectively.

The eyepieces are quite acceptable, but would better be replaced with quality Plossls down the line. Given the 1.25”-only visual-back and the long focal length, you need all the field-width you can get, so a 32mm Plossl would be a good choice for extended objects; even so, the maximum field of view is 1°. Of course the advantage of a long focal length is that even simple eyepieces can be employed for higher powers – you don’t need Naglers or the like. In this case a 9mm or 12mm Orthoscopic would be a good choice for the Moon and planets (Plossls have too little eye relief at focal lengths under about 10mm).

A 1.25” prism diagonal is provided. This has a plastic body but good optics and works well.

In Use – Daytime

My usual test of viewing branches against a bright sky at 100x gave a crisp view free from chromatic aberration.

You might be asking ‘why bother, it’s a reflector, of course there’s no CA’ but in fact Maks (like all catadioptrics) do have some false colour from the corrector plate, so it’s worth checking for; but in this case any CA is undetectable visually.

In Use – The Night Sky

Star Test

The star test is excellent, 1/6th wave or better in terms of spherical aberration.

Like any obstructed aperture, it suffers more from poor seeing than a refractor. You can really see this if you get a bright star in focus and watch the way light blurs into the diffraction pattern when the seeing spoils. In a refractor alongside, the star just twinkles.

The Moon

First impressions of the Moon through the Skymax-127 were really impressive. The resolution and overall view was similar to a good 100mm apochromatic refractor and clearly a step up from the 80mm APO set up alongside.

In good seeing an 11 day Moon showed masses of sharp detail. At 139x with an 11mm TV PlosslGassendi revealed its hummocky floor and I could make out some of its rille system. One or two craterlets were clearly visible in Plato. The terraced walls of Tycho were very obvious and I could see lots of small craters on the floor of Clavius.

If this level of performance were available at higher powers too, the Skmax-127 would be a real giant-killer, but at 214x with a 7mm Ortho the view was slightly soft and less enjoyable. By comparison, a fine 100mm refractor would take that kind of power and remain perfectly sharp, but of course a premium 100mm APO would cost four times as much or more.

Overall, the Sky-Watcher Skymax-127 makes a great lunar scope.

Mars

Surprisingly good views of Mars are to be had with the Skymax-127, with no problems of bloating in the red like many semi-APO refractors. Despite being only 7” in size, Mars early in an opposition showed the north polar cap and Mare Acidalium at 214x, a most impressive result for a cheap scope and again a step up from a 3” APO.

Jupiter

Good views of Jupiter were had at 139x with TV 11mm Plossl, including a shadow-transit. Jupiter is a very low-contrast subject, however (especially with the disk free of major storms, as it was on the night of the test) and the Skymax-127 seemed to deliver views perhaps slightly inferior to the 80mm APO set up alongside, which teased out slightly more of the subtle banding in the polar hoods.

The Pleiades

With such a relatively small field of view I couldn’t fit the whole Pleiades in, even with a 32mm Plossl. More worrying were arcing reflections from bright stars near the field edge which suggest a baffling issue. Extended objects also reveal a bit more field curvature than a really well corrected APO would have.

M42

The Great Nebula in Orion gave a surprisingly good view, with lots of whorls in the nebulosity.

Double Stars

Rigel B was not as easy to pick out of the glare as it is with a small APO, but the Double Double split really well.

Astrophotography

The slow focal ratio of any Maksutov means compromises between shutter speed and ISO. The following is about the best I managed on the Moon, without any sharpening or enhancement.

Summary

The ancient Greeks loved to balance an argument: “on the one hand … whilst on the other”. They’d have liked Sky-Watcher’s little Mak’ because it’s like that.

On the one hand … build and optical quality are first rate. I’d estimate that spherical aberration is better than 1/6th PV and the mechanical quality is good too: the focuser is smooth and creates little image shift.

The SW 127 gives very good views of the planets and Moon, especially if the seeing is good, when it will show detail roughly on a par with a 100mm APO if you never go above moderate magnifications (say 100-150x). That’s an exceptional result for a budget scope. It will split doubles quite well too and even gave a good view of M42. Meanwhile, it’s compact and easy to mount.

Whilst on the other … it takes a while to cool like any Mak’ and is surprisingly heavy for its size. A focal length of 1500mm and 1.25” eyepieces mean a narrow field of view. More seriously, it has a stray light problem with bright objects near the field-edge. This isn’t really a problem for the Moon and planets (its intended use), but it does spoil the view a bit if there are bright stars near the field edge. Could this be due to insufficient baffling (that 33% obstruction combined with F12 optics I talked about at the start)?

Though medium power views are really excellent, the Skymax-127 doesn’t take very high magnifications quite as well as a premium optic (e.g. 1/10th PV Orion OMC 140). This is not really a criticism at this price point, but it’s worth noting.

Overall then, the Skymax-127 is an excellent budget planetary and Lunar scope, but the stray light problem limits its use for the deep sky (and certainly as an astrograph). At the prices these go for used, it’s one of the cheapest ways into Lunar and Planetary astronomy.

For my own purposes, the Skymax-127 ticks the boxes as a compact planetary and lunar travel scope.

ü  High optical quality.

ü  Good, crisp, detailed planetary views.

ü  Carry-on compact.

ü  Easy to mount.

ü  Focal length similar to my big APO, so good for my Lunar phases project.

ü  Easyfoolproof DSLR attachment and accurate focusing; no extensions required.

ü  Inexpensive, so I won’t worry about travelling with it.

Highly recommended as a compact budget alternative to a refractor for the Moon and planets, but with a few caveats.

You can buy the Sky-Watcher Skymax-127 as an OTA here: