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 here might 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
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.
Optics
I am going to start with a quick review of
the Maksutov optical system, because though the Skymax-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.
In summary, 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.
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 FS-60, 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, at about 3Kg. Even so, it is still
easy to carry and mount.
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 focus 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.
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 didn’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
altazimuth mounts or Vixen’s Porta 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
would be 1°.
The corollary 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.
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 – 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.
With its naturally large image scale, the
Skymax-127 might be good for imaging smaller DSOs such as planetary nebula, but
I didn’t try it.
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 Plossl, Gassendi 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.
Deep Sky
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.
The Great Nebula in Orion gave a surprisingly good
view, with lots of clumps and whorls visible in the nebulosity.
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.
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 Skymax-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.
· High optical quality.
· Good, crisp, detailed planetary views.
· Carry-on compact.
· Easy to mount.
· Focal length similar to my big APO, so good for my
Lunar phases project.
· Easy, foolproof DSLR attachment and
accurate focusing; no extensions required.
·
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: