Sky-Watcher
Evostar 120ED DS-Pro Review
When Sky-Watcher introduced the 120 ED
it was something of a seismic shock to the industry. Here was a larger
apochromat, made with premium glasses, for the price of a tiny premium
refractor. Adding to the shock factor were optics of genuine quality.
I reviewed an early Equinox version and
was really surprised by the big-scope performance on offer, with great
high-power planetary views that smaller apertures can’t give and good deep sky
performance – imaging or visual – too.
Despite the 120 ED being available in this
budget ‘Evostar’ version from the start, I never got around to reviewing one
(even though friends had owned it and reported great results). Now, the cheaper
and more basic Evostar 120 ED DS-Pro is the only model on sale, so I thought it
was high time I tried one.
At
A Glance
|
Telescope |
Sky-Watcher
Evostar 120 ED DS-Pro |
|
Aperture |
120mm |
|
Focal
Length |
900mm |
|
Focal
Ratio |
F7.5 |
|
Length |
92cm incl.
visual back |
|
Weight |
~5.5 Kg incl rings, dovetail |
Data from Me.
What’s
in the Box?
The Evostar
DS-Pro series ship in an aluminium case:
Design
and Build
The Evostar ED range looks just like the SW Evostar
achromats, ED lens and dual-speed focuser aside, with the same build and
finishes.
The more expensive ‘Equinox’ version of the 120ED (previously
reviewed here) shares optics with this Evostar
model, but had a CNC-machined tube, rings and focuser, a sliding dew-shield and
a different finish in gloss piano black and chrome.
The Evostar ED range currently starts with a very reasonably
priced 72mm F5.8. Moving up the line is a puzzlingly more costly 80mm F7.5 and
a 100mm F9 (both of which I have also reviewed, the 80mm only in Equinox form).
The 100mm is a Scope Views best buy, though its price has gone up in recent
years.
Equinox version has a much fancier build and finish.
Optics
The 120mm
lens is an ED glass doublet, containing quality glasses (Ohara FPL-53 for the
crown and a Schott flint). It’s actually air spaced with thin foil spacers, but
these don’t intrude into the lens and are hard to spot.
I had
understood these lenses to be standard Fraunhofer (positive crown up front)
layout, but a laser test casts that in doubt. ED crowns usually scatter less
light than flints, but in this case the front element scatters much more than
the rear (see below). What’s more the rear element appears bi-convex, the front
definitely thicker at the edges. Is this a Steinheil
then, like Takahashi’s FC range? It would seem so ...
Interestingly,
that lens has very thick elements compared to the FC-100, despite being just
slightly faster at F7.5 vs F8 for the Takahashi.
These lenses
used to have the infamous ‘China Green’ coatings that weren’t as transparent as
the best, but this more recent example has different, bluer coatings that
appear very transparent.
The
objective appears to be of the flint-first Steinheil
design.
Tube
The Evostar range is finished in the current ‘Black Diamond’
finish, shared with numerous other SW products. It’s a combination of shiny
metallic black enamel for the tube and knobbly white powder coat for the cast
parts.
It’s a good looking OTA, with flawless
paint and powder-coat. But ultimately it does have a more budget feel than the
Equinox version, with cast parts and rings, rather than Equinox’s CNC. The
Equinox threaded together and had a finer level of finish (albeit a bit flashy
in its glossy black-and-chrome livery, some of which didn’t wear too well).
Whilst Tele Vue and Takahashi also have cast parts, they are also a palpably
finer build quality (as you’d expect for their premium price).
There is an upside to this basic build, though: this Evostar version
is over a kilo lighter than the Equinox and so is stable on smaller mounts, at
least for visual use.
The tube interior is properly flat-black painted and
knife-edge baffled against stray-light, with no cut-corners.
Crayford focuser has a cast body and a locking dual-speed
pinion.
Focuser
The Evostar
models have a Crayford focuser with a cast body and drawtube, finished in
off-white powder coat, unlike the Equinox models that had black CNC focusers.
The Evostar DS-Pro focuser has a black inner fine focus knob on the right side
as standard. The focuser attaches with a push fit and screws, a more basic
approach than the thread-on focuser employed by the Equinox version (and many
high-end scopes like Takahashis too).
The only
lock is on the pinion and it is pretty ineffective –the focuser seems either
locked up solid or free to rack out at will. I noted the same issue on the Evostar
100 ED.
The focuser
has enormous travel, about 138mm. This is great for imaging, because you
typically don’t need an extension. However, it means that there is barely
enough in-focus travel for a 2” diagonal and some eyepieces with long (or
effectively long) focal lengths.
Early bench
tests suggested that the long focuser tube effectively vignettes the lens to
about 114mm aperture. I can’t confirm this, but could believe it based on the
length of that focuser tube and the scope’s performance.
The focuser
doesn’t suffer from significant image shift when changing direction, but images
suggest that the weight of a DSLR causes some droop in the focusing tube when
extended (see below). What’s more, the focuser can rack out on its own or lose
traction under heavy loads – typical downsides of Crayford in general.
The fine
focus is accurate, if a little spongy, but the black fine-focus knob is too
close to the coarse-focus knob in size and makes the coarse-focus oddly hard to
use on that side because your fingers constantly snag the fine-focus knob.
All
in all, the focuser is fine for visual use with regular eyepieces like Plössls,
but can lose grip with heavy wide-angle eyepieces or imaging gear. Lack of
in-focus travel will also limit usability with some binoviewers.
Drawtube has
very long travel: convenient but flawed (note: Baader Clicklock shown is not
the standard visual back).
Mounting
One big
advantage of this telescope is its relatively small size and low weight (see
comparison below) for its aperture. In practice this means you can get away
with a smaller mount than you would for many ~120mm refractors.
My Vixen SX2
balances the 120 ED with its lightest counterweight and once mounted it feels
rock solid and vibes are low. Compare the WO-123 triplet, which needed more
counterweights and felt quite close to what the SX2 would take when fully
equipped.
SW supply a
pair of lightweight rings with one threaded hole each, but note that these have
metric threads, not the ¼-20 photo-threads that many use.
Evostar
120ED isn’t much larger than this 100mm Takahashi FC-100.
Accessories
Currently, the Evostar ED DS-Pro
range ship with an accessory bundle containing:
·
OTA
·
Rings
·
Vixen-pattern
plate
·
9x50
Finder
·
2”
Diagonal
·
28mm
Eyepiece
·
Aluminium
carry case
The 9x50 finder is the standard
Sky-Watcher unit used on most of their telescopes. It has a narrower field and
less eye relief than the best, but it is of good optical quality and is very
usable, with a sharp bright field.
The 28mm LET eyepiece looks a bit like
a rip-off Pentax XW. But … big surprise when you look through it – this is an excellent eyepiece. It’s a
simple three element design (modified Kellner), but is very sharp across 80% of
the field and commendably free from chromatic aberration. As you’d expect from
a simple design, it’s bright too. With a wide 31mm field stop, top-notch
coatings and a comfortable level of eye relief and no blackout problems, this
is a very useful basic eyepiece.
The supplied 2” diagonal has a
dielectric multi-layer mirror like expensive diagonals from Tele Vue and AP.
Why does this matter? Dielectric mirrors are supposed to be more reflective and
resistant to sleeks after cleaning. The only disadvantage with this SW diagonal
is that the body is not milled from one block, so the thread-on barrel could
unscrew with a heavy eyepiece: always securely lock your eyepiece with this
kind of diagonal!
The aluminium carry case (see above) is
the same pattern as other Sky-Watchers’ and is a good quality hard case, lined with cut ethafoam. It’s good looking and
functional, but quite large and heavy.
Generous standard accessories really
add value, especially for beginners who might not have many or all of these
essential items.
One non-standard accessory you
might want is the 0.85x reducer:
In
Use – Daytime
The 120 ED is
obviously too large for spotting, but I find viewing silhouetted branches at
100x plus a very good way of judging visible false colour. Doing this reveals
only traces of false colour in focus and just a tint of purple and green either
side and prime focus images are the same. This is truly excellent performance
for a large doublet.
Telephoto images
show good coverage and field flatness on full frame. As usual, all images are
straight from the camera, just reduced in size.
100% crop of branches
against a bright sky show modest false colour for a doublet of this size.
Prime focus
image with Canon EOS6D MkII full frame DSLR: coverage
and flatness are good for a doublet.
In
Use – Astrophotography
Good and bad
here. On the good side, for APS-C sized chips at least, the 120ED has a natively
flat field and good coverage for a doublet. Even at full-frame, coverage isn’t
bad and off-axis aberrations reasonable without a flattener. Violet bloat on
O-A stars is well controlled, thanks to the low false colour levels noted
earlier.
The Evostar 120 ED is easy to use for basic imaging
too, because you don’t even need an extension to reach focus with a DSLR. But
as mentioned before, the focuser has too much slop (you can see this in the
unequally distorted stars from top to bottom in the image below) and tends to
lose traction with heavier cameras.
Experience with the Evostar 100 ED suggests the 0.85x
reducer could yield excellent subs, but you’d need to use an extension to
reduce focuser travel (and hence slop) and lock it solid to avoid slipping.
Like most good apochromatic refractors, the flat field and sharp
optics of the 120ED produce great images of the Moon, with lots of detail
limb-to-limb even though the image you see wasn’t taken in the best seeing.
Pleaides with Evostar 120ED, Canon
EOS6D MkII 60s ISO 3200 – excellent coverage and
flatness, but some focus tube droop evident.
In
Use – Observing the Night Sky
General
Observing Notes
Its compact
size and light weight make the Evostar 120 ED easier to use than many
refractors of this size (including Sky-Watcher’s own Esprit 120 triplet) and
mean reduced shakes on a smaller mount. No, it isn’t a Takahashi FS-128 or AP
130 EDT, but it is quicker to mount up and needs less caution moving from
object to object and swapping sides on the mount. Mounted on my Vixen SX2 it’s
no problem to deploy and move around as a unit, with no need to mess about with
dovetail clamps or rings ... That’s a big advantage in the real world, in the
dark and cold with numb fingers when you’re tired.
So the Evostar 120 ED combines much of the ease of use and universally
good views that we all know and love from smaller apo’s, but with a much
broader range of capabilities, including smaller DSOs and real planetary and
lunar detail.
Cool
Down
The Evostar 120ED took ages to cool – over an hour - during
which time there was substantial astigmatism in the star test, Mars was soft at
high power and I struggled to split Rigel. Even leaving it in its open case
didn’t help much. Compare the Takahashi FC-100 Classic which cooled to produce
a perfect star test in 20 minutes on the same night.
Partially
this is just down to the greater volume of trapped air (obviously in cubic
proportion to length and diameter) and the mass of the lens, which has very
thick elements compared to the Takahashi. Indeed, after an hour warm air was
still flowing from the visual back. But it could also be partially due to the
lack of a temperature compensating lens cell, or even a quirk with this
particular lens.
The
slow cool-down would limit the 120ED Pro’s usefulness for grab-n-go somewhat.
Star
Test
Once fully
cooled (but see note on cooldown above), the star test at 200x was excellent, with
very similar, evenly illuminated diffraction rings either side of focus and not
much false colour even on Rigel or Vega.
The
Moon
Typical of
larger refractors, the Moon showed a lot of detail but still with a wide enough
field to fit in the whole disk, sharp from limb to limb (something reflective
optics often do less well). This ability to view the whole Moon, but with a
level of detail that reveals myriads of craterlets and fine rilles, might
surprise a newcomer to a larger refractor – a real ‘oh wow!’ moment I’ve heard
many times at star parties and outreach events.
I had great
views with a 5mm Nagler T6 giving 180x. At that power, the 120 ED gave a
slightly warmer toned image and with a little more false
colour in shadows than the Takahashi fluorite doublet I was using alongside,
but about the same level of false colour focusing through the limb (i.e. very
little).
On a
ten-day-old waxing gibbous Moon, I enjoyed exploring the southern highlands
around Clavius in detail, the slumped walls and central peaks of Tycho and
Copernicus. Plato’s dark floor was peppered with tiny craterlets and the nearby
Straight and Tenerife Ranges looked much more like smoothly-contoured lunar mountains
than at lower resolution and magnification.
Mars
One reason I bought this Evostar 120 ED when I did was for quick looks at Mars late in an opposition
year when its size had dropped below that where 4” refractors show much detail
in the planet’s subtle, low-contrast albedo markings.
Mars at 10.8” was very soft with a 5mm Nagler T6 until cooled (see
above). But even after an hour and more, the orange planet sloughed off plumes
of orange light in focus during poor seeing. Attempts at focusing this orange
flare away make the planet itself appear soft and slightly out of focus. This
focusing softness or vagueness on Mars is something I’ve seen with many other
ED doublets and is one reason I prefer fluorite doublets and triplets for Mars
(they don’t do this), but it’s worse than usual here, esp. when not quite fully
cooled. I also found it harder to make out low contrast albedo detail than with
a 100mm fluorite doublet on the same night.
My guess (and it’s only that) is that the price you’re paying for
excellent correction (for a 120mm doublet) across much of the visible spectrum,
is spherochromatism in the red – effectively the lens is below diffraction
limited for spherical aberration at these longer visual wavelengths.
However, when fully cooled it happily took 300x with a 3mm Nagler Zoom to
show a clean and sharp gibbous planet with some albedo markings, albeit still
sloughing that orange flare.
Saturn
I had an
excellent view of Saturn through the 120ED, with the greyish polar hood and
Cassini Division clearly visible in a way they aren’t through smaller
refractors. On Saturn, false colour wasn’t a problem, though I noted some minor
colour either side of focus.
Deep
Sky
Epsilon Lyrae (the Double Double) was a huge split at 180x, with
yawning black chasm between the components of both doubles – aperture wins for
resolution. Rigel was also an easy split, even low in turbulent seeing, with
the much dimmer companion floating free and bright from the diffraction rings
of the main star in a way you don’t get with smaller apertures.
Viewing some
of the numerous open clusters in Auriga with a 19mm Panoptic, I found myself
enjoying tiny NGC 1907 next to the larger Starfish Cluster that I usually
concentrate on. At this aperture, the smaller cluster was properly resolved
where it’s just a smudge in sub-100mm refractors.
I compared
M42 to the view minutes earlier through a TV-85. The greater aperture really
showed, with much more reach to the arcing ‘arms’ and more structure in the
boxy inner nebula. That inner region was also of a noticeably whiter-bluer hue,
the outer arms a darker tone (if not actually reddish, the way it is in
images).
The Ring
Nebula at 100x with a 9mm Nagler showed as a dense flattened smoke ring even
with direct vision.
Puzzlingly,
though, the Pleaides looked no different/better than through the smaller scope.
In fact, if anything they scintillated less dazzlingly, perhaps because the
less perfect optics of the bigger scope puts less starlight into the Airy disk
and a bit more into the diffraction rings.
Summary
The
Sky-Watcher Evostar 120 ED DS-Pro retains the near-unique status it had
on introduction a decade ago – a big apochromat for a modest outlay. Unique,
too, because it is so light and small for its aperture. The competition are
mostly heavier, slower cooling triplets.
In many ways,
performance is good. False colour just isn’t the problem you expect it could be
– levels are generally very low, similar to a 100mm fluorite doublet, thanks to the premium glasses in its
objective.
It resolves
lunar detail and double stars as well as the very finest 100mm apo’s and
the greater light gathering makes for more satisfying DSO views – more
nebulosity, fainter stars, easier with direct vision. For imaging, coverage and
field flatness are good for a doublet, even without a reducer. Violet bloat is well
controlled for a doublet of this aperture too. All good then? Not quite ...
In fact,
though I like the Evostar 120 ED, for
various reasons I preferred the Equinox version I tested previously. Why?
Partly, the CNC build quality of the Equinox; but I felt it performed slightly
better too. This may suggest lens cherry picking for the Equinox, or just
sample variation.
On planets
the Evostar
120ED gave great views of Saturn but rather less so of
Mars, especially in the mediocre seeing which is common here: often hard to get best focus,
lacking a bit of contrast and bleeding scattered light into the turbulence.
This is likely due to spherochromatism, the price for such excellent correction at shorter wavelengths for imaging.
Dedicated planetary observers might consider the 100ED F9 version instead.
Though the
star test was good, overall performance doesn’t equal a premium optic like the
similar-aperture WO-123, suggesting perhaps early reports of vignetting were
correct. Vignetting could account for that performance difference, but for the
exceptionally low false colour and off-axis aberrations too, if the outer part
of the lens were effectively masked off.
Then there is
the focuser: it has masses of travel but loses traction with heavy loads and
lacks in focus. On this example, cool-down time was very long, possibly because
of a basic lens cell.
Even if the
aperture really is vignetted to something like 114mm F8, the Evostar 120 ED is still a great-value scope in many ways, but
its price has increased since introduction (nearly doubled in fact), which
leaves it closer to the price of the cheapest Takahashi FC-100DC. And though the
120 ED Pro does come with lots of accessories and offers better performance for
many things, it cools much more slowly and is more compromised at high powers.
Which you choose might depend on your use profile and preferences.
The Sky-Watcher Evostar 120
ED DS-Pro still offers big apochromat performance for a modest price and is
still highly recommended, but it isn’t the screaming bargain (new) it once was.
And be prepared to upgrade the focuser for ‘serious’ imaging.
Buy the 100mm F9 version, the Evostar-100ED DS-Pro, from Wex here:
