Sky-Watcher (Helios) Evostar 120
Review
Sky-Watcher’s Evostar achromats
have become so commonplace that nobody seems to notice them, let alone review
them. Sky-Watcher has moved on to more
sophisticated and upmarket designs: APOs and big Dobs’
and imaging Newtonians.
Meanwhile the Evostar has changed
little in a decade and I wonder how long it can survive as Sky-Watcher pursue
more lucrative niches. So I reckoned that this Ford
Mondeo of telescopes – worthy but so familiar as to be almost invisible –
deserved a fresh look.
The range of long focal-length achromatic refractors - 90mm,
102mm, 120mm and 150mm - made by the Chinese firm Synta,
has been around longer than ‘Evostar’ or even ‘Sky-Watcher’.
My ‘Evostar’ is labelled as ‘Helios’ – the brand name
that Synta used before Sky-Watcher; the scope is the
same though and has appeared under a host of different brands over the years,
where the main difference has been colour (remember that Bresser
orange?).
I recently reviewed another Synta
achromat - the StarTravel 102, an F5 relative of an Evostar - and was impressed. Here I’m curious to find out
what an extra three f-stops does to change things.
At
A Glance
|
Telescope |
Sky-Watcher
(Helios) Evostar 120 Achromat |
|
Aperture |
120mm |
|
Focal
Length |
1000mm |
|
Focal
Ratio |
F8.3 |
|
Length |
900mm |
|
Weight |
~4 Kg |
Data from Me/SW.
Design and Build
Years ago, a friend bought one of the 150mm Synta achromats, the biggest of the range. It really was a
monster. So my initial response to the next one down,
the 120mm, was of surprise - at how small and light it was. This, let’s
remember is a refractor pushing into the 5” class, bigger than most of the
‘observatory’ achromats on sale in the 60’s.
The 120mm doublet and baffled tube behind it.
Optics
That big achromatic doublet objective lens has a focal length
of 1000mm, meaning an F-ratio of 8.33 for the 120mm aperture. The 1.22D rule
for false colour suggests a lens of this aperture needs to be about F15 for
good correction, so we can expect that the Evostar will
show noticeable false-colour, purple fringing, on the Moon, bright planets and stars.
The objective has no real cell – the foil-spaced lens
elements just push into the lens ring and are secured by a plastic ring and
five screws. Later models, I am told, have a proper adjustable cell.
The lens looks reasonably well coated on the front surface,
but it’s not the kind of deep multi-coating you get on a premium APO.
Tube
The OTA (minus its dewshield, which
is a push-fit) arrived in a little box not much longer than the one for my FS-78
and perhaps a quarter the size of the giant crate for Takahashi’s five-inch F8
doublet refractor, the FS-128.
Despite the focal length of a metre, the bare Evostar 120 OTA is just 90cm long and weighs perhaps four
kilos. This is good news: the 120mm Evostar is about
the same size and weighs less than the larger 4” apochromats.
Light-weight it might be, but overall fit and finish is
generally good, with the tube painted in a deep and glossy black, the rings and
focuser a gunmetal grey. However, the rings are a bit flimsy in typical Sky-Watcher
style and the dew-shield isn’t as well finished as the tube (but these are
minor grumbles). The tube internals are properly matte-painted and there are
knife-edge baffles.
Compact and light: the Evostar 120
is much more manageable than the 150. Longer but lighter than Takahashi FS-102.
Focuser
The Evostar is compact for its
optical spec’, but of course you can make any refractor any length you like –
just require the user to add in more extension tubes to reach focus. It’s a
common trick, making the scope short for easy transport, but that can leave a
DSLR or wide-field eyepiece hanging on by a row of set screws. The Evostar isn’t like that. Sky-Watcher have thoughtfully
provided a very long travel focuser that brings my DSLR (and any eyepiece) to
focus without an extension.
Talking of the focuser, it’s one that appears on many older
SW scopes and looks like an exact copy of older Vixen rack and pinion units.
The body and 2” focuser tube are all cast metal, the
body in hammered-finish enamel, the tube painted silver. The action is a bit
stiff, but this example is quite smooth and free from play or slop.
Lots of focus travel mean no extension tubes required.
The Evostar can go on a light-medium
mount like the Vixen GP shown here.
Mounting
It’s a long focal length 4.5” refractor, but the Evostar 120 is comfortably carried by an EQ5 or Vixen GP.
In contrast, the much bigger 150mm Evostar really pushes
an EQ6 beyond its limits.
In Use – The Night Sky
The big question for most people when it comes to larger
achromats is ‘how bad is the false colour?’. The reason is that false colour
(chromatic aberration) gets worse with larger apertures and shorter focal
lengths.
The answer is that the Evostar does
generate quite a bit of CA, just as the 1.22D formula predicts. The Moon has a
colourful limb and bright planets like Jupiter show a coloured halo,
particularly when focusing in or out. However, this false colour never becomes
a real high-power problem in the way it does with one of the Sky-Watcher Startravel F5 achromats, which can cover the Moon in a veil
of unfocussed purple at 100x. Whether you find the CA objectionable will depend
on your direction of arrival. If you are used to reflectors you may find it
off-putting, otherwise probably not.
Some owners reduce the CA by using a specialist
‘fringe-killer’ filter, but this would increase the cost of the scope by
£50-100 and I can’t comment on its effectiveness.
The Moon
Once you get used to the CA, the Evostar
delivers good views. The Moon is crisp and shows a lot more detail than a
typical 60-80mm starter-scope at 100x with the supplied 10mm silver-top Plossl (a better Plossl,
incidentally, than you get with more recent Evostars).
Prime focus snap of twilight Moon through the Evostar 120 with a DSLR. Note the false colour around the
limb.
Jupiter
Focus snap seems good and again Jupiter is a more engaging
sight than in a smaller refractor. I am interested to note that I can see much
more cloud-belt detail than with a 60mm-80mm premium APO: with the Evostar it’s possible to see several minor belts, the darker
polar region, a couple of white spots and a small dark storm. Contrast delivery
seems good. However, I do notice that the scope doesn’t take high
magnifications as well as a good APO of this size.
The reason doesn’t take long to find – the star test isn’t
particularly good. Not terrible, but not as good as that Startravel
102 I tested recently. This doesn’t mean that Star travels have better optics
in general – it just highlights the kind of variation between samples you get
with all telescopes.
Deep Sky
Most people would, I suspect, buy the Evostar
as a budget Lunar and planetary scope, but it works well for visual deep sky
too. With 120mm of aperture, open clusters, like those in Auriga, swarm with
faint stars. M45 shows off the veil of remnant nebulosity around its young, hot
stars. Smaller, fainter DSOs stand out better and show more detail than in a
smaller refractor, so that the Crab and Dumbbell show their characteristic
shapes and globular clusters reveal themselves as a myriad
tiny stars rather than a fuzzy blob.
This is a cheap scope, so it’s all
too easy to forget that aperture wins on deep sky (and to some extent on Moon
and planets as well).
Summary
The Evostar 120mm has the advantage
of a large aperture (for a refractor) without unmanageable size and heft. You
can mount it successfully on an Eq5. It may not have the finest optical quality
or the best correction for chromatic aberration, but it still shows a lot more
detail on most objects than a 3”-class refractor.
Suitor, in his book “Star Testing Astronomical Telescopes”,
writes that a refractor can ‘get away’ with a lower standard of optical quality
than an obstructed scope. The Evostar reinforces this
point perfectly.
I recently had a Maksutov of
similar aperture and mediocre optical quality pass through my hands. The Maksutov was fairly compromised and didn’t deliver great
views. The Evostar still works well, though, showing
more than a perfect smaller scope could, simply because it’s a larger aperture.
Then again, if you chanced on an example with a really fine lens (like the
Sky-Watcher Startravel I tested recently) it would
make an impressive and versatile scope indeed.
It’s a lesson that someone looking to see a lot on a budget
should learn. For the modest price of the Evostar 120
you could buy a tiny, cheap APO. It would be much more portable, but it
wouldn’t deliver such good views. But then those comments would apply equally
to a good 6” F8 Newtonian for similar cost. The choice is yours.
Recommended as a capable budget scope that will show you more
than a smaller Evostar, but is much more manageable
than the 6”.
Buy Sky-Watcher Evostar-120 Achromatic Refractor OTA from Wex here:
