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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”.