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Sky-Watcher StarTravel 102 Review

I’m always moaning how difficult it is to find a compact, lightweight 4” APO. Astro-Physics used to make one, but it’s a collector’s item now. Takahashi made one too (the Sky90), but it’s a flawed beast with collimation problems and is very expensive.

The reason for my moaning is that 4” is a magic size – where ‘big’ refractors begin. A 4” APO shows a lot more than a 3” one – real Lunar and planetary detail – and works much better for astrophotography and deep sky because it captures so much more light (over 60% more).

Now it happens that Sky-Watcher make a 4” refractor - the StarTravel 102 - which is everything I’d like in a portable refractor: small, lightweight, a doublet for quick cooldown, 2” focuser, full 4” aperture, fast F4.9 focal ratio; it’s cheap-as-chips as well (less than the finder on a Sky-90, really).

What’s the catch? It’s an achromat. Now that 1.22D theory I often quote suggests that the shortest focal length that will be reasonably corrected for false colour in a 102mm achromat is F12. So you’d think that the F5 StarTravel would be a nightmare of chromatic aberration and unusable except at low powers. But is that true? Read on to find out.

At A Glance

Telescope

Sky-Watcher Star Travel 102

Aperture

102mm

Focal Length

500mm

Focal Ratio

F4.9

Length

~550mm

Weight

~3 Kg

 

Design and Build

The Start Travel 102 is conventional SkyWatcher, with the tube design and focuser they’ve used for a decade now. Nothing wrong with it either – utilitarian but highly functional.

Optics

The objective is an achromatic (Fraunhofer) doublet. So far, so normal. But Interestingly, Sky-Watcher claim to use a wide airspace between elements in these lenses to help reduce chromatic aberration. In case you thought that’s a marketing trick, it isn’t. Wide airspaces between lens elements do allow for more freedom in correcting aberrations and are used in the Takahashi TOA, FSQ and Sky90 designs for that purpose (Roland Christen of AP reckons the TOA to be the best corrected APO of them all for this reason). One issue with this type of lens is that it can be collimation-sensitive (I have seen a couple of out-of-collimation Sky90s) and the StarTravel lens cell is not adjustable. While we’re on the subject, the lens doesn’t have the kind of ‘proper’ cell that you get with some refractors, but once again that can be good for ruggedness (AP’s Traveller takes a similar approach, as do most Tele Vue refractors).

The optics are multi-coated - from what I can see from the reflections, on all surfaces - but the coatings aren’t as good as a premium APO’s.

The StarTravel objective fills the tube and employs a special design to reduce CA.

Tube

Mine is one of the older blue tube designs. Newer models have a swanky white and metallic black livery that looks well and matches all their other scopes and that Sky-Watcher call ‘Black Diamond’; finish aside, it’s the same scope.

The OTA is very compact and lightweight because they use all the available space – the objective fills the tube. Finish is good and is all metal. Interior of the tube is properly matted and has a couple of knife-edge type baffles.

Focuser

Focusers are often an issue on cheaper scopes, but this one is an all-metal rack and pinion with good travel and a 2” visual back. It is reasonably smooth and accurate, if a bit stiff and with some play when focusing. There is some vertical slop too, but you can remove it with the tensioner.

It’s worth repeating that this focuser has the ability to take 2” eyepieces (unlike the smaller 80mm Star Travel): great for really wide deep-sky views. This means that with, say, a 55mm Plossl you will get a binocular-sized 5.3 degree true field at 9x magnification. It also makes for wider-field astro-photos too.

Mount

The supplied AZ3 mount is lightweight, stable and quite sturdy and has slow motion controls that are easier to use than just pushing and pulling the scope around. A tripod-tray that clips onto the leg-braces is supplied to hold your bits and pieces whilst observing. The fold-in legs and light weight mean almost anyone can move the complete scope setup around – it’s an ideal grab-n-go.

The OTA comes with rings that are the basic Synta type, with a hammered enamel finish like the focuser. The rings bolt straight to the AZ3 mount with a single ¼-20 threaded hole, but will fit straight on a Vixen-fit dovetail plate if you want to upgrade to an equatorial mount (like an EQ5) later.

Accessories

The OTA comes with two basic but serviceable (i.e. decent optics, coatings and no plastic barrels) Plossl eyepieces: 10mm giving a magnification of 50x and a 25mm giving 20x. A 2x barlow lens is also supplied, that effectively gives you 40x and 100x through the same eyepieces (though perhaps with slight loss of image quality).

The red-dot finder is very usable and easy to align. I much prefer it to a cheap optical finder on this type of telescope.

Sky-Watcher supply an erect-image diagonal … yes you read that right! It seems they think the alt-az version is going to get used as a spotter. But in any case, the focuser will take a 2” star diagonal, so with the wide field on offer, you’ll want to upgrade and maybe get a 2” eyepiece (Sky-Watcher’s own 28mm LET is a great eyepiece and can be bought cheaply).

 

25mm and 10mm Plossls are supplied.

Sky-Watcher’s 28mm LET 2” eyepiece is not supplied with the StarTravel-102, but it would make a great, inexpensive upgrade.

In Use – Daytime

Since Sky-Watcher seem to be marketing the alt-az mounted and OTA-only versions as a terrestrial scope, that’s where I’ll start.

I didn’t bother much with the erect-image prism diagonal as it’s 1.25” and it vignettes (stops down the aperture), as they all do. But it works fine if you really can’t stand the mirror image view (left and right swapped but upright) that an astronomical mirror diagonal gives.

With a 2” star diagonal installed, the daytime view is very crisp indeed: wide, bright and with a snap to the focus. It takes about 20 seconds to realise this is emphatically not some 3rd rate optic. I can watch ducks swimming on the bay in vivid detail and Autumn leaves in the woods across the way are sharply defined all the way to the field stop using a 13mm Ethos at 38x. Prismatic or reflective optics rarely deliver daytime views this fine.

Only when looking at high-contrast subjects do problems show up and you know in advance what they are going to be. So, when I turn to the family of squabbling rooks that inhabit the tall tree opposite, black feather against blue sky, I get a lot of unfocused purple light around their black plumage. However, the problem is little worse than Stellarvue’s Nighthawk 80mm or Sky-Watcher’s own Star Travel 80 (or ST80s from various other brands), even though theory suggests that at 102mm it should be. This may be down to that wide inter-element gap in the objective.

So, make no mistake, the chromatic aberration is quite severe under some circumstances, but here’s the thing: it doesn’t spoil the view nearly as much as a poor optic would. I would rather have this for daytime viewing than a fuzzy APO scope with quality issues.

Overall, the StarTravel-102 makes a very nice terrestrial spotting scope.

This gives an idea of the level of chromatic aberration on high-contrast targets, but don’t let this put you off: in most terrestrial views it’s much less noticeable.

 In Use – The Night Sky

A star test on the StarTravel sets the scene for much of what follows. Basically, it’s excellent, with lovely round rings on either side of focus and nice tight concentric rings around bright stars at high magnification, just as it should be. Collimation was perfect and optical quality high.

The StarTravel 102 excels as a wide-field scope as you would expect and does the job very well indeed. All the refractor hallmarks of wide sharp field and diamond pinpoint stars are there in abundance. In fact, apart from a little extra field curvature and off-axis coma, this could be mistaken for a premium refractor when looking at the Orion Nebula or a cluster like M36 in Auriga. The extra aperture really makes a difference for deep sky when compared to most 3” grab-and-go scopes (around 45% more light gathering than a 76mm, for example). Note that for visual use on Deep Sky, a premium 4” APO from one of the top makers really won’t give a much better view than this cheap achromat!

“Ah, but …” I hear you say “it’s not going to be much good for the Moon and Planets is it?” Well that’s what I thought too, so imagine my surprise when I first set the StarTravel on Jupiter at 100x with a 5mm Type 6 Nagler. This was something a scope like this just wasn’t designed for, but the image was really superb – crisp, sharp and detailed with perfect focus snap. Yes, there was a bit of a violet halo around the planet, but it didn’t matter as much as you’d think and a lot of cloud-belt detail was visible; I could even clearly make out some knots and the Great Red Spot.

If Jupiter was a big surprise (and I tried on several occasions to make sure), the Moon was more what I was expecting from a short-focus achromat. At lower powers (up to about 50x) the view was fine, albeit with a bright purple haze runnin’ all ‘round the limb (apologies to Hendrix). However, when pushing the magnification up to 100x, whilst the image remained sharp, everything was washed with purple light, destroying contrast and really spoiling the view.

Mars was less satisfactory than Jupiter too, as it so often is, perhaps because the lens is better corrected at shorter wavelengths to cater for imagers on a budget and reduce the violet bloat around stars that can plague achromats. In fact, though I didn’t try it, I would guess the StarTravel would make a good budget imaging scope as long as you keep away from bright O-B stars where possible.

Overall the contrast with another scope I’m testing – the Vixen VMC95 Maksutov – was interesting. The Vixen of course shows little CA, but in almost every other respect its view was inferior, even on Jupiter where you’d think it would excel.

Summary

What’s not to like about Sky-Watcher’s simply excellent StarTravel 102? It is cheap, light, fast and grabs a lot of light. Its compact size makes it easy to mount and the optical quality, on my sample at least, was very good indeed – better in fact than you have any right to expect from a scope of this type and price. Sky-Watcher’s use of a wide gap between crown and flint seems to have done a good job at keeping chromatic aberration down to ST80 levels.

The Sky-Watcher Star Travel 102 is highly recommended and is a really great beginner’s scope which will happily stay-on as a grab-n-go when you get something bigger.

You can buy the Star Travel as reviewed (but in its new livery) on its AZ3 mount here:

 

 

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