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 StarTravel
102 |
|
Aperture |
102mm |
|
Focal Length |
500mm |
|
Focal Ratio |
F4.9 |
|
Length |
~550mm |
|
Weight |
~3 Kg |
Data from Me.
Design and Build
The StarTravel 102
is conventional Sky-Watcher, 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 StarTravel): 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 StarTravel 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.
The purple rinse is quite severe 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
Star
Test
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
Moon
The Moon was what I expected 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.
This is important, because when I (and others)
accuse semi-apos like Tele Vue’s Pronto of being ‘just an
achromat’, this proves I’m wrong – the Pronto does show some
false colour, but will happily take much higher powers on the Moon without
purple wash spilling over everything.
Jupiter
“Ah …” I hear you say
“so, it’s not going to be much good for the 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.
Mars
Mars was less satisfactory than Jupiter, 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, or use a filter (or set
your DSLR to b&w).
Deep
Sky
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!
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 StarTravel
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 StarTravel
as reviewed (but in its new livery) on a newer altaz
mount here: