Astro Physics (AP) Traveler
Review
The Traveler is legend. Introduced in the early Nineties and
discontinued about ten years later, AP made less than a thousand I’ve heard. No
small APO is made today that matches its combination of features. For though
the Traveler is a true four-inch (just over in fact), its size and weight
belong in the class below – it is a really compact scope, easily carry-on
portable without any disassembly.
You might well ask why the Traveler remains peerless, just as
it was when AP stopped production over a decade ago, why no one else has
reproduced it. I suspect the reason is that it is only possible to make a four
inch APO this good and this small if you make and integrate everything as a package.
To my knowledge only AP and TEC have ever done this; only their four-inch APOs
end up being so small and light. Integrating third-party lens cells and
focusers inevitably adds weight and bulk.
So why did I finally buy a Traveler, having always wanted
one? The answer isn’t just some urge to own the best, nor even simply to write
this review at last. No, I bought a Traveler to do a job – as a travel scope
(no surprise). I took it on a month-long trip to the US this year, including
visits to star parties at the Grand Canyon and Bryce, and for the Mars
opposition. Having used it under some of the best seeing and darkest skies on
Earth I am now in a good position to tell you if the Astro Physics Traveler
lives up to its huge reputation.
At A Glance
|
Telescope |
Astro Physics Traveler EDFS |
|
Aperture |
105mm (4.1”) |
|
Focal Length |
610mm |
|
Focal Ratio |
5.8 |
|
Length |
48cm (19”) / (46.5cm minus VB) |
|
Weight |
4 Kg incl cap (~5.8 Kg as
shown below) |
Traveler mounted on my TMB 175/AP1200
for testing
Design and Build
The Traveler was manufactured in-house by AP, all of it –
lens, tube, focuser, dew-cap. It is much higher quality than it needed to be,
something I’ve noted with just about every AP product that’s come my way.
Everything is CNC-milled and screws together with no adapting rings; the lens
fits straight into the tube with no heavy cell or lens ring. It is this total
approach that allows the Traveler to be so small and light.
The lens quality is amongst the very best too, with minimal
false-colour despite very challenging numbers (F5.8 is very short for a 105mm
triplet). But still, it’s the size and weight that make it so special: even the
equivalent LZOS lens – the 105/650 - adds 40mm to the focal length for the same
aperture and several kilos more weight (and a lot more bulk) when assembled
into an OTA.
The Traveler was reputedly designed to be extremely rugged in
line with its intended use, but I am not about to test it. Mine is certainly in
perfect collimation after lots of travel (try that with a Sky 90!)
Optics
The 105mm triplet lens has an all-spherical design with a
centre element of ED glass and the gaps between the elements filled with
mineral oil. This design allows an exceptionally good optical figure, despite
the severe curves that the short focal ratio of F5.8 (610mm focal length)
requires, because spherical surfaces are relatively easy to make well and
because the oil ‘fills’ small surface imperfections on the inner surfaces.
However, this design does mean that some off-axis aberrations are less
well-corrected than the more common air-spaced approach.
I have discussed performance for chromatic aberration and
field curvature in the ‘In Use’ sections of this review. Suffice to say here
that though one tester has called the Traveler a ‘semi-APO’ on account of its failings
in the far red, it performs more like a ‘super-APO’ in use.
Unlike an LZOS lens, Astro Physics don’t supply test
certificates for their lenses. However, the lens has been checked on a Zygo interferometer
and a test certificate exists – back on a computer at AP. Roland Christen has
published example Zygo reports on various forums in
the past; all his lenses are pretty much hand-made and are reputedly of very
high optical quality (1/8th PV, 98% Strehl
is supposedly typical).
One quirk of the oil-filled design is that it may only give of its very best when
stored with the lens horizontal, something I do when I can.
Although this is quite an old lens, it has very high quality
multi coatings of a deep purple hue.
Again, unlike the finest triplets from LZOS, Takahashi et al,
the Traveler’s lens has no separate cell – one of the
ways it saves weight and girth. Nonetheless, the design is supposed to be very
rugged and it certainly cools quickly.
Traveler
lens and tube baffles behind it
Tube
The CNC tube is slim - just over the bare 4” for most of its
length, flaring towards the objective. The resulting OTA is just 19” long and
weighs about 4Kg including the dewcap. That makes it
one of the lightest and smallest 4” APOs, one of the very few that is
definitively carry-on portable in its case. However, adding the rings,
attachment dovetail and top accessory plate/dovetail (the configuration as
shown) brings that to 5.77 Kg.
The Tube is beautifully made, with numerous baffles machined
into it and then flattened with the blackest paint. The interior build quality
is superior to just about any other refractor I can think of, more evidence of
AP’s peerless attention to detail, even where it’s not obvious.
The dew-shield slides with just the right weight and has been
carefully flocked. It has a machined-in stop to prevent it moving too far. The
dew-cap is a beautiful thing – machined from solid stock, it slides on and off
with just the right resistance.
The Traveler was introduced in 1991 and earlier ones like
mine have a glossy-black painted tube with anodised focuser and dew-shield.
Later ones have a textured black powder coat finish that may be more
chip-resistant, but which looks slightly more utilitarian to me.
Focuser
Astro Physics makes their own
high-quality rack-and-pinion focusers. Once again, the focuser is a beautifully
machined and finished item, both inside and out (the draw tube contains
numerous machined-in baffles to kill stray light, just like the main tube).
My Traveler’s
focuser is an early one which would originally have been a single speed. Like
the more recent versions, it has a 2.7” drawtube with a generous 4.1” (105mm)
of travel. It includes a very finely-made thread-on visual back with three generously-sized
thumb screws acting on the locking ring. The focuser has another large thumb
wheel on the side of the focuser body to lock the draw tube (rather than the
more usual top). The only feature
lacking is a rotator.
The brass rack is over-sized and
cross-cut, but narrow and deep, unlike the wide, fine-toothed rack fitted to
Starlight Instruments’ larger Feather Touch focusers. Consequently, the AP
focuser’s action is a bit less smooth and fluid than a Feather Touch, but is
reassuringly precise and positive. There is minimal image shift when changing
focus direction at high power and likewise the lock mechanism is progressive
and effective with only minor image shift. The focuser will comfortably accept
heavier eyepieces and cameras without becoming unstable.
This particular Traveler has been
fitted with a dual-speed pinion assembly that was developed with AP’s help and
fitted by their German AP distributor Baader
Planetarium. The dual-speed unit is precise and well-engineered; it has a
locking screw underneath. Later Travelers had a similar focuser, but take an
AP-supplied dual-speed ‘Feather Touch’ pinion made by Starlight Instruments.
You can tell which is which from the style of focuser knob: the Baader unit has black knobs with a silver fine-focuser; the
later Feather Touch pinion has silver knobs with a gold fine-focus.
Focuser Upgrades
For owners of older Travelers (shipped
before July 2001) who want to upgrade to the Feather Touch pinion, a new
focuser is required (part number 127FOC3E-FT) that incorporates the Feather Touch
pinion and other improvements. It’s a simple screw-fit upgrade, though note
that it lacks the engraved ‘Traveler’ logo.
For newer Travelers without the Feather Touch upgrade, the
pinion can be bought and fitted to the existing focuser (part number 27FMTU)
for exactly half the price of the complete new focuser. If you are wondering
why you can’t just fit the Feather Touch pinion to the older focuser (which
appears very similar), the answer is that the bolt pattern is different. AP
publish details of the different bolt patterns so you can check which upgrade
is appropriate for your Traveler.
One small quirk is that the focuser has
no finder mount or dovetail: AP intend you to use the little dovetail plate
which fits atop the rings to mount a finder.
Overall,
the existing focuser is an excellent piece of engineering that has lasted well
and is only a little less smooth than the market-leading Starlight Instruments
focusers.
The Traveler on a Vixen mount at the
Grand Canyon Star Party
Mounting
One of the
huge benefits of the Traveler is that its small size and low weight mean you
can use just about any really compact mount. An EQ5 would take it fine
(sacrilege maybe, but if you’ve just spent big to buy a Traveler you don’t
immediately need to buy a premium mount to go observing with it).
I use the
Traveler with a Vixen SX2 mount that is very stable and well-damped, whilst
remaining very portable. I got some negative comments from scope-snobs (‘you
should have a Mach One mount with that’) at a recent star party, but the Vixen
mount is a perfect functional match for the Traveler. Astro Physics’ own Mach
One mount is vastly superior, of course, but is also much costlier, heavier and
less portable.
The Traveler
comes with a pair of very slim (to save weight) CNC rings, machined to the same
quality as the rest of the scope.
Accessories
The Traveler comes as standard with a semi-rigid
case made by Tenba. The case has a document pocket
and some small Velcro-divider compartments for diagonal, eyepieces and reducer.
It is very easy to carry and stow onboard and currently conforms to most
airlines’ size limits. The case is very ruggedly made and has a hugely
oversized zip with flaps covering it to prevent dust ingress – more of that
total design that sets AP products apart.
AP make lots of (expensive, but good) accessories
for the Traveler, including dovetail bars for their own and Losmandy
plates (but curiously not a Vixen compatible one – mine is third party) and
focuser upgrades (see above). My Traveler has a small AP dovetail plate
attached to the top which can be used to attach a finder (if you really need
one on such a short focal length scope) or guide scope.
AP used to make a dedicated flattener for the Traveler (AP part number 67PF462), however this is now
out of stock. Instead, AP now offer two options for imaging:
1) A custom 1.0x flattener – Part 92FF - with an image
circle of 50mm (!) that’s colour corrected deep into the violet for modern CMOS
chips
2) A 0.8x reducer/flattener – Part 92TCC - to reduce
the focal length to 488mm (F5.3) with an image circle of 40mm
AP make a
wide camera adapter with a 47.6mm bayonet to fit these, parts DSLR25EOS for
Canon or DSLR25NIK for (you guessed) Nikon.
Both the
flattener and reducer fit into a circular 2.5” dovetail called ‘DoveLoc’ designed to pull the reducer/flattener into
perfect alignment. For the Traveler you would need a threaded
DoveLoc adapter, part EC2725, to fit the focuser.
In
Use – Daytime
The really small size of the Traveler
makes it one of the few four-inch class refractors you could readily use for
birding or general nature viewing.
Daytime views with a wide-angle
eyepiece – an Ethos or Nagler say – are very sharp, high-contrast and full of
natural colour. Chromatic aberration is virtually absent. The Traveler would
likely outperform even the very best prismatic scopes, giving the potential of
much higher powers (120x plus is perfectly useable in the daytime). It would
however need a very sturdy tripod and head and of course it isn’t waterproof.
They say the ability to distinguish
between Rook and Crow is the sign of a true British countryman – well there’s
no trouble doing so at 200m plus with the Traveler: watching a crow sitting in
a tree at the top of the field opposite, I see every feather and blink at 47x
with a 13mm Ethos. There is no chromatic aberration around the Crow’s black
feathers. Even out-of-focus branches are all but free of false colour and the
field is usable to the edge. Contrast is of the very best. Even the finest F8 triplet would struggle to outperform the Traveler
here. Semi-APO? Certainly not for daytime use.
In
Use – Astrophotography
The short focal length of the Traveler
makes it a great astrograph for extended objects. However, you really would
need a flattener for ‘serious’ astrophotography because it has a little more
field curvature and off-axis coma than the best air-spaced triplets (and a lot
more than a native flat-field design like a Takahashi FSQ-106). There is also,
as usual, some light drop-off due to vignetting in the corners of a full-frame
image.
You can see these off-axis aberrations
in this full-frame image (unprocessed, as usual) of M31 below. Centre-field is
very sharp, though, giving very good snaps of the Moon or smaller DSOs without
a flattener.
Full-frame unprocessed image of M31
through the Traveler with a Canon EOS 5D: off-axis curvature is quite strong
Cropped, but otherwise unprocessed
frame of the Moon through AP’s Traveler
I discuss the false colour issue of the
Traveler in detail below, where I claim it’s hard to distinguish from a
super-APO. Don’t believe me? Alright, well above are two shots of silhouetted
branches– one through the Traveler, the other through a premium F8 triplet ‘super-APO’.
You try to decide which is which.
Takahashi FS-102 and Traveler are from
the same era, give similar views, but the Traveler is much smaller
In Use – The Night Sky
General
Observing Notes
At the GCSP
this year I was set-up next to one of my other favourite four-inch refractors –
a Takashashi FS-102. I thought the FS-102 and the
Traveler performed very similarly in every way, giving very similar high-power
views of Mars, Jupiter and Saturn, confirming something fellow reviewer Ed Ting
noted a decade ago. Of course, the Traveler is about half the length of the
FS-102 and so much more portable, as well as being photographically faster and
giving a much wider maximum FOV.
Stars are
perfect airy disks surrounded by complete diffraction rings in focus at high
power. Perfect focus is of course an absolute snap and the micro-focuser helps
in finding it.
Some small
amount of coma/curvature is evident from about 80% field width when using an
otherwise flat-field eyepiece like a Nagler or Ethos. The steep F5.8 light cone
does cause trouble for some eyepieces (not Tele Vue’s), as it does with any
fast scope; long focal-length Plossls generate more
off-axis aberrations than with longer focal ratios (or flat field designs like Petzvals). Performance in this respect is slightly inferior
to the best F8 air-spaced triplets, as discussed.
A lot of the
critical viewing described here was at the Grand Canyon Star Party under good
to superb seeing conditions and very dark skies. For the planets, I used a
2-4mm Nagler zoom eyepiece. I had thought the eyepiece too high powered, but I
mostly used settings between 3.5mm (174x) and 2.5mm (244x) and occasionally the
maximum 305x at 2mm – much higher powers than I can usually use at home and
proving the outstanding optical quality of the AP lens.
Cool
down
The Traveler
cools quickly for a triplet, much more rapidly than the WO 123mm with its heavy
lens and cell. Twenty minutes usually seems plenty.
Star
Test
In-line with
published Zygo reports, the star test appears perfect
– identically round concentric rings either side of focus. I could find only a
trace of false colour in the star test, even on the brightest O-B stars.
False Colour
In theory, the short focal length and all-spherical,
oil-filled design of the Traveler’s lens should give
slightly more chromatic aberration than an air-spaced and perhaps aspherised triplet from the likes of LZOS. This is where
things get a bit strange. Reviews of the Traveler say it’s virtually free of
false colour visually and I agree. Meanwhile, a well-known bench-tester
announced that the Traveler is effectively a semi-APO based on his measurements
that it is less than perfectly corrected in the far red (though to be fair
other famous refractors like the TEC 140 would be similarly compromised by his
standards). Who is right?
Well I can tell you that visually the
Traveler has very low levels of chromatic aberration, even at high power. The tester in question acknowledges
this, saying it is due to the low sensitivity of the eye in the red at night –
the red colour blur just isn’t visible. Fair enough. But, in fact, the Traveler seems virtually colour free during the day
too, even viewing silhouetted branches– a stiff test that most ‘APOs’ fail.
Even more strangely, the Traveler gave me really excellent high-power views of
Mars this opposition; ditto Venus.
So, what are we to make of all this? Perhaps the Traveler has
indeed been carefully tuned to tightly control all visible wavelengths but the
furthest red; if so it’s a strategy that really works.
Ah, you say, but what about imaging? Well I looked at
exposures of M42 I’d made with a number of fine APOs, including my TMB 175 and
NP 127, comparing them with the Traveler. Zooming in on bright O-A stars shows
the Traveler giving a level of violet blur as low as (perhaps better than) the
rest. Meanwhile red stars showed no bloating that I could detect.
The bottom line: you are likely to
find the false colour correction of the Traveler very good indeed, even if you
are used to high-quality APOs from other makers. It emphatically performs
nothing like a ‘semi-APO’.
The
Moon
The Moon fits
nicely into the flat, wide field of a 5mm Type 6 Nagler giving 122x with the Traveler.
Viewed like this, a half Moon is sharp from limb to limb and full of detail –
rilles, craterlets, shadows of peaks and Mare wrinkle ridges; substantially
more than you get in a 3”-class scope. Chromatic aberration is effectively
absent in focus, with just a hint of amber focusing through the limb.
The high
optical quality of the Traveler means the boundary with the blackness of space
is very sharp, with none of the hazy bleeding of light you can get from cheaper
scopes.
In short, the
Traveler gives wonderful views of the whole Moon that keep the sharpness and
contrast of a smaller APO, but with quite a lot more resolved detail.
Mars
Views of Mars at 18” size and 23
degrees altitude, under the thin skies and sharp seeing at the Grand Canyon
south rim, showed significant detail at 244x with a Nagler 2-4mm zoom eyepiece.
I noted lots of albedo detail, including Mare Acidalium,
a hint of northern cap, Terra Sirenum and hints of a
very dark area north of Argyre; bright limb cloud
too. No problems with lack of sharpness or significant chromatic aberration
that semi-APOs exhibit on Mars (see discussion of chromatic aberration), but there is a little red blur -
out-of-focus only - at high power on Mars.
Jupiter
Already high in the sky at dusk,
Jupiter was the first object in everyone’s eyepieces at the GCSP, with the
Summer air still hot at dusk. Lots of cloud belt detail was readily visible through
the Traveler at 174x, along with an easily identified GRS, several darker
storms and subtle banding in the polar hood.
Saturn
At 244x, the view of Saturn from the
Grand Canyon south rim was like a miniature Cassini space-probe image: the
Cassini Division gaping all the way around; fine banding on the disk and rings;
clearly visible ring shadow. This view of Saturn really wowed my star party
visitors, many of who commented that it was their best view of Saturn on the
night. I spent ages just gazing at it in quiet moments.
Venus
At 22” in size and at 153x, Venus looks
like a tiny first quarter Moon in dazzling white. There is almost no chromatic
aberration – not in focus, not out of focus. To put this in perspective, both a
Takahashi FC-76 (a 3” F7.5 fluorite doublet) and an LZOS 123 (a 123mm F6
triplet) show more false colour on Venus than the Traveler.
You
don’t expect an F5.8 four-inch triplet to be a planetary ‘scope, but the Traveler
is.
Deep
Sky
The C14 set
up alongside me, under the dark skies at the Grand Canyon, delivered structure
in deep sky objects that I’d never seen before (in particular the Rosette and
Swan nebulae). Still, the much more modest 105mm aperture of my Traveler gave
some wonderful deep sky views.
The dumbbell
Nebula showed off its shape much more clearly than usual. M56 and M13 were
packed with stars. The Ring Nebula was impressive too and managed to please
some late visitors at the GCSP, who were used by then to much larger scopes. I
had one of the best splits of Epsilon Lyrae (the
Double Double) ever, with lots of black space between. Meanwhile, overhead, the
Milky Way was almost cartoonishly bright.
M42
Back home,
the Great Nebula in Orion is a wonderful winter sky object through the Traveler,
which shows a mass of nebular detail in the core region and more than a hint of
colour – reddish in the ‘arms’, greenish in the core. This is where you remember
that, though the Traveler looks like an 80mm scope, it really is 105mm and so
collects much more light.
Rigel’s faint
companion is particularly easy to pick out with the Traveler, perhaps because
so much of the starlight is thrown into the Airy disk and doesn’t bleed into
the space around.
AP Traveler vs LZOS 123mm F6 (in
William Optics tube)
I bought the
Traveler and the W/O LZOS 123mm at the same time, intending to use both as
travel scopes. My intention was to keep the one I preferred and sell the other.
I still own both. The reason is that, of course, they do things slightly
differently. A summary of their relative merits follows.
·
The W/O LZOS
123mm is one of the very smallest 5-inch class APOs, but it’s still
significantly bigger and heavier than the Traveler and feels much less
portable. The Traveler would go on an EQ5; the W/O 123mm would probably break
one.
·
Whilst both
are theoretically airline portable,
you would need to take the focuser off the W/O and make a thin-wall case,
whereas the Traveler goes straight on with no messing. My airline portable
Vixen SX2 mount takes either scope just fine, but the W/O needs a heavier
counterweight, pushing you well into excess baggage territory when you fly with
it.
·
The more
modern, air-spaced LZOS lens should be slightly better. It has less chromatic
aberration on some things; more on others. I did notice that the LZOS lens
delivered slightly whiter, perhaps higher contrast Lunar views.
·
Both lenses
have a similarly outstanding figure and polish, allowing higher magnifications
than you would think a fast lens could. But note that my LZOS 123mm is effectively
the ‘deluxe’ version with a higher-than-standard Strehl
and would be very expensive to buy new.
·
The LZOS
123mm is a superb lens, but it is a heavy thing – the lens and cell are 2.8kg
on their own - that cools slowly and makes the scope front-heavy.
·
The wonderful
and expensive 3” Feather Touch focuser fitted to the W/O is a bit smoother and
more fluid than the AP focuser; it also has a much-appreciated rotator, that I
love for visual or imaging use.
·
The W/O is
beautifully made, in the sense that it’s been built up from the best
components. But the Traveler has been designed and built as unit with
everything artisan-made in the USA – it has a feel of uncompromised integration
and quality that the W/O can’t quite match (to me anyway).
Overall, the LZOS
is probably the better lens, but the Traveler is much lighter and more compact
(even than the 105mm LZOS in any available tube).
Summary
I hate mindless elitism (hey, I drive a
Toyota!) So you can trust that my high opinion of the Traveler isn’t due to
snob appeal.
When I asked AP about the Traveler,
they told me, ‘You’ll love it’. I do. In fact, the AP Traveler is my favourite
ever four-inch refractor. The lens and focuser are as good as, but no better
than, the best of the rest. But because AP fabricated everything themselves
they’ve created a scope that is ridiculously compact compared with the
competition. It is, of course, absolutely perfect for travelling. But should
you buy one?
It all comes down to whether you need
the flexibility of a carry-on-compact scope with fast optics that can also take
high powers too. If you don’t, then a regular four-inch triplet or a fluorite
doublet may be as good for visual, or a flat-field quadruplet for imaging. But
in truth most astronomers need one really portable telescope and the Traveler
is just so much more capable than most APOs of similar size.
For me the Traveler’s
portability is such a wonderful thing. The scope and accessories all easily fit
in the little Tenba case and then slide effortlessly
through airport security and into an overhead locker. Meanwhile, a suitable
mount fits in a large suitcase and comes in under the 20 Kg weight limit for
most airlines. I can easily lug the Traveler, accessories, mount and some clean
undies through airports and rental car offices on my own. Back home, I can just
pick the Traveler up and carry it out, all setup on its mount, wait a few
minutes for it to cool and use it.
Despite its portability, the Traveler
is much more capable than almost any other refractor of similar size. It gives
great views of everything and works well as an astrograph (with a flattener).
There may be very slightly better APOs (at least theoretically), but none of
this size and versatility: the Traveler is the only four-inch APO I’ve owned
that will do everything in a really
small package and that’s why I love it.
The AP Traveler is my favourite
telescope of my favourite type (the four-inch APO) and it really is tiny –
small enough to use as a birding or spotting scope if you want. So, it’s
arguably the most useful general purpose telescope ever. Too bad they
don’t make ‘em anymore and probably never will again.
