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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 make a focal reducer for the Traveler (AP part number 67PF462) that should easily cover a 35mm chip and is even designed to cover huge Pentax 6x7 film frames.

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.

 

 

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