Home

 

 

Vixen FL80S Review

Thank goodness, we’re over the worst: in terms of Mars oppositions, at least. The 2010 and 2012 oppositions were the most distant in this particular cycle, with Mars achieving a maximum angular size of 14” (compared with 25” in 2003). The 2016 and 2018 were the closest in this cycle, but much too low in the sky to give good views from the northern UK.

So, if you’re keen to observe Mars over the coming oppositions and you want a small, portable travel scope to do the job, what should you buy? The problem is that Mars makes particular demands on your scope system. Why? Because Mars is small, bright and its surface markings are low in contrast. Also, because it’s red!

The fact that Mars is red (or yellow-orange anyway) is a problem because many modern refractors (and even catadioptrics) are optimised at short wavelengths for imaging (to avoid bright blue-white stars ‘bloating’ in CCD images). As I noted in my review of the TV-76, when you turn a fast ED doublet on Mars you realise this all too well: it’s hard to find best focus and the image is soft and slightly blurry. This is a deeper problem than simple chromatic aberration (false colour fringing).

The following graph, of optical quality, in the guise of Strehl ratio, at different wavelengths for a fast APO, explains why such scopes can be poor on Mars. To put those Strehl figures into context, 0.95 is excellent, 0.80 barely acceptable. So, in the red (and even yellow to some extent) your expensive, beautifully figured APO lens turns into a piece of bottle glass!

If you want a small (i.e. 60-90mm) refractor which avoids that Strehl cliff beyond the yellow and with premium optics for high magnification on Mars (and a good small APO can show surprising detail on Mars), what options do you have? Surprisingly few, it turns out.

Even Takahashi’s latest FC series are F7.5 or 7.4 (the old FCs were F9) and the ‘D’ for ‘digital’ suffix suggests concessions to imaging. One well known telescope tester describes the new FC-100D as having ‘a little too much’ chromatic aberration. There are now the FC-100DL and DZ, but these are large and expensive respectively.

Takahashi’s old FS-78 is a superb performer on Mars and quite light weight, but is big and bulky so not very portable; it certainly isn’t carry-on portable. An interesting alternative is the long-discontinued Vixen FL series.

I first saw the diminutive FL70 at a friend’s and one look was enough to tell me it was an outstanding APO. Vixen made the FL series in a variety of sizes, from 55mm to 102mm. All have Japanese fluorite doublets at F8 or F9. All are compact and light weight as well. The sweet spot is arguably the FL80 and the one on test here is the later ‘S’ model. Let’s see if it lives up to the promise of fluorite performance in a compact package and how it stacks up to its main rival when new, the Takahashi FS-78.

At A Glance

Telescope

Vixen FL80

Aperture

80mm

Focal Length

640mm

Focal Ratio

F8

Length

650mm (600mm w/o visual back)

Weight

2.3 Kg

 Data from Vixen.

Design and Build

Comparisons with the FL80’s big Japanese rival at the time, the Takahashi FS-78, are inevitable. Compared to the luxurious finish of the Takahashi, the Vixen FL80 has the same workaday mid-range look common to all Vixen scopes right up to the present.

The FL80’s focuser knobs are black plastic, as is the focuser lock-knob; on FS-78’s these are anodised metal. The tube has too many stickers and the rings are formed rather than Takahashi cast. The crinkle green finish on the metal parts aren’t as premium as Tak’s enamel either. The Tak’ has that remarkable (and heavy) cast iron manhole cover to shield the lens; the Vixen has a cheapo plastic cap, like a £100 Vixen kid’s scope. Whilst every Takahashi has an individual serial number, Vixens like the FL80 are not serial numbered.

That’s not to say the FL80 is poorly made, far from it: everything is well finished and threads together like the Tak’, rather than push-fit with screws like a cheaper Vixen. But the FL80 it lacks the thick tubing, lustrous paint and superb castings of the Takahashi.

However, the good news is that optically the FL80S is right up with the Takahashi and it’s usefully lighter too, as we will see.

The rear fluorite element in the FL80 is uncoated, as you can see from the reflections. This isn’t a problem in use.

Laser test confirms the FL80S is a Steinheil doublet with a fluorite rear element.

Optics

Some manufacturers claim fluorite, but use high fluoride glass. Roland Christen of AP has said much the same. Does this matter? It does, because fluorite has higher optical performance in some key areas than any glass; and it’s a much more expensive material. So what about the Vixen FL80S – is it really fluorite? The answer is an emphatic yes! In the test above you are looking at the back of the lens, i.e. the fluorite element. The laser disappears in the fluorite, then reappears in the glass element: glass scatters laser light, fluorite dos not.

So, like the FS-78, the FL80 has a premium F8 Japanese-made fluorite doublet lens (probably also made by Canon/Optron). But there are key differences:

·        The FS-78 has a Fraunhofer objective, with the fluorite at the front, the FL80 is a Steinheil configuration with its positive fluorite element at the back (as you can see above), like FC Takahashis old and new.

·        In the FS-78, all elements in the objective were multi-coated, whilst the FL80’s rear fluorite element is uncoated. This results in a noticeably more reflective lens in the FL80 (see photo).

·        The Takahashis all had fully adjustable cells, whilst this later FL80 has a fixed-collimation screw-in cell, like the Takahashi FS-60 (earlier FL series Vixen did have adjustable cells). The advantage of this minimal cell and lens ring is reduced weight and a narrower tube and dew-shield (though one unexpected disadvantage is that the FL80 is back-heavy).

Does this difference of approach to a fluorite doublet make much difference? Well, lore says the Steinheils were better, which is why Takahashi have resurrected their older FC design for the new FC-76 and FC-100. Then there is the Tak FS manual which agrees with optical-bible Rutten and van Venrooij in claiming the Fraunhofer to be the ‘better’ design.

In either case, fluorite in combination with an ideal mating element has the potential for delivering very high performance in a small F8 doublet – performance approaching that of a triplet super-APO. What this means is minimal chromatic aberration and optical quality that remains high across the whole range of visual wavelengths. For general viewing this doesn’t matter much, but for imaging and high-power planetary viewing it does.

I should mention that the FL series have legendary optical quality. That little FL70 was sold to a chap who got it tested on an interferometer; he was thrilled to find that it had effectively perfect optics (Strehl above 99% as I recall), but I had guessed as much from its star test and general performance.

Bench tests results I have seen suggest that the smaller FL-series have exceptionally low false colour, well into super-APO territory, which suggests performance on Mars should be good.

The FLs were expensive scopes new and would be expensive to make now – check out the prices of Optron fluorite doublets in Taks and Borgs today.

Tube

The upside of the Vixen’s thinner tube, less substantial castings and lack of adjustable cell is that the Vixen is much smaller and slightly lighter (600mm long, 90mm diameter, 2.3kg) than the Takahashi (750mm long, 95mm diameter, 2.6kg), despite having a bit more aperture. In use the weight difference is much more marked, because all the Vixen ancillaries (rings, finder etc) are lighter too.

Note: The specs claim 650mm length for the FL80, but with the 2” visual back it’s actually 600mm. By removing the dew-shield the FL80 would easily pack in a carry-on bag.

Despite its small diameter, the tube has numerous knife-edge baffles tapering to fit the light cone and Internally everything has been carefully matte-painted to reduce reflections; this is one of the most carefully baffled tubes I’ve come across (so much so the baffles are really hard to photograph).

The Vixen FL80 is significantly smaller than the Takahashi FS-78 and actually weighs less than a TV-76.

Focuser

The unit on the FL80 was Vixen’s premium focuser in its day: it has a big drawtube, a cross-cut rack and a Takahashi-like M60 threaded visual back to allow 1.25” or 2” eyepiece holders and various other accessories to be threaded on. Like the tube, it has knife-edge baffles to kill ghosting and help contrast.

That said, it’s single speed and cast-bodied with plastic knobs. In use the focuser lacks the fluid feel of a Takahashi, but is light, smooth and accurate and has plenty of travel. It has some image shift on changing focus direction (just like the FS-78 I tested), but this isn’t excessive.

The focuser does suffer from a tendency to rack out under even light loads (though an effective locking knob is provided). In focus travel is a bit lacking too, and may not be enough for some purposes.

The visual back has a flexible configuration. There are 2” and 1.25” backs provided. The 1.25” is stopped down in stages from M60 to M54 and then M42 via adapters to allow various accessories to thread in.

Meanwhile, the 2” visual back has some clever features. You can reverse it to create a thread-in extension tube. Also, the lock ring acts as a clever camera rotator. If you want to rotate an attached camera or diagonal, just loosen the ring, rotate the visual back and lock it up again (in comparison, a Takahashi camera rotator costs ££Big).

The visual back permits several configurations: 1.25”, 2”, 2” plus extension.

The lock ring for the 2” VB acts as a camera rotator – clever!

Mounting

The Vixen comes with a couple of split (not hinged) rings that have a single ¼-20 thread in each and a Standard dovetail that fits numerous mounts of all types and sizes. Again, these rings are less elegant but lighter weight than the trademark Takahashi clamshell.

For testing, I used a Vixen GP mount (on which the FL80 was originally marketed). On the GP it’s easily possible to use the smallest counterweight which makes the whole assembly highly portable as a unit (much more so than the FS-78 on its matching P2Z mount, for example).

I also piggy-backed the FL80 on my AP1200 to really put it through its paces on a range of targets.

Finder

The FL80 comes with Vixen’s usual small finder that is almost identical to the SkyWatcher equivalent. Again, it lacks the premium look and feel of a Takahashi 6x30 or 5x25. The eyepiece holder is plastic, compared to the knurled metal focuser of the Takahashi 6x30 finder and indeed the field of view is narrower too. In other respects, though, the Vixen finder works well: sharp, bright, with decent eye relief.

In Use – Daytime

You probably wouldn’t choose the FL80 as a daytime spotter, but terrestrial performance can reveal a lot about optical performance.

My usual test of tree branches against a bright sky at ~100x yielded minimal chromatic aberration: in focus there is no false colour; even focusing through gives a barely detectable cast of purple on one side, green the other. This level of performance is a significant cut above something like a TV-76 or a SkyWatcher Equinox 80.

This shot of silhouetted branches through the FL80 (with inset detail) shows no chromatic aberration.

In Use – Astrophotography

Images of the Moon and M42 with a Nikon APS-C follow. The image of the Moon has been cropped; M42 is straight from the camera.

The FL80 produces images of the Moon that are very sharp and full of contrast. M42 is good too, with well controlled violet bloat. Whilst curvature is well controlled for a doublet too, you would need a flattener for serious imaging, especially at full frame.

The Moon in mediocre seeing through FL80 at prime focus with Nikon 5100.

M42 at prime focus with some sky glow through FL80, straight from the camera (Nikon 5100).

In Use – The Night Sky

Cool Down

This is an area where doublets often really win over more complex designs. The FL80 cools very quickly and benignly. On one recent evening, I had just a few minutes between storms to enjoy Jupiter in fine seeing. The FL80 was happily delivering razor-sharp views at 180x just ten minutes or so from a warm room.

Star Test

The FL80 has an excellent star test with near-identical patterns either side of focus; I would estimate at least 1/6th PV.

Despite that non-adjustable cell, collimation is perfect.

Trying the star test on a red star (Albireo) produced the same result, suggesting the good optical figure is maintained at longer wavelengths.

There is only the merest hint of chromatic aberration in the star test, even out of focus on Vega. In this respect the FL80 is as good as all but the very finest F8 ED triplets.

The Moon

The FL80 delivered some of the best views of the Moon I have had in a sub-100mm scope: super sharp, with no chromatic aberration, even at high power. In the FL80, shadows and space are absolutely black with no flare, just as they should be. Detail is excellent for the aperture and at 128x with a 5mm Nagler, the Moon just fills the field to give a superb ‘Lunar Module Porthole’ view.

In fine seeing, the 3.5mm Nagler giving 182x is quite usable: Like the FS-78, the FL80 takes high magnifications better than a typical F6 doublet like the TV-76, though at this magnification getting perfect focus would be easier with a micro-focuser. Perfect focus has an absolute snap to it.

 At 182x the Moon through the FL80 is still bright and pin-sharp and shows significant detail (I was able to make out Hadley Rille – a difficult target for a small scope).

Planets

Mars

The red planet was still early in its opposition cycle and so very small (6” x 9”) when I was reviewing the FL80, however the disk was sharply resolved with none of the chromatic aberration or softness of focus that you get with short focal length ED doublets. In good seeing I was able to make out the bright polar cap at 182x.

Jupiter

At 128x with a 5mm T6 Nagler, Jupiter was very clean and sharp, with no flare or CA, no ghosting and considerable detail. At least four belts, the Great Red Spot and polar hoods could be seen, along with detail in the NEB and SEB at moments of good seeing.

At 182x Jupiter showed masses of detail: fine banding in the polar hoods, changes of thickness and colour in the main belts and any shadow transits or dark storms are easy to pick out. The crisp view at 60x per inch of aperture is a credit to the optical quality of the FL80 and it could doubtless take 200x or more with a suitable eyepiece in steady seeing.

Neptune

Clearly resolved into a tiny grey disk at just 107x with a 6mm Ethos

Uranus

A small, buff-coloured disk at 107x, with a noticeably warmer colour than whitish-grey Neptune.

Overall, the FL80 delivers more planetary detail than many would imagine possible for such a small, lightweight scope. It’s one of the best planetary grab-n-go’s I’ve tested.

Double Stars

You would expect a high-quality doublet to be good at splitting doubles and the FL80 certainly is. A 3.1” double in Aquarius from the ADS catalogue was too easy.  The Double double (Epsilon Lyrae) at 2.3” gave  a perfectly clean split at 107x on both components to yield four tiny hard Airy disks with black space between - the  best I have seen in any scope under 100mm aperture.

Albireo was particularly beautiful with the orange and blue components bright and vivid. Star colours seem to be a strength of the FL80 for some reason; the twin orange ‘eyes’ in the centre of the Pleiades were similarly vivid. This may well be because its perfect optical figure throws all the available light into the Airy disk, intensifying colour.

Star fields/Clusters

The field of view with an Ethos is virtually coma free to the edge, with just a little curvature, so star fields and clusters remain bright and sharp across their extent. The Pleiades were wonderful: dazzling and with the nebulosity visible  at 59x with the Ethos 13mm.

DSOs

The FL80 gave perhaps the finest view of the Ring Nebula in Lyra I’ve had in a small scope: at 107x with a 6mm Ethos it really showed its shape and stood out very well from a flat field of pinpoint stars. The view was so good I found myself just gazing at it for ages before remembering I was supposed to be in test mode.

The Dumbbell Nebula was similarly very clearly defined against a dark sky, with the smoky nebulosity almost 3D. The Crab Nebula was easy to pick out from its background star field and again looked as good as I have ever seen it in a small scope.

Autumn is open season for globular clusters, so I tried a few with the FL80: M02 and M13 were excellent, with the outer stars resolved into diamond dust; M15 and M56 were less impressive, due to the smaller aperture.

The FL80 must rank amongst the best small refractors I’ve used for deep sky. The extra 10% light gathering compared to a 76mm and superb contrast giving better results than I’m used to in a small scope.

Summary

The FL series are a bit of a legend, but if you expected a harsh reality check in the face of cheap modern competition, you’ve come to the wrong place.

True the FL80 is, frankly, rather ugly. The dew-shield looks oddly small and truncated and I’m not keen on all those big red stickers either. The Tak’ FS-78 is in another league if living-room beauty is your concern and has more of an ‘heirloom’ feel.

The FS-78 also impresses with general build quality and that fully adjustable lens cell. But here’s the rub: in terms of performance, the FL80 is on a par with the FS-78, whilst being a lot more compact and yet still very thoughtfully built where it counts. The two scopes are hard to separate in terms of performance. The larger aperture of the FL80 – 5% more light gathering than the FS-78 - easily makes up for that uncoated fluorite element for reach on deep sky.

Of course, the FL80 is a small scope, but every object I set the FL80 on (and I set it on lots by mounting it on my AP1200 and letting that find stuff) looked just as good as it possibly could with 80mm aperture. The planets (Mars included) show more detail than you might expect from a 3-inch class refractor.

The FL80 would benefit from a better focuser, but the existing one is adequate, keeps weight down and is very adaptable. That threaded visual back would allow a slim helical (like a Borg) to be fitted downstream for fine focussing.

The FL80 is, in my view, another example of where fluorite matters. Turn up the magnification on a double star or planet (especially Mars) and the FL80 performs noticeably better than a fast ED doublet, even an excellent one like the TV-76 or TV-85.

Highly recommended, especially for refractor buffs who value a really perfect view through their scope (buy the FS-78 for more living-room appeal).