Vixen FL80S Review

Vixen FL80S on GP mount.

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). In truth this opposition (maximum April 8th, 2014) won’t be much better, but thereafter (2016 and 2018) we are in for a couple of good ‘uns. Personally, I aim to make at least one trip to somewhere with really good seeing for one of those oppositions (last chance for me – I’ll be ancient by the peak of the next cycle).

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 works well for 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; presumably the FC-76 will be much the same.

What about an F6 triplet, you ask? The superb (now discontinued) LOMO 80 might be an option, but is relatively big, heavy and slow to cool – not ideal for a grab-n-go; it’s expensive too. Meanwhile Takahashi’s old FS-78 is a superb performer on Mars and quite light weight, but is big and bulky so not as portable as I could wish; it certainly isn’t carry-on portable.

One possibility that I’ve long known about is the (also discontinued) Vixen FL series. I first saw the diminutive 70mm version at a friend’s and one look was enough to tell me it was an outstanding APO. Vixen used to make the FL series in a variety of sizes, from 55mm to 102mm and all have Japanese fluorite doublets at F8 or F9. As I’ve explained many times before, small Fluorite doublets at F8+ have the potential to achieve super-APO performance and unlike the Takahashi, the Vixen variation on this theme is typically compact and light weight as well.

So I vainly advertised for a 70-90mm FL for years; then when I was least expecting it, I was offered a late model FL80S in a trade. Let’s see if it lives up to the promise of fluorite performance in a compact package and in particular how it fares in comparison to its main rival when new – the Takahashi FS-78.

Design and Build

Optics

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

Fluorite or high-fluoride glass?

Some optics I have seen that claim fluorite actually 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 of course 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, as I have explained before, 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

Compared to the luxuriously finished FS-78, the Vixen FL80 has the same workaday mid-range look of cheaper Vixens. That’s not to say it’s poorly made, far from it: everything threads together like a Tak’, rather than push-fit with screws like a cheaper model, and the OTA has been very carefully designed and put together. But the FL80 it does lack the thick tubing, lustrous paint and superb castings of the Takahashi.

The Tak’ has that remarkable (and heavy) cast iron manhole cover to shield the lens; the Vixen has a cheapo plastic cap, just like a £100 Vixen kid’s scope. Every Takahashi has an individual serial number, ditto TeleVue; Vixens like the FL80 are not serial numbered.

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 also has the unfortunate tendency to rack out under even light loads (though an effective locking knob is provided). In focus travel is a bit lacking though, and may not be enough for some purposes.

Visual Back

  

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

The visual backs provided are quite clever. 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. For one thing, you can reverse it to create a thread-in extension tube. But the thing I like best is that having threaded it in, you lock it in place with a knurled ring. If you want to rotate an attached camera or diagonal, just loosen the ring, rotate the visual back and lock it up again: a camera rotator at no extra cost (Takahashi take note, theirs cost £100s) or image shift.

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 lighter weight than the trademark Takahashi clamshell.

For testing purposes 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. 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

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 – 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 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.

Astrophotography

As usual, un-retouched  images of the Moon and M42 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, but you can see some field curvature creeping in at the edge – you would need a flattener for serious imaging.

The Moon in mediocre seeing through FL80 at prime focus with Nikon 5100 – cropped but not sharpened or enhanced in any way and with an inset enlargement of Mare Nectaris.

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

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 Tak 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, but I suspect the FL80 takes high magnifications slightly better, though that may be variation between samples, rather than a design issue. 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.

Optical performance of the FL80 must be pushing at the limits for an 80mm refractor, it’s that good. Contrast and sharpness are supreme for this aperture. When a doublet is this free from chromatic aberration, this sharp and contrasty and quick-cooling, why go to the complexity and weight of a triplet like the LOMO 80/600? The FL80 confounds the idea that triplets are better for small APOs. The only area in which a more complex lens might better the FL80 is on field curvature, but serious imagers will want to use a reducer/flattener anyway.

Of course, the FL80 is a small scope and has limitations as such, 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. In this case the magic mineral provides a level of correction for chromatic aberration that I have never seen in an ED doublet of similar spec’ (though I’m willing to stand corrected if one turns up). Make no mistake about it, the difference is quite subtle, but 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.

For me the FL80 is a very useful and highly portable second scope and may well be a long term keeper, because nothing I know that does so much is so lightweight. Maybe I’ll even take it somewhere for the 2018 Mars opposition.

Highly recommended, especially for refractor buffs who value a really perfect view through their scope (buy a brass TV-85 for spouse/living-room appeal).