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Vixen FL70S Review

Vixen made their classic 1990s apochromatic refractors - the FL series - in a range of sizes, from 55mm to 102mm. All have Canon fluorite doublets at F8 or F9. All are light weight for their aperture.

The sweet spot in the FL range is arguably the FL80, a scope I’ve tested already and found to be excellent. But Vixen made a couple of smaller models as well – the FL55 (not the rebooted imaging scope) and this, the FL70.

The FL70 was the one that got me interested in the FL series. I first saw one at a friend’s many years ago and one look was enough to tell me it was something special. The views it gave of Venus were some of the best I’d ever had back then, with any scope.

But that was a long time ago and I was curious to see how the FL70 stacks up today. This is a rare telescope now. People hang on to them. So to find out, I sourced a near-mint example from its homeland for this review.

At A Glance

Telescope

Vixen FL70S

Aperture

70mm

Focal Length

560mm

Focal Ratio

F8

Length

510mm excl. visual back

Weight

~2.5 Kg

 Data from Me.

Design and Build

All FLs have the same workaday mid-range look as Vixen achromats from that era, less premium than a Takahashi of the time.

The focuser knobs are black plastic, as is the focuser lock-knob; whereas on a contemporary FC-series Takahashi they are anodised metal, or at least a classy-looking plastic imitation. The Vixen rings are formed rather than Takahashi cast. And the tube has too many stickers for my taste. The cast parts don’t quite exude quality like a Takahashi somehow, either.

Though this earlier FL looks the same as later examples, it does have some subtle differences. Whilst later FLs seem to lack a serial number, this FL70 has one, much like a Takahashi, even if here it’s punched into a sticker, rather than a screw-on metal plate.

Note that some of Vixen’s FL models were re-badged with the Orion brand (the US importer, not Orion Optics in the UK).

Optics

Like all the ‘fluorite’ lenses produced by Canon, this really does use the mineral, not some high-fluoride glass like FPL-53. I confirmed this with a laser, which disappears completely only in fluorite, not glass.

So, like a contemporary Takahashi FC60, the FL70 has a premium F8 Japanese-made fluorite doublet lens made by Canon/Optron. But note:

·        Like the Takahashi FC-series (old and new), the FL70 is a Steinheil configuration with its positive fluorite element at the back, where it’s protected from dew and lens cloths.

·        In modern Steinheil doublets the fluorite is coated, but the FL70’s rear fluorite element is uncoated as was the FC-60’s. This results in lower transmissivity and a noticeably more reflective lens.

·        This older FL70 has a colimatable cell like larger Takahashis of the time, complete with engraving in yellow where a Tak’ would have had green. But the closest contemporary Takahashi, the FC-60, has a fixed-collimation screw-in cell, like the modern Takahashi FS-60 and the later FL80S I reviewed.

This FL70 was made back when flint glasses were doped with lead and other heavy metals. Some leaded flints made excellent mating elements for fluorite, but were deemed hazardous and were discontinued in favour of ‘eco’ glasses. In my experience, only a few recent high-end scopes - think Tak’s FOA-60 and FC-100DZ - have fluorite doublets with ‘eco’ (in these two cases, special dispersion) flints that match the performance of these FL doublets.

I should mention that the FL series have legendary optical quality. That little FL70 I saw years ago was sold to a chap who got it tested on an interferometer; he was thrilled to find that it had a Strehl above 99% (Tak’ fluorite doublets of the time were made to a minimum of 95% Strehl – still excellent). Bench tests indicate exceptionally low false colour, into super-APO territory.

Note: 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 later FL80S I reviewed reminded me of a big Tak’ FS-60C – a compact tube with a short dew-shield and black-anodised fixed cell of the same diameter. This older FL70 is classic Japanese refractor by comparison: a large-for-the-aperture tube finished in off-white enamel, with oversized dew-shield and a traditional lens ring in hammer-green to match the focuser. In both cases, focuser and lens cell attach with threads.

The classic FL70 looks more attractive, to me at least, but it’s almost as big as the later FL80S: 510mm long to the end of the focuser drawtube and 75mm diameter.

Internally, the FL70 has 3 knife edge baffles and is painted flat black.

Focuser

The FL80S I reviewed had a larger focuser with a 2” visual back and clever built-in rotator suited to imaging. This one is more basic and aimed at visual use.

Note that the fluorite scopes have thread-on focusers, whilst the similar-looking achromats used basic set-screws.

The focuser doesn’t have the famously buttery feel of a Takahashi unit, but it’s still a high-quality, all-metal (well apart from those knobs) item. The feel is smooth and precise with minimal image shift.

The chromed 55mm diameter drawtube sports a thread-on visual back, three internal knife-edge baffles and a cross-cut rack. The 80mm of travel is fine for most eyepiece/diagonal combinations. Two internal bearings with adjustment screws and side-mounted lock knob mean it’s stable and won’t rack-out on its own with a heavy eyepiece.

The supplied visual back is 1.25” only and has a set screw rather than a lock-ring. It’s the same item that came with the later FL80S. It screws into the drawtube with an adapter on what looks like a thread about M52, so a 2” visual back might fit. A pair of thread-in extension tubes are provided for straight-through viewing.

The 1.25” visual back adapter has a standard thread and could be swapped out for a different one, such as Takahashi’s twist-lock.

Mounting

This FL70 is labelled ‘Super Polaris’ and this means it originally came on the small Vixen equatorial of that name, a forerunner of the similar and more commonly encountered GP mount.

The FL70 is too heavy and bulky for most photo heads (unlike, say, a smaller 70mm TV Ranger from the same era), but works really well on a small alt-az mount like a Vixen Porta (not the mini version), where slightly heavier and longer scopes (even the TV-85 I was testing at the time) struggle with excessive vibes.

If you want a period-correct alt-az mount, then you could seek out an old Vixen example with matching green enamel, but bear in mind that the rings bolt-on to these mounts and don’t hinge open, so you’ll be moving the whole setup as a unit.

For greater flexibility I’d prefer the old ‘D’ (for dovetail) version with (you guessed) a Vixen dovetail built-in but also weighted to counterbalance large eyepieces.

The Vixen Porta is later than the FL series, but works well.

Accessories

This FL70 came with the original formed rings and Vixen dovetail plate. The rings have the advantage of light weight, but aren’t a match for the quality of a cast Takahashi clamshell and don’t hinge open which means removing the focuser if you want to take the tube out!

The original 6x30 finder has quality optics (not stopped down like many cheap optical finders) and though it has a plastic eyepiece with less eye relief than the peerless Takahashi 6x30, field of view (8°) is the same.

The finder mount is unusual: it allows the finder to rotate to either side, presumably to clear the eyepiece area when used on an alt-az mount.

The period-correct eyepieces would have been Vixen’s own Circle-V 0.965” Orthoscopics. These are of excellent quality (if not up to Tak’ or Pentax Ortho’s) and can still be had quite cheaply if you want to assemble a full retro’ rig.

In Use – Daytime

The size and style of the FL70 means you probably wouldn’t use it in a hide for nature viewing or birding, the way some do a small Tele Vue; but daytime views are simply outstanding.

I tried using the FL70 to view waders – Curlews and Oystercatchers - out on the bay sands near my home. The view was almost hyper-real: sharp and vivid and completely free of false colour in a way few spotting scopes manage. But I did get asked by a local birder why I was using an astro’ scope!

At 100x, silhouetted branches (my usual ‘test’) are basically free of false-colour fringing: none in focus and the faintest (really) tint focusing through. This is a rare result for a doublet. Sharpness meanwhile is perfect even at this power.

It may be an astro’ scope, but the FL70 works outstandingly well for nature viewing.

In Use – Astrophotography

Lack of a 2” visual back (and I couldn’t find one to buy as an accessory) precluded deep-sky imaging, but I did take a snap of the Moon, which is sharp and free from flare or false colour.

In Use – Observing the Night Sky

General Observing Notes

The superb optics are let down slightly by the focuser – it’s not worn or sloppy or imprecise, but heavy at room temperature it gets pretty stiff when cold and may need re-lubricating.

Cool Down

A small doublet in a proper cell cools fast and this is no exception – completely usable within a few minutes and giving its best in ten to fifteen, with no pinching or severe spherical aberration that I’ve seen from simpler cells. This is such an important feature for a quick-look scope.

Star Test

The star test is effectively perfect with evenly illuminated, identical rings either side of focus and no perceivable under or over correction. In focus at high power yields a perfect Airy disk and dim single diffraction ring.

The Moon 

A low first quarter Moon in turbulent seeing across the bay nonetheless shows all the major features you can see with a ~3” refractor, including the dramatic crater grouping of Theophilus and Catharina near the terminator.

Focusing through the bright limb at 112x reveals no false colour fringing at all – this is a perfect Moon of only blacks, whites, greys and buffs.

I managed to see a two-day-old Moon from the top of the fell – only a few degrees above the horizon, it’s a difficult phase. The FL70’s peerless contrast revealed more features than I’ve ever seen in this thinnest of crescents, including Mare Marginis and Smythii on the very limb, dark-floored Mare Humboldtianum and spectacular crater Gauss.

Venus

Perhaps the ultimate visual test for any refractor, a razor-sharp crescent Venus was my first taste of the FL70’s capabilities all those years ago.

Now, a dazzling magnitude -4.7 crescent at just 20 degrees altitude and 112x with a 5mm Nagler shows only a hint of gold on the limb that might be atmospheric and no significant false colour focusing through and no flare either.

Jupiter

A large and bright Jupiter shows its creamy hue to perfection, with no false colour and perfect sharpness. All the main details are on show, including the equatorial belts, polar hoods, a hint of storms and of course the tiny disks of the Galilean moons. The maximum power I could get - 187x with the 3mm setting on a Nagler zoom - was still perfectly sharp and contrasty.

Saturn

Again, Saturn has that buff, almost pink hue unsullied by false colour or flare. The Cassini division is visible in the ring ‘handles’, along with the polar hood and a hint of cloud-belt banding. It’s a perfect view at this aperture and noticeably more feature-rich than through a 60mm apochromat of similar quality.

Deep Sky

The superb contrast and very tight stellar images from the high-strehl objective make the FL70 a surprisingly good tool for visual deep sky. I had a most enjoyable evening with it just after New Year, viewing out over the bay from a secluded and dark spot on the Prom’.

The Great Nebula in Orion revealed significant structure at 112x – as good as I’ve seen in a small scope. I easily picked the strange oval misty patch of the Crab Nebula out of the darkness above Zeta Tauri too.

Open clusters seem to be a particular strength. The Pleiades were the glitteringly brilliant blue I’m used to in a quality refractor, but I was surprised at the amount of nebulosity I could see, including a hint of structure around Merope. Similarly, the Starfish in Auriga showed its sweeping arms of stars more clearly than I recall in a scope of this aperture.

The Owl Cluster in Cassiopea is one of my favourites, one of the few objects that really looks its name, with ‘spooky’ (according to my daughter) eyes and outstretched wings. The FL70 gave a wonderful view of it, ditto the nearby Double Cluster.

Epsilon Lyrae was a perfect split in both components at 112x, but upping the power to 187x with the 3mm setting of a Nagler zoom revealed four perfect Airy disks with overlapping first diffraction rings and black space between – startlingly good for a 70mm optic. I split Rigel from its dim companion easily at 112x, even when low in the sky.

Summary

Most small apochromats these days are optimised for imaging. The Vixen FL series (like Takahashis of the time) were mainly built for visual use, especially on the Moon and planets.

What does this mean in practice? This: an F8 fluorite doublet like the FL70 gives better sharpness at high magnifications and lower in-focus false colour and spherochromatism than a modern ~F6 ED doublet. This was brought home to me again when viewing Mars near opposition recently. A TV-85 gave a slightly softer view, with more unfocussed red light bleeding into the seeing.

F6 Triplets can offer similar correction to a fine fluorite F8 doublet, but are heavier and slower to cool. So, if you want a small lunar/planetary scope, for travel and quick looks, then an F8 fluorite doublet of 70-80mm aperture is ideal.

Takahashi’s FC-76D is the only widely-available option new, but this old FL70 offers similar (or perhaps even slightly better) correction in a similar-sized OTA and with a wonderful classic vibe. The only downside is its focuser, which lacks the fluidity and absolute precision of a Takahashi (or a modern dual-speed unit).

Finally, the FL70’s in-between-sized aperture is unusual, but I really like it: noticeably more capable than a 60mm, it’s still very light and portable.

Vixen fluorites like the FL70 are getting rarer, but still make a superb alternative to a small Takahashi, especially if observing is your thing.

 

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