Vixen FL-102S SP Review
Vixen’s FL-102S was one of the first fluorite-doublet 4” apochromats. It has a big reputation, especially in Japan, for outstanding visual performance. I’ve long wanted to try one and see how it compares – both with its contemporary, the original Takahashi FC-100 that I reviewed here, but to more recent doublet apochromats too.
A big thanks to Justin for the loan a of magnificent example, in pristine condition and equipped with all the original Vixen accessories, for this review.
At A Glance
Vixen SP FL-102S
900mm (later 920mm)
F8.8 (later F9)
940mm (37”) w/o EP Holder
3.75Kg incl. rings
Data from Vixen/Me.
Early Vixen fluorites have different graphics (above on an FL-70S) from later ones.
Later F9 version of the FL102S with aftermarket focuser (image credit: Alex Shen, with thanks).
Design and Build
Vixen once made a whole range of Steinheil fluorite doublet refractors in competition with Takahashi’s FC-series. The range included 55mm, 70mm, 80mm, 90mm and 102mm models; but no equivalent of the FC-125.
Vixen’s fluorite OTAs came in two distinct models, earlier and later, with different tube and focuser styles. Optically there were some changes too, but Vixen never went over to a front-surface Fraunhofer design like Takahashi.
This is the earlier F8.8 ‘SP - Super Polaris’ version, named after the mount that it shipped on as a package. At the time, Vixen made SP versions of all their fluorite OTAs, including the FL-70S I reviewed here. These early scopes were uncompromisingly aimed at the visual astronomer interested mainly in the Moon and planets.
The later FL102S had optics, focuser and tube that were all subtly different (see above). It was likely packaged with the later GP or GPDX mount, but wasn’t named as such like the older version.
The early F8.8 model was sold under the Celestron brand in the US at the time and the later F9 version as an Orion.
FL-102S and ED102S lens cells.
F9 FL102S lens cell.
Laser disappears only in fluorite, not in glass.
The FL-102S has an F8.8 (900mm focal length) Steinheil doublet with a positive fluorite crown element at the back (see above). This is the same arrangement as the current Takahashi FC-100D and the older FC-100, but different from many refractors which have their crowns at the front. Unlike the modern FC-100D, here the fluorite is uncoated due to coating technology limitations at the time.
The later model had a slightly longer 920mm focal length (F9). I’m reliably informed that there is little difference in optical performance and the change may have been to rationalise production with the ED102S which had the same optical specs as the later FL102S.
In either case, ~F9 is slow for a 4” fluorite apochromat and should give excellent correction.
Indeed, the main reason for the FL-102S’ strong reputation is the lens - for its correction, but quality too. Designed by a famous Japanese optical guru, it was typically very finely figured and finished, as were all the other Vixen fluorite objectives (I know of a random FL-70S found to be 0.99 Strehl on an interferometer).
The FL-102S lens cell has collimation screws and is more sophisticated than the ED102S’, with a temperature-compensating gap in its micro-baffled lens ring, where the ED102S (and later SD103S) get a plain lens ring with neither ridges nor flat black paint.
Whilst the lens and cell are comparable to a Takahashi FC-100 of the same era, everything is different in detail, from the mating element to the coatings and cell construction. Whereas Tak’s objectives were (and are) made by Canon/Optron, I believe these Vixen objectives were made in-house.
The OTA is finished in gloss off-white and has a fixed dewshield which threads on, unlike the later ED models’ which are push-fit. The lens cell threads on, but the focuser is attached by three screws.
This is a notably light OTA for its size: I measured 3.75Kg including the rings.
The later FL model sports a shorter dew-shield and different graphics that match other fluorite doublet models at the time, e.g. the FL-80S I reviewed here. Like all the later scopes in the FL series, the later FL-102S omitted the serial number on the OTA sticker.
This version has a serial number; later ones don’t.
Early FL-102S has a smaller focuser. Later ones’ like the ED102S’ on the right.
The focuser is a small, single-speed unit with a cast body finished in Vixen green ‘Hammerite’. The chrome drawtube has a narrow, cross-cut rack and is 55mm diameter with 80mm travel. It terminates in a (I think) M50 female thread.
The knobs are Vixen’s standard black plastic ones that graced all their focusers from this era: they look a lot like Takahashi’s apart from the colour and I’ve seen them sprayed silver (please don’t!)
The FL-102S is specialised for visual use on the Moon and planets and so includes an adapter for a regular M36.4 0.965” or 1.25” eyepiece holder, or in this case a 1.25” diagonal that threads straight in; but there’s no obvious way to attach a 2” eyepiece holder.
The focuser has a pivoting mount point (so you can move it out of the way of the eyepiece) for the finder and there is no finder dovetail.
The later FL model also got the ED102’s heavier focuser with a wider (64mm) drawtube that is better suited to imaging and dovetails to suite a range of finders. It’s a focuser that continues on Vixen’s current SD103S and AX103S and has been cloned on countless Sky-Watchers.
As the OTA serial plate says, the FL-102S came on Super Polaris mount with an early HAL-130 tripod. It’s a lightweight mount, driven in RA, with a clutch so you can operate the RA manually with a slo-mo knob. The SP has no dovetail: the rings attach directly to a fixed plate.
The split-rings are Vixen’s standard formed items that look a bit basic compared to a Takahashi clamshell, but have the advantage of lightness and security – the OTA can’t fall out. You just loosen the thumbscrews to slide the OTA. Want to get them off? You’ll need to remove the cell or focuser!
The later F9 FL102S (including the Orion-branded one) came on a GP or GPDX mount like the one below with an FL80S.
In terms of modern Vixen mounts, it would work on an AP in terms of weight, but an SX2 would be better for killing vibes at high power.
The F9 version would have come with a GP (or GPDX) mount, like this one with an FL80S.
Some of the period accessories bought with the FL-102S by its original owner.
Vixen’s Lanthanum eyepieces, from the same period as the FL-102S.
The FL-102S in this review was originally supplied by the UK importer, Broadhurst Clarkson &Fuller, as a full luxury set: the SP mount, 60mm guide scope and 6x30 finder you see in the photos.
The 60mm/910mm guider is a fine long-focus achromat in its own right and gave excellent views. It even has a serial plate with a number that matches the main scope. Nylon-tipped screws hold it securely in the rings that attach to the top of the main ones.
This particular FL-102S was also equipped with a range of optional Circle-V accessories that included a T-2 camera adapter, an eyepiece projection adapter, the standard 22mm and 10mm silver-top Plössls that shipped with the OTA, a 25mm Circle-T Erfle, and more. Sadly (though not for the new owner!), none of these appeared to have been used.
The owner also loaned me a set of period Lanthanum eyepieces for the full retro-Vixen experience.
The finder is a standard, quality Vixen 6x30 unit that attaches by an odd (but useful) angle-adjustable stalk.
In Use – Daytime
My usual daytime shot of branches – shown below as a 100% crop - shows no false colour at all and is pin sharp: more proof of the FL-102S’ outstanding correction and puts it in a rare group of doublets with Takahashi’s FOA-60, perhaps slightly ahead of the original FC-100 and even of the recent FC-100DZ which still revealed the faintest trace of false colour.
In Use – Astrophotography
The M36.4 visual back has a T-2 adapter. It’s too narrow for wide-field AP, but results are good with very low blue bloat on hot stars, confirming this objective is unusually well corrected (for a doublet) at shorter wavelengths.
As you would expect, the FL-102S is much better suited to imaging the Moon, which is crisp and sharp and full of detail. But it’s interesting to note just a little false colour on the bright full Moon at prime focus below.
Both images below are reduced in size, but not processed in any way, so you can compare them with my other reviews.
Pleiades: 30s ISO3200 Canon EOS 6D MkII full-frame.
In Use – Observing the Night Sky
General Observing Notes
The focuser has a heavier feel than the later one on the ED102S, partially because it’s had very little use, but suffers from no image shift. It’s generally sufficiently precise, but is at the limits at the 200x and above that the objective easily copes with.
True to reputation, optical performance is very fine for a doublet. The FL-102S takes high powers with aplomb and generates minimal false colour under any circumstances.
A reader (Alex Shen, with thanks) who owns both the later F9 FL-102S and a super-ED triplet Takahashi TSA-102 (one of the very best 4” apochromats) has compared them extensively and reports very little difference, just a trace more false colour from the FL-102S at over 300x.
The original Lanthanum eyepieces proved excellent – narrow of field, but very sharp and well-corrected to the edge, with lots of eye relief.
A doublet in a proper temperature-compensating cell is a recipe for fast cooldown and the FL-102S doesn’t disappoint. Cool down is fast: just ten or fifteen minutes. I’ll say it again, don’t discount the much slower cool-down of a triplet - like a TSA-102 or LZOS 100/800 - if you want to use it for quick looks.
The star test was excellent, with sharp and evenly illuminated rings, just a little under corrected.
257x with a 3.5mm Nagler T6 brought a gibbous Moon into closer and more intimate detail than I’m used to with a 4” aperture. High in the sky and in good seeing, it revealed the intricacies of the slumped walls in Copernicus, Lots of subtle wrinkle ridges in the Ocean of Storms on the terminator, close-up views of the Pitatus area and several tiny craters in Plato.
A few days later, peaks on the very limb of a full Moon showed themselves as lunar mountains really are: rounded and smooth, not the exaggeratedly jagged peaks you get on the terminator.
A super-sharp view of the limb was helped by minimal flare and no in-focus false colour (interestingly, unlike a prime focus image – see above – due to the different sensitivity of a camera sensor). The FL-102S is a little better than the ED102S in this regard.
A bright but small Venus high in the sky showed a perfectly clean gibbous disk with no flare and only a hint of false-colour-gold out of focus. Only the finest triplets remove that last tinge of false colour.
A diminishing (10.8”) and now gibbous Mars in fine seeing really surprised me by remaining bright and completely sharp, with perfect focus snap and not a hint of softness or false colour anywhere, in focus or out. It looked like the high-power view through a triplet super-APO.
In still moments, bright Hellas and dark Syrtis Major, the dark strip of Mare Sirenum came into sharp focus with 7mm and 5mm Nagler T6s, giving 129x and 180x respectively.
Upping power to 225x with 4mm Zeiss Abbe Ortho’ gave the best view of all, with excellent contrast, no false colour in or out of focus and little light bleed into surrounding space. A perfect little gibbous planet.
On another occasion, even the 3mm setting on Nagler Zoom giving 300x showed a clean, sharp image. This is another instance where the FL is noticeably better than either the ED102S or SD103S, which go slightly soft on Mars above 200x.
Castor was a perfect split, with narrow stellar images giving wide black space between at 180x with a 5mm Nagler T6.
Despite the uncoated fluorite element, the FL-102S gives high-contrast views of DSOs like the Crab Nebula. At 47x with a 19mm Panoptic, The Pleiades looked wonderfully brilliant and sparkling, with a touch of misty blue only from the stars themselves, not from chromatic aberration.
The Double Cluster filled the field of the 19mm Panoptic, with sprays of pinpoint stars of subtly differing hues.
Vixen FL-102S vs Vixen ED102S
Vixen ED102S (above) and FL-102S from the same era.
These two Vixen 102mm refractors – ED and FL - are superficially very similar and hail from the same era, so let’s compare them…
· This early FL objective is 102mm/900mm, the ED’s 102mm/920mm (same as the later FL)
· The FL objective is a Steinheil fluorite doublet in a temperature-compensating, collimatable cell
· The ED objective is a Fraunhofer ED doublet in a basic fixed cell
· False colour in my 100x branches test is similar, with just a hint out of focus in the ED
· At 200x the ED shows a trace of false colour on a bright lunar limb, esp. out of focus; the FL does not (visually anyhow)
· At high power, the ED suffers more light bleed and stray light
· Mars was soft and mushy in the ED at 300x with some red blur, sharply defined and false-colour-free in the FL minutes later
· The ED has a slightly shorter OTA, a push-fit dew-shield
· The ED has the later style focuser, same as the F9 FL, with a wider drawtube
Refractors don’t come more classic than the FL-102S; but though it’s a beautiful retro scope, it’s still an outstanding apochromat today.
The objective remains one of the best corrected doublets of all, with virtually no false colour under any circumstances. Designed for the Moon and planets, it gives outstanding high-power views, including of Mars which troubles many doublets that suffer spherical aberration in the far red.
The OTA is perhaps not as finely constructed as a Takahashi FC-100 of the same era, but has the big advantage of being much lighter to handle and mount. And though the focuser isn’t as imaging friendly as later Vixens’ (hardly a problem for what is primarily a visual scope), it is smooth and precise for visual use.
Now consider that as well as being easier to mount than a typical 4” triplet, the FL-102S is also much quicker to cool for brief sessions. All in all, if you want a fine 4” refractor for high-power viewing there are still few better, classic or not.
Vixen’s FL-102S is a true classic, but it’s still a superb lunar and planetary specialist today: if those are your interests there are few finer refractors at this size.