Takahashi FS-102 Review
The FS-102 is one of a range of long discontinued Takahashi refractors. In my view the FS series are some of the finest small refractors ever made for visual use (as opposed to imaging, for which more complex designs – triplets and quadruplets - may have an advantage).
In my opinion, a fine four inch apochromat is the most versatile small telescope you can buy, which is why I prefer the FS-102 to both the FS-128 and FS-78. The FS-78 is big for a three-inch scope and is little more portable than the FS-102, but considerably less able due to its smaller aperture. The FS-128 is magnificent, but much less portable than the FS-102 and requires a much bigger mount due to its length, yet on all but the finest nights the FS-102 does a similar job.
I have owned two FS-102s: the older version with a green lens ring (see above) and a newer model with the blue lens ring which features in some of the other photos in this review; optically they are the same.
At A Glance
Fluorite doublet (fluorite at front)
Data from Takahashi FS Manual.
Design and Build
The FS range of refractors included 78mm, 102mm, 128mm and 152mm models, all with F8 fluorite doublets. The odd one out is the FS-60 which is an F6 and the only one still in production.
As well as a shared optical design, the F8 models all had a similar tube and focuser and family ‘look’. Most had fixed dew-shields, all were bulky for their aperture. All had a strange but classy cast ‘manhole cover’ dew cap that slid into the dewshield with a perfect fit on a felt shim.
The FS-102 objective is 102mm aperture, 820mm FL (F8.1 to be exact). It has superb coatings and a high quality, collimatable, temperature compensating cell. The lenses for the FS series were made by Canon-Optron to Takahashi spec’s, like other Takahashi objectives.
The FS-102 shares the same lens design and cell with its larger and smaller siblings, all of which are ~F8 fluorite doublets with the positive fluorite element on the front (hence the ‘FS’ – Front Surface – moniker). This means the lens is a conventional ‘Fraunhofer’ doublet, like a basic achromat, but with fluorite as the crown.
In recent years, Takahashi have brought back a fluorite doublet at this size, but it’s rather different with a fluorite-at-the-back Steinheil design, faster f-ratio (F7.4) and optimised for imaging (hence the ‘D’ for digital tag).
Does fluorite offer a genuine advantage over high-fluoride glass like Ohara’s FPL53 or OK4 (the stuff LZOS make and use)? Possibly. Fluorite is not a glass, it’s a crystalline mineral and it has optical properties that no glass can quite match, including very high transmissivity and low scatter. The FS manual has this to say:
“… the use of multi-coatings further increases light transmission over any ED glass. This makes any fluorite objective brighter than any comparable glass objective …”
Does putting the fluorite at the front make for a worse optic, as some have claimed? I believe not. Check-out the excerpt from the FS manual below.
Independent tests have found the FS-102 to be a true apochromat, whereas ED doublets (think TV-102) often aren’t. Takahashi’s own spot diagrams suggest that the FS-102 is less well corrected at the violet end than the TSA-102 triplet (so less good for imagers), but better in the yellow, orange and red (great for Mars).
Overall, don’t think of the FS-series as inferior to a triplet, or ‘old-fashioned’ compared to the Steihnheil FC-100D. Instead think of them as being optimised for visual use, offering razor-sharpness and maximum contrast.
Excerpt about front-surface fluorite from the FS Series manual.
All the FS series share a simple, beautifully finished white enamelled tube with a blue lens ring (early FS102s had green ones – see title photo) and a long fixed dewshield and that cast “manhole cover” lens cap. The look of the FS-series has a kind of ‘Zen’ simplicity to it.
The tube has a number of knife-edge baffles to prevent stray light reaching the focal plane and these, along with the big fixed dew-shield, mean that the tubes are physically large (the FS-102 is almost a meter long and 114mm in diameter), but light due to the doublet design (the FS-102 weighs about 5 kg).
Size is perhaps the only real drawback of the FS series. The FS-102 is much larger than the newer FC-100D and heavier too (the basic FC-100DC is only 2.8 Kg) – see photos below.
Takahashi later made a sliding dew-shield version of the FS-102 (the FS-102NS). This is much more compact (but still heavy) and looks like a large Sky-90 (see photos below).
The fixed dew-shield FS-102N is by far the most common variant. It’s a big scope for a 4”.
Sliding dew-shield version is much more compact. Shown with Sky-90 and FS-60Q for scale.
All the FS models had simple, high quality single speed rack-and-pinion focusers with cast bodies and traditional Takahashi green finish (a 10:1 dual speed unit is available as an accessory). The version fitted to the FS-102 has a 2.7” drawtube and those signature silver-finished knobs. On the FS-102 those knobs are solid metal, but on some of the cheaper models (the FS-78 and FS-60) they are hollow plastic look-alikes.
Overall finish is top-notch, but plain and functional, like the tube. The shorter bodied focusers on the FS-60 and Sky-90 can have problems with image-shift in my experience, but the longer focusers much less so. The Takahashi focuser at its best has a creamy smoothness unmatched even by a Feathertouch in my opinion. Many people upgraded the focuser with a dual speed unit – either Tak’s own or a complete Feathertouch pinion assembly - but for visual use the original was perfect.
The FS-102 came with a 1.25” visual back with Tak’s characteristic twist-grip as standard. A 2” visual back was an accessory, but so expensive that some 3rd party ones got fitted (as in the scope here, by its previous owner, not by me). The drawtube terminates in an M72 thread, so the whole visual back can be replaced by 3rd party components using Borg’s M72 adapter.
FS-102NS fitted with a Starlight Instruments FeatherTouch focuser pinion.
The OTA is mounted in a beautifully-made, cast clamshell which directly bolts onto Takahashi mounts via a standard pair of M8 holes, although you can fit a dovetail – Takahashi make a slim silver one that works with a Vixen/CG5 shoe or dovetail and others are available from various manufacturers. It’s not widely available, but Takahashi do in fact make a dovetail to fit their mounts so you can easily swap scopes.
The light weight of the FS-102 means it works fine on my Vixen GP, but I found that a Vixen Porta struggled. The title photo shows it on Takahashi’s own EM200 mount – an over-mounted combination so stable I once used it at high power in a very high wind with no tremor at all. The FS-102 should work perfectly on various medium mounts, including Vixen’s Sphinx and derivatives.
The FS-102 would likely have been bought with the 6x30 or the 7x50 finder. The 6x30 is one of the very best finders available, with loads of eye relief and a wide sharp field. It’s an expensive option in the UK; but in Japan, Takahashi list it as a point-of-sale upgrade for just Y8000 (about £55) in their catalogue.
The 7x50 finder was often spec’d on the FS-102, it has less eye relief and a smaller field, but can be fitted with an illuminator.
Takahashi made both a 1.6x extender and a 0.74x reducer for the FS Series refractors. The FS reducer was a premium product that delivered a 610mm focal length and large image circle, but it was an expensive option.
Takahashi’s excellent 60x30 finder and view.
In Use – Daytime
The FS-102 is too large for use as a spotter, which is a shame because it gives great daytime views and photos too. I idly spent an hour snapping some deer in the copse opposite. One of those images (through a float glass window pane) is shown below.
In Use – Astrophotography
The FS-102 makes a good astrograph, though the TSA-102 and FC-100D are probably better tuned for deep sky imaging (see spot diagrams above). The only prime-focus image taken with the FS-102 I can find from a decade ago is the one of the double cluster below. It shows good star colours and very modest bloating.
The FS-series reducer speeds the FS-102 up to F6 (610mm FL) with an image circle of 51mm (from the days when 35mm film was standard for astrophotography). I haven’t tried it.
Like other FS Series refractors, the FS-102 takes a great snap of the Moon – sharp and detailed. The image below was taken with an FS-128, but is very similar.
Sharp Moon through FS-series (slightly enhanced contrast and cropped).
In Use – The Night Sky
General Observing Notes
The reason the FS-102 is one of my favourite telescopes is that it does everything superbly. On every type of object, the FS-102 offers razor sharp, high contrast views with no in-focus chromatic aberration (there might be a trace out-of-focus on O-B stars).
If you believe that simple eyepiece designs can offer a bit more sharpness and contrast than complex ones like Naglers, then you have to concede that an FS fluorite doublet must deliver just a bit more contrast than an ED triplet. Not only are there two less optical surfaces, but fluorite (which the light hits first) simply scatters and reflects less light than glass. I think this theoretical advantage is visible. The difference is small and subtle, but it is there.
The focuser may not look as fancy as some of the modern CNC units, but it has loads of travel, is very stable under big loads and is super-smooth and precise. Like most Tak’ gear, it just works. The wider draw-tube focuser on the FS-102 is a little better – more stable and smoother - than the narrower unit on the FS-78.
When Ed Ting compared the FS-102 with an AP Traveller years ago he found they offered a very similar level of performance. When I did the same comparison many years later at the Grand Canyon Star Party, I found a similar result, though I thought that perhaps the FS-102 had the edge in contrast.
The FS-102 cools quickly, thanks to being a doublet with a quality cell. In comparison, a TMB 100/8 – one of the best 4” triplets - took perhaps twice as long.
Takahashi’s spec’ to Optron was rumoured to be a minimum of 95% Strehl. The optical quality of the two FS-102s I have owned was very high and both showed a virtually perfect star test.
The Moon is achingly beautiful through the FS-102 (as it was with the FS-128), with a clarity and brilliant white sharpness few scopes can match and a much more involving level of detail resolvable than with a 3” refractor: fine rilles, craterlets and domes. The lens scatters so little light that mountains on the Moon’s limb seem etched in 3D against the pure black of space, with no flare at all.
I recall one early morning with exceptionally steady seeing and the FS-102 mounted super-stably on the EM-200: views of the Moon were almost unbelievably crisp and detailed, with lots of craterlets and rilles resolved down to its theoretical limits.
To quote the FS manual again, “The FS refractors are particularly suited for planetary observation. Their ultra high contrast, sharp images will reveal a wealth of detail.”
My first FS-102 (the one with the green lens ring) was an old scope which had been well used, but it still delivered some of the best planetary views I have had from a four inch refractor.
Real Martian detail is on offer with the FS-102 - the planet showing as a proper crisp disk with well-defined albedo markings and caps where most scopes show a fuzzy orange ball. As I said, the spot diagrams indicate the lens corrects unusually well into the red and this shows on Mars.
Views of Saturn and Jupiter were similarly excellent, with Saturn’s Cassini division easily visible in a way it isn’t with smaller refractors. Venus displayed a perfect, dazzling phase, refusing to show flare or chromatic aberration.
These ‘scopes were designed to excel at delivering planetary and lunar contrast and detail at high-powers (it says so in the manual), but the FS-102 has a short enough focal length to do star fields and extended DSOs as well. Stars have that perfect sparkling diamonds-on-velvet look that only the best APOs give and the field is very flat.
A four inch APO is one of the best all-round small telescopes and the FS-102 is one of the very finest four inch APOs.
People now seem to view doublets as inferior, but for visual use in small apertures, that’s nonsense. They also seem to regard all doublets as equal – again nonsense. The Canon/Optron lenses in the FS series used crystalline fluorite (not fluoride ED glass) and are a no-compromise design for visual use that work well for imaging too.
My TMB 100/8 had slightly more perfect colour correction (the best I’ve ever seen in any refractor), but was much slower to cool, heavier and – perhaps – delivered just a little less contrast (but I’m willing to concede that might be imagination).
The biggest disadvantage of the FS-102 is just that – it’s big and bulky for its aperture, a ‘fault’ shared by the whole FS range. If you can find an sliding dew-shield FS-102NS, that doesn’t apply and its only downside is that it’s heavier than, say, a modern FC-100D.
So the FS-102 is something of a used bargain, if you can still find a nice one – especially if visual is your thing. If you can accept the slower cool down, the replacement TSA- 102 is doubtless a superb telescope and would be a good alternative, as would the smaller FC-100D.
The guy who bought my second FS-102 did so because he had been comparing one to a 132mm Chinese triplet APO at a star party. He told me that the bigger scope looked prettier, but the Takahashi gave nicer views; says it all really.
Is there any problem with the FS-102? Two actually: I stupidly sold both of mine.
For visual use, the Takahashi FS-102 is one of the most perfect small telescopes ever made and is very highly recommended. Can I buy mine back now, please?
Updated by Roger Vine 2019
FS-102 on EM-200USD with a Takahashi 7mm Ortho’ nestling on its eyepiece tray – a perfect setup for planets.