Takahashi FS-102 Review

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Perfection: FS-102 on EM-200.

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).

At various times I have owned the FS-78, two FS-102s and a FS-128, all of which have been superb. I have never owned an FS-152: I dithered, then finally decided to buy one just as they had just been discontinued – the last new one had just been sold when I tried to place my order back in 2004.

Design and Build

Lens

The FS series have now been replaced by the TSA and TOA series of ED glass triplets. Why? Because the triplets’ perfect CA correction gives a small advantage for imaging and they may also actually be cheaper to make. They are also heavier, slower to cool and may lose just a little visual brilliance and sharpness.

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). The FS-102 objective is 102mm/820mm FL.

For a full description of fluorite and the FS optics, see the review of the smallest in the range, the FS-78. Here I will just say that the lens has superb coatings and a high quality, adjustable, temperature compensating cell. The lenses for the FS series were made by Canon-Optron.

Borg now sell a 50mm F8 fluorite lens made by Optron for their Mini Borg line. As I write the objective alone retails at $695. Make no mistake, the Tak’ FS series were always expensive lenses, never merely a cheap alternative to a triplet.

Does fluorite at the front 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.

Don’t think of these ‘scopes as cheaper, inferior alternatives to a triplet. Instead think of them as being optimised for visual use, offering razor-sharpness and maximum contrast.

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Superb coatings and a very fine cell characterised Optron’s FS lenses.

Tube

All the FS series share similar mechanical and visual features, in addition to similar optics. This means 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 with a cast “manhole cover” lens cap.

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The FS-102 is large for a 4” refractor, but light in weight.

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. When Ed Ting compared the FS-102 with an AP Traveller years ago he found they offered a very similar level of performance. The difference of course if that the Traveller is tiny by comparison (smaller than the FS-78)and so much more portable  … and now about four times the price on the used market.

Focuser

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). Overall finish is top-notch, but plain and functional. The shorter bodied focusers on the FS-60 and Sky-90 have problems with image-shift in my experience, but the longer focusers have very little. Only absolutely premium CNC units - like the Feathertouch - better it in practice.

 

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The FS-102 focuser is super-smooth and stable.

Mounting

The OTA is mounted in a beautifully-made, cast clamshell which directly bolts onto Takahashi mounts via a standard pair of holes, although you can fit a dovetail – Takahashi make one that works with in a Vixen shoe and others are available from various manufacturers.

The light weight of the FS-102 means it works fine on my Vixen GP, but I found that a Vixen Porta struggled a bit. 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.

In Use

The reason the FS-102 is one of my favourite telescopes is that it does everything superbly. For a start it cools very 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.

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 (personally I’m still unconvinced about this), 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.

In my opinion a fine four inch APO 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 3 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.

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 bigger draw-tube focuser on the FS-102 is a little better – more stable and smoother - than the narrower unit on the FS-78.

Planets

The optical quality on the two FS-102s I have owned was very high and both showed a virtually perfect star test. One of them 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. 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.

The Moon

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.

Deep-Sky

Obviously 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.

So what is there about the FS-102 not to like? 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.

Summary

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 that would be fiendishly expensive to make today. Cost was almost certainly a major factor in their demise. As I have said elsewhere, Roland Christen reckons these lenses would be several times the cost of an ED triplet to fabricate now.

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 (though not heavy) for its aperture, a ‘fault’ shared by the whole FS range.

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.

Very highly recommended. The Takahashi FS-102 is one of the most perfect small telescopes ever made. Can I buy mine back now, please?