Takahashi TSA-102S Review

When Takahashi discontinued the FS series, almost a decade ago now, they effectively gave up on the idea of having a single scaled design. The FS-series encompassed every major aperture class from 60mm to 152mm; now a number of different designs fill the same space. For the smaller apertures, Tak’ still makes fluorite doublets: the FS-60 and FC-76. At the upper end they introduced a radical new configuration in the TOA130 and TOA-150, a triplet with a very large air space that gives the best colour correction of any APO. But the TOA requires a heavy, expensive cell, so for the middling apertures, the TSA-102 and TSA-120, they have adopted a conventional ED triplet of moderate focal length.

The old FS-102, which the TSA-102 replaced, has long been one of my all-time favourite telescopes: light-weight and fast-cooling, it gives super-crisp, high contrast views that come close to triplet super-APO levels in terms of chromatic aberration. I have always suspected that replacing it with a triplet might have been more about cost and fashion than any genuine technical improvement.

Having said that, Tak’s own data suggest that the TSA-102 is a genuine improvement over the FS-102 in all measurable ways (except weight) – point spread function, polychromatic Strehl, etc. But what about the things that are harder to measure – cool-down time, contrast delivery, sharpness, stray light? Let’s find out …

 

Design and Build

Optics

I take a fairly lengthy look at the TSA’s optics in this section, particularly with a view to answering the question I posed above by starting with a technical comparison to the FS-102. Skip this section if the optical ins and outs aren’t of interest.

The acronym TSA stands for Takahashi Super Apochromat and is significant. A ‘Super APO’ is supposed to be one of a small breed of refractors that is essentially free from chromatic aberration across the visual spectrum and beyond. A super-APO should bring all visible wavelengths to the same focal point and maintain a high Strehl across the full visible spectrum.

If you have read many critical refractor reviews (whether mine or elsewhere) you will know that this ideal is rarely met in practice. Most ‘APOs’, even triplets, are far from CA-free if you know how to push them a bit and most have performance that falls off in some part of the spectrum (usually the red these days as a nod to imagers who want their O-B stars un-bloated).

As we will see the TSA is indeed a Super-APO, one of very few I have tested. Certainly for visual purposes and to my eyes, the TSA is completely free from chromatic aberration. Does this matter? Well for high-power use on the planets or critical imaging it might.

So how does the TSA-102 achieve this feat? The answer is a carefully designed air-spaced triplet made in Japan by Canon/Optron. The quality of both the cell and the glass is of the very highest, but the design is quite conventional, unlike the larger 130 and 150 tak’ APOs which use a very large air gap to allow even further control of aberrations. Why doesn’t the TSA get this design? Quite simply because at this aperture it is unnecessary: the job can be done with a conventional F8 (816mm in this case) ED triplet that uses a centre element made of top quality S-FPL53 sandwiched between two crowns with small air spaces between. It’s essentially the same kind of design that AP and LZOS use for their own outstanding APO refractors.

Takahashi don’t supply a certificate of optical quality (unlike APM/LZOS with their competing product – the 100/8), but nonetheless bench tests indicate Strehls of 98% for production TSAs – a result every bit as good as the LZOS lens and better than the modal quality of the old FS series. As you would expect, Takahashi use the finest coatings (on every surface – I checked) and that cell is a work of art that makes for very rapid cooling as we will see.

FS102 vs TSA102

A comparison of the two designs, using Tak’s own data is interesting. The poly-Strehl values tell their own tale. For one thing we can see the effect of the triplet most in the violet: The FS-102 falls off a cliff below about 450nm, whereas the TSA-102 still manages a decent showing at 420nm which is at the limits of the visible. This is a clear benefit to imagers. At the other extreme – the far red – Strehl performance is very similar.

The other telling difference is the wavelength for which the theoretical performance is highest. For the FS-102 it’s in the yellow; for the TSA-102 it has been pushed to higher wavelengths (in the green); again the design has been optimised for imaging.

In terms of spot sizes, we get some more evidence that the TSA has been designed with the imager in mind. Its spot sizes are all smaller (better) than those of the FS-102 in the shorter wavelengths … until you get past green. But for yellow and red, the FS-102 actually manages consistently tighter spots, tending to confirm my experience that the FS series are great for Mars.

Overall, it’s clear that the TSA will be the imager’s friend, but the FS-102 runs it surprisingly close for a doublet, on paper at least. Again it’s worth stressing that the old FS series were no ‘run-of-the-mill’ ED doublet.

Tube

Simplicity is the theme: Takahashi Zen. Perfect white paint on the outside, matte-black baffles on the inside. Build quality is palpably superb. The focuser screws in and is locked with a tiny grub-screw: no push-fit and protruding screws here.

This is the TSA-102S, so the dew-shield slides smoothly back to shorten the tube for transport, but sadly that means the old FS-series cast ‘manhole-cover’ cap is replaced with a conventional pressed-tin item. The fixed dew-shield TSA102N is cheaper and lighter (400g), but almost 19cm longer than the TSA-102S with dew-shield retracted.

To make it shorter, the TSA-102 is designed to be used with an extension tube (supplied), but this is only really needed for straight-through viewing or the longest focal-length eyepieces.

Remove the extension and unscrew the eyepiece holder from the focuser drawtube, slide the dew-shield fully back and you end up with an OTA that is barely 25” long. That’s the same as an NP101 and is very compact. But, unfortunately, it’s not definitively carry-on portable. To get down to the ~20” length required for cabin baggage, you need an F6 design like the Traveller or Eclipse (both rare and expensive compared to the TSA).

Focuser

The TSA-102 uses Takahashi’s long standing 2.7” focuser, much like that fitted to the FS and FC series of past times. The focuser body is cast and painted the usual delicious Tak’ lime green; the knobs are the equally delectable standard silver metal items (no plastic here, unlike the FS-60). The focuser is a rack-and-pinion design, but with a very wide rack and cross-cut teeth for smoothness and stability. The visual back uses threads onto which you can fit various adapters including 2” and 1.25” eyepiece holders, T-rings, camera rotator, reducer, etc.

Takahashi are apparently very proud of their focuser and with this example it’s not hard to see why. It is a superb unit: beautifully smooth and precise with virtually no image shift and capable of handling heavy eyepieces and cameras with ease. No it isn’t a Feathertouch and no it doesn’t have a microfocuser (they make one as an add-on), but for visual use at F8 you don’t need one – the single speed is so accurate getting critical focus is easy.

I have tested other Tak’ focusers that have significant image shift, but I am starting to suspect that examples suffering from this defect may have been tinkered with or simply worn from carrying heavy CCD cameras. Certainly this TSA doesn’t suffer significantly from the problem and neither did my (owned from new) FS128. Best to try before you buy on the secondhand market.

Mounting

The TSA uses the typical Takahashi cast clam-shell tube ring with two M6 bolt holes. Again, it’s a beautifully made thing with thick green felt, double hinges and a heavy, thickly chromed lock knob. Do I sound like a Takahashi fetishist?

TSA102 Clamshell with M6 mounting holes

The mount holes allow direct attachment to the top of any Takahashi mount. For other mounts things are a bit trickier, but it’s easy enough to find Vixen dovetail bars from third party suppliers with the correct holes and threads. If you get stuck, Takahashi make a dovetail bar for one of their dovetails that also works in a Vixen fit – ask your dealer.

One small issue with the traditional Takahashi way of using castings, rather than CNC, is that orthogonality won’t be sufficient for critical alignment of a GOTO mount. In particular, those M6 holes in the clamshell just allow too much free play.

Portability

The other issue that I have with the TSA is portability – an issue it shares with many 4” refractors.

The TSA packs up small, but as I have bemoaned at length elsewhere, the problem is weight. Add ring, finder, diagonal and eyepiece and the basic weight of 5.4kg soon creeps beyond the ~7Kg limit of most small German mounts. The fact that the TSA-102 is front heavy like all triplets doesn’t help. It is really a bit unwieldy for EQ5-size mounts and the Takahashi P2Z in these tests. A Tak’ EM11 would be fine, as would an EM200 of course. But then you no longer have a truly portable system. Need to move it around to a different spot in the garden? Time to start disassembling. And if you’re going to do that, why not get the TSA-120 – the TSA-102’s larger (but not much larger) sibling?

A truly portable, truly available 4” super-APO remains an elusive thing; regrettably.

Accessories

Tak’ have always been strong on (good but costly) accessories, so much so that they have to publish ‘system charts’ so you can see how all the possible combinations fit together. Combinations, you ask? The range includes an extender, flattener, F6 reducer, camera-angle adjuster (also great for visual use, by the way), T-mounts and more. All will be optimised for the TSA and the spot-diagrams suggest both reducer and flattener work superbly (though the field is pretty flat to start with).

Finder

Another obvious accessory decision is a finder, but should you choose the 7x50 or the 6x30?

It’s a tough choice.

The 6x30 is a minor legend and with reason: it is small, light and has a wide bright sharp field with masses of eye relief and a smooth focuser at the eyepiece end. If weight or cost is an issue (and a fully fitted-up TSA-102 is at the top end of the weight capacity for a small mount), then you’d go for the 6x30. Its knurled silver focus ring also looks a great match for the focuser knobs, but you don’t care about such vanity … do you?

The 7x50 is a much larger, heavier and more costly item. What’s more it doesn’t have as much eye relief. However, it is ‘probably’ the best 7x50 finder on the market with a really excellent reticle that leaves a small gap for the target, rather than obscuring it in the cross-hairs like most do. You can thread in an illuminator too, which makes it even easier to use.

 

In Use

Cool-Down

The TSA has textbook cool-down characteristics. Put it out into a frosty night from a warm room and it shows quite severe under-corrected spherical aberration for the first few minutes only – no astigmatism, no pinching. This spherical aberration fades with the tube currents in the star test for the first thirty minutes or so of use; after that … perfection. Overall, cool-down is impressively fast for a triplet.

Star-Test

Once cooled for half an hour the star test is pretty simple to describe: it’s perfect. Really. I can’t see any difference either side of focus and the zingy rings are perfectly evenly illuminated and very well defined; the outer ring has no fade-off.

When you see a star test that good, you know the in-focus views won’t disappoint and that is certainly true of the TSA-102. In good seeing, an in-focus star reveals a perfect Airy disk with a perfect faint diffraction ring around it.

In this respect at least, the TSA beats its FS-102 forbear. The FS-102s I have seen are typically 1/5th to 1/6th PV with a noticeable trace of spherical aberration (like most premium scopes, my TMB 175 included, in fact). The TSA is significantly better – I reckon my example is up at 1/8th wave, again in-line with bench-test figures I have seen.

Does this extra optical perfection matter? Probably not. Still, The TSA has the best optical quality of just about any refractor I have tested.

Daytime

The easiest way to push a refractor to reveal its true colours is to turn it on a high-contrast subject during the day: tree branches against a bright sky are perfect. Now ramp up the magnification to over 100x and most refractors, even APOs, will show some colour fringing either side of those branches. Even if you don’t see any colour in focus, try racking in and out of focus and you’ll see it. But not with the TSA. It shares the distinction, with the APM/LZOS 100/8, of being the only refractor to show absolutely no colour in this test, in or out of focus. Bench tests have confirmed this excellence and match Takahashi’s own published data on the lens.

Zoom of prime focus branches against a bright sky: near perfect correction for CA here.

The Night Sky

For use on the Moon and planets the TSA-102 may surprise you – honestly, on many nights in seeing-poor northern Europe, it will show you as much as any scope available and high-power images are always sharp, high-contrast and absolutely free from chromatic aberration or stray light bloat.

I have always been a big fan of the old FS fluorite doublets and it remains true that they have a small theoretical advantage in terms of contrast. However, in practice the TSA is as good as the FS-102 and in some ways slightly better (higher optical quality and even less chromatic aberration).

The Moon

The Moon is a magnificent sight through a really good 4” refractor – with much more detail than you get with a 3”. Through the TSA-102 the Moon is absolutely crisp – all hard whites and blacks and grey scales – with no false colour whatsoever, even focusing through the limb at high power.

Combine the zero CA with minimal stray light and you can enjoy seeing  mountains picked out in silhouette against the black of space, or a peak shining out from dense shadow beyond the terminator.

In terms of detail, in good seeing numerous rilles can be discovered and you can play at trying to find the end of Rima Sirsalis. Craterlets and dome fields can be explored in a way not possible with smaller scopes. By pushing the magnification on a night of fine seeing (the TSA-102 soaks up high powers), you can investigate those dark patches in Alphonsus, count craterlets in Plato, glimpse Hadley Rille near the Apollo 15 landing sight, follow the rays from Tycho out across the highlands.

I’ve said it before – a 4” APO is the entry level for really involved Lunar exploration.

Jupiter                

Jupiter is a tough object due its brightness and low contrast, but the TSA-102 gives one of the finest views of it I’ve seen. With a 5mm Type 6 Nagler or 3-6 Zoom on the 5mm setting giving around 163x, Jupiter is absolutely crisp, full of detail and shows no chromatic aberration whatsoever. One advantage of the longer focal length (compared to say an NP101), is that getting high powers is a bit easier and you’re less likely to have to invest in specialist eyepieces for planets.

Numerous cloud belts are visible, some with dark storms, whorls and bifurcations. The GRS and both grey-blue polar hoods could easily be picked out, with a hint of pinkish-buff colour in the GRS (as much as you ever get these days). At times of good seeing there was the hint of more detail to come – banding in the hoods, more dark and light spots in the belts, the thickening and vortices downstream of the GRS.

At high power, the Galilean satellites are easily resolved and distinguished based on size and colour.

In comparison with something like an NP101 – good though the Tele Vue is - I do think the extra optical quality of the TSA-102 is noticeable at high powers on planets in good seeing.

Deep Sky

Four inches is of course small for a deep sky scope, but the tight point spread function, flat field and inherently high contrast really help. Star fields and clusters give that brilliantly sparkling effect, with OB stars dazzlingly luminous against a perfectly black sky (I promised myself I wouldn’t mention diamonds on velvet ever again). The double cluster has an almost 3-D appearance with the fainter central stars seeming recessed from the outer ones; that red star between the clusters is a vivid colour like a hot coal.

The brighter Messier objects  show more structure than with many small scopes and again that visually flat field really helps, so that you’ll easily pick out the dark lane in Andromeda, see the hour-glass shape of the Dumbbell Nebula standing out like grey smoke and make out the eye shape of the Ring Nebula.

At this focal length, wide fields are still easily possible and the whole Pleiades, Bodes’ pair of nebulae and other extended objects and clusters easily fit in at lower powers. But the F8 ratio makes for easier focusing and places less strain on eyepieces, so the TSA-102 is singularly unfussy about the glass you choose to pair it with.

I would expect the F6 reducer to convert the TSA-102 into a superb imaging machine, but for casual use the field is very flat to start with. Both curvature and off-axis coma are very well controlled. With a flat-field eyepiece like an Ethos you get pin-sharp stars to the field stop.

Summary

“Honestly, three thousand quid for a basic 4-inch APO when you can get a Chinese doublet APO for under a thousand? Who do these snob-scope manufacturers think they’re kidding?”

If that’s what you think, I’d politely suggest you’re dead wrong.

Perhaps more than any premium scope I’ve tested, the TSA-102 is worth its price. Why? Because what you get is just perfection: perfect star test, perfect build quality, a perfect view of every kind of object. This could really be that buy-it-once-last-a-lifetime only scope.

Even the focuser is as good as any. No, it’s not a Feathertouch: it has a touch more image shift and no dual-speed, but it’s arguably got an even nicer, smoother, more fluid feel. And the threaded visual back is much more flexible than just a 2” compression ring. For imaging, an add-on electric focuser that threads onto the drawtube is arguably a better solution than a dual-speed anyway.

The question of which is the better scope – FS-102 or TSA-102 - is moot. The FS-102 is a tad lighter and cools more rapidly. It may deliver a very tiny contrast advantage and may be just a little better at the red end (so particularly for Mars). However, in all other respects the TSA is the better refractor – higher optical quality, better suited to imaging. But, for visual use at least, FS-102 owners can continue to enjoy their scopes without feeling the need to upgrade.

Just about the only downside to the TSA-102, like most 4-inch triplets, is its weight. Which brings us to its main competitor. For just a (surprisingly) little extra cost, weight and bulk you can have the TSA-120, which is likely quite a step up (~40% more light gathering, 20% higher resolution). Given that the TSA-102 pushes the limits of a small mount anyway, I might be tempted to opt for the larger model; from reports I’ve seen it’s been tweaked to deliver super-APO performance every bit as good as it’s smaller sibling.

The TSA-102 is very highly recommended: with it Takahashi have honed the small APO to exquisite perfection.