When Takahashi discontinued the FS series, which
spanned 60-152mm apertures, they effectively gave up on the idea of having a
single scaled design. For the smaller apertures, Tak’
continued with fluorite doublets, retaining the FS-60 and re-introducing the FC-76.
At the upper end they introduced the TOA-130 and TOA-150, triplets with a very
large air space and two ED elements that are heavy but give the best colour
correction of any APO. For the middling apertures they deployed conventional F8
ED triplets - the TSA-120 and the TSA-102 on review here.
Very recently, the TSA-102 has been
discontinued, replaced in turn by a brace of 100mm fluorite doublets, but
that’s a shame because the TSA-102 turned out to be a very different scope from
the old FS-102 (and the newer FC-100D too) with different strengths and
weaknesses. As we will see, if you can find a good used one it’s still a great
buy.
At A
Glance
|
Telescope |
TSA-102S |
|
Aperture |
102mm |
|
Focal Length |
816mm |
|
Focal Ratio |
F8 |
|
Length |
~ 625mm (25”) minus extensions |
|
Weight |
5.4 Kg |
Data from Takahashi/Me.
What’s
in the Box?
The TSA-102 came in a typical Takahashi double box:
Design
and Build
Like other Tak’
refractors, the TSA-102 was available in fixed (TSA-102N) and sliding
(TSA-102S) dew-shield versions – the fixed version longer but lighter. The
cream tube and green enamel livery was the same, but the dewshield
had green writing in place of the traditional Takahashi blue. Otherwise, The
TSA refractors are typical Takahashis, plain and
simple but beautifully made.
Optics
The acronym TSA stands for Takahashi Super
Apochromat and is significant. A ‘Super APO’ is supposed to be essentially free
from chromatic aberration. A ‘Super APO’ should also be free from spherochromatism - maintaining a high Strehl
across the full visible spectrum. As we will see, in these respects the TSA
label is a statement of fact.
The TSA-102’s lens is an 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 and twin ED elements 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 glass sandwiched between two crowns with small air
spaces between. It’s essentially the same kind of design that LZOS use for
their own outstanding apochromats.
Air-spaced triplets like this one are
generally harder to make (because all glass surfaces must be finely polished) and
generally perform better than the oil-spaced variety (apologies to AP, TEC et
al), but may take longer to cool. Also, the air spaces ‘breathe’ so there is
more chance of internal fogging or mould.
Takahashi don’t supply a certificate of
optical quality (unlike APM/LZOS), 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.
So how does the ED triplet TSA-102 compare to
the model it replaced, the fluorite doublet FS-102? 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: neither suffers the spherical aberration in the
red you often get with faster ED doublets.
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 for reduced
violet bloat in images.
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-102 will be the imager’s friend, but for visual the
FS-102 runs it surprisingly close for a doublet, on paper at least.
Spot diagrams suggest TSA-102 is better for
imaging, FS-102 (perhaps) for Mars!
Tube
The TSA-102 tube is typical Takahashi – rolled
aluminium coated in white enamel with a cast lens ring in knobbly green powder
coat. Internally, the tube has a number of knife-edge baffles and is flattened
with super-matt black camera paint.
Simplicity is the theme: Takahashi Zen. Build
quality is palpably superb. The focuser screws in and is locked with a tiny
grub-screw: no push-fit and protruding screws here. Likewise, the ample
dew-shield threads onto the sliding lens ring which moves smoothly and clicks
into place.
The old FS-series cast ‘manhole cover’ cap is
replaced with a conventional pressed-tin item. The FS manhole cover was
complete overkill, but I loved it.
The fixed dew-shield TSA-102N is cheaper and
(400g) lighter, but almost 19cm longer than the TSA-102S (reviewed here) with
its 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 DSLR imaging.
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 a Tele Vue NP-101 -
very compact for an F.L. of 816mm. But, unfortunately, it’s not definitively
carry-on portable.
Again, comparisons with the sliding dew-shield
version of the FS-102 are inevitable – the tube and focuser are very similar,
but despite being exactly the same focal length, the FS-102 may be slightly
shorter (see photos).
For comparison: sliding dew-shield FS-102 has
a very similar tube and focuser to the TSA-102 (the dual-speed focuser pinion
is an after-market upgrade).
Focuser
The TSA-102 uses Takahashi’s medium 2.7”
focuser, much like that fitted to the defunct FS and FC series. The focuser
body is cast, not CNC, and painted the usual delicious Tak’
lime green; the knobs are the equally delectable standard silver metal items (no
plastic imitations here, unlike the FS-60 and FS-78).
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. The big silver knob on top locks the focuser and it too works
perfectly, with no image shift.
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 suspect this is due to wear from carrying heavy CCD cameras. This focuser
doesn’t suffer significantly from the problem and neither did my (owned from
new) FS-128. Best to try before buying used, though.
Mounting
The TSA uses the typical Takahashi cast
clam-shell tube ring with two 35mm spaced M8 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?
The clamshell attaches directly 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 traditional Takahashi
castings, rather than CNC, is that orthogonality won’t be sufficient for
critical alignment of a GOTO mount. In particular, those M8 holes in the
clamshell just allow too much free play.
The TSA-102 is quite
heavy (due to its triplet lens). Add ring, finder, diagonal and eyepiece and
the basic weight of 5.4kg soon creeps beyond the ~7Kg limit of most small
German mounts, so you’ll need a medium
sized mount: an EQ5 will struggle. The Takahashi P2Z shown here coped, but was
straining a bit. A Tak’ EM-11 will be fine, an EM-200
super-steady.
TSA-102 on P2Z mount.
Accessories
Accessories include an extender, flattener,
reducers, camera-angle adjuster (also great for visual use, by the way), wide
T-mounts and more. Two reducers are available: F6 and the much more expensive
F5.6 TOA-35 reducer.
Another obvious accessory decision is a finder
- 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 helical 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, as I’ve said), then you’d go for
the 6x30. Its knurled silver focus ring also looks a great match for the
focuser knobs.
The 7x50 finder 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 an 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
– 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 apochromats, 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 one of the few refractors I
have tested 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.
In Use
– Astrophotography
Perhaps surprisingly, the TSA-102 has been the tool
of choice for some of the best imagers, usually equipped with one of the
reducer or flatteners options. I didn’t have a chance to try it, but suspect
violet bloat would be much less of a problem than it often is with doublets
(and even Petzvals).
The TSA-102 certainly takes sharp, contrasty and detailed snaps of the Moon.
In Use
– The Night Sky
General
Observing Notes
For use on the Moon and planets the TSA-102
gives high-power images that 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 on this example
and even less chromatic aberration on most things).
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 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, thanks to the sophisticated cell,
but a doublet still cools a bit faster.
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.
The
Moon
The Moon is a magnificent sight through a
really good 4” refractor like the TSA-102 - absolutely crisp, all
blinding-white uplands, blacks shadows and grey scale Maria, 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 Lunar 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, numerous rilles can be traced in good seeing 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.
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
and detailed, all creamy buffs and no false colour. One advantage of the longer
focal length (compared to say an NP-101), 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 NP-101 – 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 of aperture is 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 reveal more structure than through
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
All in all, the TSA-102 is worth its premium
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.
The question of which is the better scope –
the TSA-102 or its FS-102 forebear - 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 other
respects the TSA is the better refractor – higher optical quality on my sample,
better suited to imaging.
Just about the only downsides to the TSA-102,
like most 4-inch triplets, are weight and cool-down.
Very highly recommended: with the TSA-102, Takahashi
honed the small triplet apochromat to exquisite perfection.
Updated by Roger Vine 2020
