Takahashi FS-102 Review
Older style FS-102 with green lens ring.
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
|
Aperture |
102mm |
|
Lens design |
Fluorite doublet
(fluorite at front) |
|
Focal Length |
820mm |
|
Focal Ratio |
F8.1 |
|
Length |
930mm (37”) |
|
Tube diameter |
114mm |
|
Weight |
5.3Kg |
Data from Takahashi FS Manual.
Signature cast ‘manhole’ dew cap.
Design
and Build
The FS range of refractors included
78mm, 102mm, 128mm and 152mm models, all with F8 fluorite doublets except the
FS-60 which is an F6 and the only one still in production.
The FS series were preceded by the
original FC series, which look similar but have their fluorite at the back. The
FS fluorite doublets have in turn been superseded by the new FC-100D range,
that also have their fluorite at the back but have smaller tubes and optics tuned
for digital imaging.
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.
Optics
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.
Tube
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.
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). 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.
Mounting
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.
Accessories
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.
6x30 finder and mount in more recent
blue.
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.
Cool
Down
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.
Star
Test
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
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.
Planets
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.
Venus
Venus displayed a
perfect, dazzling phase, refusing to show flare or chromatic aberration.
Mars
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.
Jupiter
Lots of cloud belts
and changes in darkness and thickness within them are easily seen with the
FS-102, as is the Great Red (now pink) Spot.
Saturn
Views of Saturn were
similarly excellent, with Saturn’s Cassini division easily visible in a way it isn’t with smaller refractors.
Deep
Sky
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.
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 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 a 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 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.