Takahashi FS-78 Review
The FS-78 was the smallest in a scaled
line of Takahashi refractors that have been out of production for over a
decade. Nonetheless, the FS-78 was one of the best 3” refractors ever made,
something confirmed in bench tests that suggest it has outstandingly good
correction for a doublet. That makes it still relevant today – as a great used
buy, but as benchmark for just how well corrected a doublet can be. How was
that outstanding correction achieved? It's all about the crown and where you
put it …
At
A Glance
|
Telescope |
Takahashi
FS-78 |
|
Aperture |
78mm |
|
Focal
Length |
630mm |
|
Focal
Ratio |
F8.1 |
|
Length |
740mm |
|
Weight |
2.6 Kg |
Data from FS Series manual.
What’s
in the Box?
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.
Optics
The FS-78 objective is
78mm aperture, 630mm 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-78 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, the FC-76D, but
it’s rather different with a fluorite-at-the-back Steinheil
design, faster f-ratio (F7.5) and optimised for imaging (hence the ‘D’ for
digital tag), as well as having a much more compact tube.
Does fluorite offer a
genuine advantage over high-fluoride glass like Ohara’s FPL53? 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-78 to be a true apochromat, whereas ED doublets often aren’t. And
it’s not surprising the FS-78 corrects false colour so well, after all the
FS-128 and even FS-152 are very well corrected too, even though false colour
increases substantially at larger apertures for a given design.
Don’t
think of the FS-78 as inferior to a triplet, or ‘old-fashioned’ compared to the
Steihnheil FC-76D. Instead think of it as being
optimised for visual use, offering razor-sharpness and maximum contrast for the
Moon and planets.
Tube
The FS-78 looks just like its larger siblings (a friend who
owns an FS-128, just said “Ahhh”, the way you might
about a cute kitten, when he saw it). So, like the others, the FS-78 has a
long, glossy white tube with a fixed dewshield and a
blue lens ring. To cap it off is a cast “manhole cover” which slides into the dewshield with a perfect fit and which has a 50mm port for
solar viewing.
Takahashi started off as a specialist casting firm, so the
focuser, lens ring and that dew cap are high-quality castings; no CNC here. The
castings are finished in the traditional (and to me, beautiful) lime green
enamel. As with the larger FS models, the FS-78 is not a compact instrument for its aperture: at 740mm (almost 30”) it is longer than many
4” refractors and has a 95mm diameter tube. Below are photos of the FS-78 next
to a Takahashi Sky-90 and a Tele Vue TV-76 so you can see what a big OTA the
FS-78 is.
Compared to a triplet it is light at 2.6 kg and well balanced,
but still 800g heavier than the fixed-dewshield version
of the FC-76D. In size and weight terms, the FS-78 is surprisingly similar to
the newer FC-100D [sic].
The OTA contains a number of knife-edge baffles in super-matt
black (no flocking paper here) to kill stray light and help with contrast,
which in part explains its bulk.
Focuser
The focuser is a heavy duty rack and
pinion unit with a single speed and long travel, with a single big tensioning
knob on top – typical of the FS series and Takahashis
in general. However, the FS-78 has a 2-inch focuser, not the 2.7-inch or 4-inch
models found on the bigger scopes.
Fortunately, the action is every bit as creamy smooth and
precise as the larger focusers. The only down-side was a bit of image shift at
high power. This seems to be a problem with some
Takahashi 2-inch focusers (much less so on the larger focusers), but
emphatically not all of them. I believe this may be because folks are wont to
hang big CCDs and DSLRs off them and eventually the bushings wear. The 2-inch focuser on my FS-60, owned from new, remains perfect.
One sign of cost-cutting, on this the bottom of the FS range,
are the focuser knobs. These are a Takahashi trademark (perhaps even literally)
and are usually heavy anodised metal, but in the FS78 they are cheap plastic
imitations. I replaced them with proper FS-102 knobs.
Takahashi sell a 2” visual back (you can see it below on an
FS-60 and on the Sky-90 pictured above).
Mounting
The tube-ring is another typical Takahashi item. It has a
very easy-to-use double-hinge design and is lined in thick snooker-beize-green felt. It has both the standard pair of 35mm-separated
M8 bolt-holes for Tak’ mounts as well as a central
¼-20 thread. Takahashi make a narrow plate with matching M8 threads that fits
Vixen/CG5 dovetails. The central thread is meant for photo tripods
(optimistically I would have thought given the size of the FS-78).
I mostly used the FS-78 on Takahashi’s own little German
equatorial, the P2Z; occasionally atop the AP1200 in my dome. I am reminded of
what a pleasure it is to use a driven mount for high powers: you can just relax
and gaze without constantly having to shift the image, an important factor in
seeing detail.
By way of comparison, I used a TV-76 (another 3” APO) mounted
on a TeleVue Panoramic mount – a simple push-pull
alt-az mount, a kind of Dobsonian
for refractors. At low powers, the Panoramic is ideal for sweeping star fields,
but at high powers it’s a pain. The alt-az mount
requires constant pushing and jiggling to keep up with the Earth’s rotation;
frequently you lose the object altogether and have to scan frantically or swap
back to low power to get it back.
Superb views of Jupiter with the
FS-78 mounted in my observatory reminded me how important a good mount is for
high-power viewing of planets – I had never before seen so much detail with the
FS-78.
Accessories
The FS-78 as shown is fitted with the
6x30 finder - 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.
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 470mm focal length and a 42mm image circle, but it was
an expensive option.
Takahashi’s excellent 6x30 finder and
the view through it.
In
Use – Daytime
The FS-78 is just too big and bulky for
use as a daytime spotter, the way you can with a Tele Vue TV-76, but it
delivers sparkling high-power terrestrial views. In daytime use there is no false colour in focus and
just a trace either side. This confirms bench test results.
In
Use – Astrophotography
Before the FC-76D was introduced, the
FS-78 was widely used for imaging, but not by me. My only experience of imaging with
the FS-78 is on the Moon with a DSLR. Unlike many scopes, where you struggle to
get a really sharp Lunar images that will stand enlargement, the FS-78 makes
this task easy with its absolute sharpness and snappy focus. Lunar images are among the best I have taken
with a small scope. This sharpness, combined with its flat field and
reasonably short F.L. make the FS-78 great for snapping conjunctions like this
one (be sure to check out Jupiter - it actually shows the NEB and SEB).
In
Use – The Night Sky
General
Observing Notes
In action the FS-78 delivers on the fluorite doublet promise.
It’s all about contrast, contrast, contrast.
The FS-78 has a modest aperture considering it’s an F8
fluorite doublet and you would expect minimal false colour; you’d be right. the
same is true on bright white stars at high power.
Perfect focus on the FS-78 is a precise point, an absolute
snap; fortunately the super-precise focuser is up to
the task.
Cool
Down
Cool down is very rapid, great for a
quick-look scope.
Star
Test
The star test is good, but both the FS-128 and two FS-102s
I’ve owned had slightly better star tests: they were effectively perfect whilst
the FS-78 shows slight under-correction. This is academic: the FS-78 works
superbly in focus, as I’ve said.
The
Moon
The moon is just pure blacks and whites and greys through the
FS-78. One of the things which the FS series do better than any other ‘scopes I
have tried is picking out features on the very limb of the moon, where the
bright disk meets black space. In many ‘scopes this area is a bit fuzzy from
scattered light, but through the FS-78 you see mountains in stark silhouette
against space.
This telescope surprises in how much detail it shows for such
a small aperture and how much magnification it can take without loss of
sharpness. A 4mm eyepiece (I tried both a 3-6 Nagler
zoom and a 4mm Takahashi Hi-ortho) gives 157x, which seems ideal for Lunar and
planetary detail. To my surprise, the FS-78 still gave a very pleasingly sharp,
contrast-filled image at 225x with the 2.8mm Hi-ortho, a magnification way over
what I would normally use with a three inch scope.
Mars
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.”
On Mars, I was surprised to see significant detail, despite
the fact that the planet was still a month from an unfavourable opposition
(2010) and so still pretty small. Normally I reckon a 4 inch refractor is the
minimum for seeing detail on Mars, but the FS-78 faithfully showed me Syrtis Major, the bright region of Hellas and the north polar
cap. Incidentally, at the same time I used the Nagler
zoom to look at Mars with a Televue TV-76 (the 3mm
setting giving virtually the same magnification as the 4mm setting in the
FS-78). The image in the TeleVue was noticeably
fuzzier and lacked the contrast of the FS-78. The difference was small, but
enough that I wouldn’t have been able to pick out the details in the same way
with the TeleVue.
Jupiter
As promised, Jupiter is very impressive through the FS-78 and
on many nights reveals as much detail as any ‘scope, period. I recently had the
FS-78 mounted atop my TMB 175. With the absolute stability and perfect tracking
afforded by the big mount, I pushed the magnification to 180x with a 3.5mm Nagler and was rewarded with a pin-sharp view of Jupiter that
included several belts, polar hoods, the GRS and several dark storms, along
with a beautifully defined shadow transit. The TMB showed little more and in
fact the FS-78’s smaller aperture gave a more stable image in the mediocre
seeing.
Deep
Sky
I you want to enjoy clusters, bright DSOs and star fields
with the FS-78, you can. Although it is an F8 (compared to the faster F6.3 of
the TV-76), the actual focal length is still just 630mm, so very wide fields
are possible. What’s more, the field is flat, the contrast superb with the
typical diamonds-on-velvet of a good apo’. The only limitation for use on deep
sky is lack of aperture.
On a night of decent seeing, the double double
was an easy split at 90x, whilst at 225x each pair was two little hard balls of
light with a big dark space in between – an impressive result for a 3” scope.
Rigel was a very easy split for the FS-78.
Summary
Optically, the FS-78 remains among the
very finest of three inch apochromats – razor sharp,
high contrast and effectively false colour free. The FS-78’s overall performance is undoubtedly a notch
above a regular 3” doublet APO (something like a TV-76 or SW Equinox 80).
There is a problem though – size. It’s
a problem the FS-78 shares with others in the FS Series. Bulk and weight are
more of an issue for the FS-78, though: mostly people buy refractors of this
aperture because they are small and ultraportable, but the FS-78 really isn’t.
It’s as large as a current Takahashi four-inch and really needs a medium sized
mount. If you want a Takahashi three-inch, the newer FC-76D is much smaller
(and the FC-76DCU even splits for airline transport) and perhaps 90% as good.
However, if the portability thing
doesn’t bother you, then the FS-78 remains a wonderful small refractor and one
of the most false-colour free doublets ever made.
The FS-78 is very highly
recommended, but the FS-102 is a better all rounder
and not that much bulkier.
Updated by Roger Vine 2018
