Takahashi FC-100DC and FC-100DL Review
Takahashi FC-100DC
Takahashi FC-100DL
This review covers two of Takahashi’s
fluorite doublet 4” apochromats – the FC-100DC and FC-100DL.
I can often complete a binocular review
after a few good nights of observing, but telescope reviews typically take much
longer – usually months, given my crummy weather and seeing. In contrast, I
sometimes spend a lot of time with telescopes that belong to other people and
are able to form an opinion of them, even though I haven’t done a full review
of them myself.
Takahashi’s current 4” doublet
apochromat offering – the FC-100 – is a case in point. I’ve observed with several at star parties
and recently spent a fabulous evening comparing the FC-100DC and the super-rare
F9 FC-100DL version with friends. So I thought I would
write up my findings here as a mini-review, even though I can’t provide all my
usual comprehensive tests and images.
A big thanks to Mike Hezzlewood and Paul Yates for letting me play with their
toys and for allowing me to use their photos and sketches.
At A Glance
|
Telescope |
Takahashi
FC-100DC and FC-100DL |
|
Aperture |
100mm |
|
Focal
Length |
740mm /
900mm |
|
Focal
Ratio |
F 7.4 / F9 |
|
Length |
815mm /
940mm |
|
Weight |
2.8 Kg /
3.8 Kg |
Data from Takahashi.
Design
and Build
The shorter focal length (F7.4) FC-100D
objective is found in two different models, the FC-100DC and FC-100DF. They
differ only in tube and focuser; the lens is the same.
The FC-100DC reviewed here has a fixed
dew shield and a small focuser with only 1.25” visual back as standard (a 2” VB
is available as an accessory), whilst the DF has a beefier focuser suited to
heavy cameras and a slightly shorter tube for more in-focus. The pay-off for
FC-100DC owners is that it’s a kilo lighter and cheaper too.
So, what about the FC-100DL? It is an
F9 lens of similar design, in a similar fixed dew-shield OTA to the DF, with
its larger focuser (originally in black, but traditional green for the second
run). Takahashi only made about 100 units of this model in the original run.
If you missed out on the DL, there is
some good news. In July 2019, Takahashi announced a third FC-100D lens variant
that offers an alternative to the DL in a tube similar to the DF’s.
Laser test on the new FC-76D, showing the
Steinheil fluorite-at-the-back design it shares with
the FC-100D (laser disappears in fluorite).
Optics
The first thing to understand is that
the recent FC-100D line on review here isn’t the same as the old FC-100. The
clue is in the ‘D’. Confused? Read on, or skip this section if you know this
stuff …
The original FC-100 was actually two
generations back. Prior to the latest FC-100D, Takahashi’s 4” apochromat was
the FS-102, an F8 fluorite doublet with a ‘conventional’ Fraunhofer design that
put the fluorite element up front.
Unlike the FS-102, the original FC-100
was a Steinheil design that put its fluorite in a
protected position at the back. Back then, fluorite couldn’t easily be coated,
so this made perfect sense – fluorite is fragile and degrades in moisture. Like
the FS-102, the old FC-100 was an F8 optimised for visual use.
By comparison, the new FC-100D is an
F7.4 Steinheil optimised for digital imaging (hence
the ‘D’), whilst the FC-100DL is an F9 intended primarily for visual use at
high powers.
The reasons for Takahashi’s return to
the FC-100’s Steinheil design (see laser test above) are
unclear. I used to think maybe they (or rather Canon/Optron
who actually make Taks’ lenses) couldn’t manufacture
the front-surface fluorite designs anymore, perhaps due to the crack-down on
mating element glasses containing heavy metals. But not so: one of Takahashi’s
very latest models – the FOA-60 – returns to a front-surface design (confirmed
with a laser test), albeit using a ‘new’ type of mating element.
So, is it that the Steinheil
design is better? There are certainly those who think so; but if so, why would
Takahashi have returned to front-surface for the FOA-60, their ‘best corrected
ever’ fluorite doublet?
In any case,
fluorite doublets are found in some of my favourite scopes. Fluorite is a
mineral and scatters less light than any glass (which is why anti-reflection
coatings are made of fluorides). The simple design and low scatter make
fluorite doublets especially transparent and sharp, something I think you can
notice in critical viewing.
The F8 FS-102
was pretty much free of false colour, so I would expect the F7.4 FC-100DC to
have some chromatic aberration, whilst the FC-100DL should also be almost
perfect in this respect. In fact, the FC-100DC had less false colour than I was expecting – just a trace more than the
F9 FC-100DL, which is all but colour free.
I say ‘all
but’, because though you would be forgiven for thinking the FC-100DL is
Takahashi’s last word on apochromatic 4” doublets, check the crossings and spot
diagram below for the newest (third and last) FC-100D variant, the FC-100DZ.
The DZ is an
F8 (c.f. F7.4 and F9 for the DC/DF and DL respectively) that uses an exotic
glass mating element like the FOA-60 to achieve better correction than the DL:
Lenses and
aberrations (crossings and spot diagrams) for the FC-100DL and DZ compared.
Tube
The FC-100
has the kind of simple but classy white-enamelled aluminium tube Takahashi have
been using for years, except that these days the lens ring is silver rather
than the blue or green used for the older FS-102. The FS series were all big
for their aperture, but the FC-100DC is both lightweight at 2.8 Kg and compact
with it. Thread-off the focuser and dew-shield and the FC-100DC is carry-on
portable at under 19”.
The FC-100DL
is 12.5cm longer and a kilo heavier than the DC, but is still lighter in weight
than the old FS-102, despite its greater focal length.
Focuser
These two
refractor siblings have different focusers. The DC’s unit is based on the 2”
focuser from the FS-60C: it’s a nice focuser, free and smooth and precise with
just a touch of image shift. In comparison, the larger 2.7” focuser
on the DL is heavier in operation, but free of image shift on the sample under
review.
Both focusers
handled a heavy load of eyepieces and binoviewers with ease and without having
to resort to the tension adjustment. The FC-100DC’s focuser is a bit short on
travel, like the FC-76’s, but it’s enough to get most eyepieces to focus with Tak’s own 1.25” prism diagonal. The DL’s focuser has a bit
more travel to suit its longer focal length, but neither had enough in-focus
for my Denkmeier binoviewer.
FC-100DC focuser, based on the FS-60C
unit.
FC-100DL Focuser, based on the larger
FC-100DF unit.
Mounting
The FC-100DC
and DF are light enough that a small mount like the Vixen GP shown below (or a
modern EQ5) handle them without problems, even at high magnifications. For
mounting on Vixen-compatible mounts, Takahashi make a dovetail bar (intended
for the Mewlon-210) with two threaded M8 holes at 35mm spacing to match
Takahashi clamshells.
For the
evening I spent with it, the longer FC-100DL was mounted on a Sky-Watcher AZ4,
which was fine, although it did vibe a bit at really high powers.
Accessories
Like most
Takahashi refractors, these come with a clamshell-type ring that has two M8
holes at 35mm spacing. The FC-100DL clamshell is longer and is compatible with
the GT-40 guidescope.
Both scopes
have the excellent 6x30 finder that goes in a Tak’s
own proprietary ring (you can get a black one to match the FC-100DL).
In keeping
with the ‘D for digital’ tag, various reducers, flatteners and extenders are
available for imaging.
FC-100DC mounted on a Vixen GP with the
Mewlon-210 dovetail bar.
In
Use – The Night Sky
General
Observing Notes
Both scopes
performed very well and were really very similar. In the end, it was critical
observations of Jupiter that revealed any real difference.
Cool
Down
Both fluorite doublets cooled quickly
and had no trouble keeping up with temperature changes during the night.
Star
Test
Star tests
were excellent on both.
The
Moon
Given that
the three of us are keen Lunar observers, much of our night comparing the FC-100DC
and FC-100DL was spent on the 6-day-old Moon between crescent and first quarter
with lots of interesting craters and formations on the terminator. Most of the
viewing was done with bino’d ~17mm Plossls giving between 150x-200x in barlowed
binoviewers from Revelation, Baader and Denkmeier, with some wider views in a 6mm Tele Vue Ethos
and critical high-power views with a 5mm Super Monocentric giving 148x in the
DC and 180x in the DL.
The seeing
varied from good to truly excellent (Mike is lucky with his seeing), allowing
these fine refractors to reveal stunning detail.
Formations
studied included:
Theophillus, Cyrillus and Catherina – a triplet of large craters and a
favourite of mine since childhood observing sessions with my 3” Tasco. To the north of Theophilus lies Sinus Asperitatis and an area of extraordinarily rough terrain –
hummocks and craterlets and blocks – presumably ejecta from Theophilus.
Nearby rilles Hypatia and Gutenberg and the Crater Madler, source of a bright ray that sweeps out across Mare Nectaris and seems to have deposited a bright patch in
lava-flooded Daguerre.
Rimae Burg in the Lacus Mortis, with the strange scarp that forms a ‘v’ with
it.
Posidonius and its
rille system.
Strange
keyhole-shaped Torricelli, it’s floor in inky darkness and the walls of the
surrounding ghost crater picked out in the low sun.
Plinius
(after Pliny the Elder, who died in the eruption of Vesuvius that destroyed
Pompeii) and its unusual cratered central peak.
The eastern
wall of Albategnius just catching the sun and looking
like a twisted laurel wreath, or as Paul put it, a swarm of bees.
Curious
Ritter and Sabine with their inner walls near the Apollo 11 landing site,
‘Tranquillity Base’.
I was
astounded at the level of Lunar detail that these refractors delivered in fine
seeing – enough to last a lifetime of exploring and sketching, just as Mike
said.
In focus, I
could find no difference between the FC-100DC and the FC-100DL when observing
the Moon, except for the obvious difference in image scale for a given
eyepiece, despite swapping back and forth between the two.
I did notice
that at approaching 200x, when focusing through the lunar limb the F7.4
FC-100DC produced a trace of false colour, whilst the F9 FC-100DL remained
colour-free.
Mars
The owner of
the FC-100DC, Mike Hezzlewood, specialises in the
Moon and planets and like me is particularly interested in Mars. Unlike me,
though, Mike is a very accomplished sketcher and has made the following
outstanding map of Mars by transferring his eyepiece sketches onto a
cylindrical projection. This map says much more about the FC-100’s ability to
reveal fine planetary contrast than I can convey in words, so I’ll shut up and
let Mike’s drawing skills speak for it.
Jupiter
On a night of very good seeing quite
close to an opposition, Jupiter yielded up an astounding level of detail for a
4” refractor – variations in the belts and small dark storms, a shadow transit
and banding in the polar hoods. On the night, we all agreed that the FC-100DL
revealed a touch more low-contrast detail.
Once again, I will leave it to Mike Hezzlewood’s sketching skills to demonstrate just how much
planetary detail the FC-100DL can reveal on Jupiter:
Sketch of Jupiter by Mike Hezzlewood, combining observations with both FC-100DC and
DL.
Deep
Sky
I was able to
check out the deep sky abilities of an FC-100DF at a star party recently under
very dark skies. I had fantastic views of the Veil, Swan and Rosette nebulae
that showed more detail than a 4” refractor has any right to and that kept up
well with my larger-aperture AP Traveler on the night. Meanwhile, Mike reports
having seen the nebulosity around the Horsehead with his FC-100DC.
Certainly,
the outstanding contrast of these fluorite doublets make for better deep sky
scopes than you might think; the high optical quality throwing so much
starlight into a narrow PSF must surely help too.
Mike reports
resolving doubles down to their theoretical limits with his DC.
Summary
The FC-100DC is that rare thing – a
high performance 4” apochromat that is also very portable (even carry-on sized,
if you unthread the dewshield and focuser). The
standard F7.4 FC-100 is super-sharp, delivers outstanding contrast and makes a
great all-round scope.
By extending the focal ratio by one and
a half f-stops, the FC-100DL removes the last trace of false colour and
sharpens things further to deliver just a bit more high-magnification,
low-contrast detail for dedicated planetary observers and imagers, though on
the higher-contrast Moon I could see little difference.
However, the FC-100DL is longer and
heavier and much less portable (no chance of getting the FC-100DL on plane).
If you want a fabulous general purpose
4” that is very portable, buy the FC-100DC. If you love the Moon and planets
and don’t need to travel with it, I’d splash out for the FC-100DL if you can
find one (or if not, perhaps the new FC-100DZ will prove a good compromise).
Both the FC-100DC and FC-100DL are
highly recommended.
