Takahashi
FC-100DL Mini Review
Takahashi’s current four-inch
fluorite doublet, the FC-100D, comes in four varieties of which this, the FC-100DL,
is the rarest.
I haven’t been able to obtain one
for a full review, but I spent a memorable evening comparing an FC-100DL with
the FC-100DC in the company of two friends who are keen planetary and lunar
observers. So I thought I would write up my findings here as the first in a new
series of mini reviews, 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.
(Note that this was originally part of
a combined review with the FC-100DC, but I’ve since completed a full
review on the DC here).
At
A Glance
|
Telescope |
Takahashi FC-100DL |
|
Aperture |
100mm |
|
Focal Length |
900mm |
|
Focal Ratio |
F9 |
|
Length |
940mm |
|
Weight |
3.8 Kg |
Data from Takahashi.
Design
and Build
The first FC-100D variants to market
had the shorter focal length (F7.4) FC-100D objective in two different models,
the FC-100DC and FC-100DF. They differ only in tube and focuser; the lens is
the same. I’ve reviewed the FC-100DC here. Most recently, Takahashi brought out
a new premium FC-100DZ model with an F8 objective incorporating a special flint
element for the best possible correction at high powers, which I’ve also
reviewed here.
So, what about the FC-100DL on review?
It was a limited-edition version with an F9 lens 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 in the original
run and presumably a similar number in the second. It was intended as a classic
lunar-planetary scope, but has been effectively replaced by the DZ.
Laser test on the FC-100DC, showing the
Steinheil fluorite-at-the-back design it shares with other FC-100D models (laser
disappears in fluorite).
Optics
The first thing to understand is that
the current FC-100D isn’t the same as the original 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. The original FC-100 preceded the FS-102.
Unlike the FS-102, the original FC-100
was a Steinheil 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 quite fragile. Like the FS-102, the old FC-100 was an F8 optimised
for visual use.
By comparison, the regular FC-100DC and
DF are 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. Most likely it’s just to keep the fragile mineral away from dew
and wiping at the back, even now it’s coated.
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 old F8 FC-100
was pretty much free of false colour, so the FC-100DL should also be almost
perfect in this respect. Even the FC-100DC had less false colour than I was expecting – just a trace more
than the original FC-100.
You might
think the FC-100DL is Takahashi’s last word on apochromatic 4”
doublets, but check the crossings and spot diagram below for the newest (third and
last) FC-100D variant, the FC-100DZ.
Lenses and
aberrations (crossings and spot diagrams) for the FC-100DL and DZ compared.
Tube
The FC-100DL
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 green used for the original FC-100 (in the DL version that lens ring
is longer too).
The original
F8 FC-100 had a big tube for its aperture, but though the FC-100DL is 12.5cm
longer and a kilo heavier than the FC-100DC, it is still lighter in weight than
the old FC-100, despite its greater focal length.
Focuser
The FC-100DL
has the larger 2.7” focuser from the DF and DZ (originally derived from
the Sky90) with more travel than the DC’s. Another small difference from
the cheaper DC model is that those focuser wheels are the proper cast version,
not the cheaper hollow plastic ones.
FC-100DL Focuser, based on the larger
FC-100DF unit.
Mounting
For the
evening I spent with it, the FC-100DL was mounted on a Sky-Watcher AZ4, which
was fine, although it did vibe a bit at really high powers. A medium sized
equatorial like an HEQ5 would suit its high-power potential perfectly.
Accessories
Like most
Takahashi refractors, all the FC-100D variants come with a clamshell-type ring
that has two M8 holes at 35mm spacing. The FC-100DL clamshell is longer than
standard and is compatible with the GT-40 guidescope.
The finder is
the excellent 6x30 that goes in a Tak’s own proprietary ring (you can get
a black one to match the FC-100DL), or you could spec’ the larger 7x50
model with its optional illuminator.
In keeping
with the ‘D for digital’ tag, there is a reducer and a flattener available
for imaging, though you might prefer the faster DF or DZ for that role.
In
Use – The Night Sky
General
Observing Notes
I compared
the FC-100DL side by side with an FC-100DC. 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. On a DC I’ve viewed with more recently, the star
test was perfect.
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
On a recent session comparing various
scopes on Mars at opposition, an FC-100DC gave excellent views of Mars at 160x
with the 4mm setting of a Nagler Zoom: no false colour, softness or the red
blur often found with ED doublets, just excellent contrast and albedo-marking
detail. I have also had outstanding views of Mars with the DZ version, so I
would expect the DL to be a great scope for Mars.
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-100DL 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 the 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. Contrary to some popular
opinion, the longer focal length of the DL variant doesn’t give dimmer
deep sky views.
Mike reports
resolving doubles down to their theoretical limits with his DC. A recent
comparison session gave a perfect split of the Double Double
at 160x, with bright and sharply defined Airy disks, black space between.
Summary
By extending the focal ratio by one and
a half, the FC-100DL removes the last trace of false colour from the F7.4
models, to deliver just a touch more high-magnification, low-contrast detail
for dedicated planetary observers and imagers, though the FC-100DC is already
excellent on the planets and the differences are honestly very subtle.
However, the FC-100DL is longer and
heavier and much less portable (no chance of getting the FC-100DL on plane).
Still, 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, the new FC-100DZ should prove a good compromise).
The FC-100DL is the most specialist
FC-100D variant, but a very desirable planetary scope and something of a
collectors’ item now.
FC-100DL and DC.