Takahashi 100-DC/DL Review

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 spent a fabulous evening comparing the FC-100DC and the super-rare F9 FC-100DL version with friends. More recently, I spent another evening comparing an FC-100DC with a range of other scopes on Jupiter and Mars.

So I thought I would write up my findings here as a review, even though I can’t provide all my usual comprehensive tests and images.

A big thanks to Mike Hezzlewood, Paul Yates and Richard too, 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 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 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. 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 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-100D 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-102 and the original FC-100 had big tubes for their aperture, but the FC-100DC is both substantially lighter at 2.8 Kg and more compact, with a narrower tube and smaller dew-shield. 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 quality unit, 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.

A friend has hacked his DC by replacing the focuser with one from an original FC-76 (not the recent FC-76D) – it threads on in place of the green extension you see in the photo below. The result has more travel, is super-smooth and offers more in-focus too, but does then need a downstream extension to focus with some diagonals and eyepieces.

Both OEM focusers handle a heavy load of eyepieces and binoviewers with ease and without having to resort to the tension adjustment. The FC-100DC’s is a bit short on travel, 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.

For a recent viewing night, I mounted it on Vixen’s latest APZ altaz – a heavier duty alternative to a Porta that’s still very small and light and vibed less than the AZ4.

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. 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, a different 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.

The owner of the FC-100DC in the original test, 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.

On a more recent session comparing various scopes on Mars at opposition and Jupiter too, an FC-100DC gave the best views of the night, with perfect sharpness and loads of cloud belt detail on Jupiter at 160x with the 4mm setting of a Nagler Zoom.

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. 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

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 to deliver just a bit more high-magnification, low-contrast detail for dedicated planetary observers and imagers, though the FC-100DC is already excellent on the planets, the differences honestly very subtle.

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 very highly recommended, but the DC is a do-anything bargain and one of my very favourite four-inch APOs.