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Takahashi FC-100DC and FC-100DL Mini Review

This review compares 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 don’t have enough for a full review. 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 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 first thing to understand is that the FC-100D range isn’t the same as the old FC-100. Confused? Read on, or skip this section if you know this stuff …

Takahashi’s previous 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 and 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. What’s more, the old FC-100 was a slower design optimised for visual use.

The reasons for Takahashi’s return to the FC-100’s Steinheil design 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: Borg’s newer fluorite doublets – think 90FL, 71FL, 55FL - are front-surface and very much still available.

So, is it that the Steinheil design is better? There are certainly those who think so; but surely Borg, starting a few years back with a clean sheet, would have chosen the layout that best suited their challenging (as fast as F4.5) designs. It will be most interesting to see which design the new FOA-60 – supposedly Tak’s best corrected doublet ever – employs.

In any event, the newer FC-100DC and DF are shorter focal length fluorite doublets optimised for digital imaging (hence the ‘D’). They differ only in tube and focuser; the lens is the same. The FC-100DC reviewed here has a fixed dew shield and a 2” focuser, whilst the DF has a sliding dew shield and a beefier focuser suited to heavy cameras. The pay-off for FC-100DC owners is a very light and compact scope, ideal for travel; it’s a bit cheaper too.

So, what about the FC-100DL? It is an F9 lens of similar design in a similar fixed dew-shield OTA, but with the DF’s larger focuser (uniquely in black). Only about 100 units of this model were made and the lucky few already own them. If you decide you want one by the end of this review, join the queue behind the rest of us who should have bought one when they had the chance (update 2017: Tak’ are supposed to be making another batch, apparently minus the black enamel but plus quite a bit of cost).

Optics

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 doublets, check the crossings for the new ortho-apochromat FOA-60 below. Could we one day see an FOA-100? Or even an FOA-100Q? Count me overenthusiastically in!

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 Denks.

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 here (or a modern EQ5) handle them without problems, even at high magnifications. 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 M6 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.

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). When the original batch of 100 FC-100DLs were released (this is one of them), they weren’t much different from the standard model in price. Now (late 2017) Takahashi are supposed to be making some more, but the price has climbed well above the F7.4 FC-100 models, even for the Japanese home market. Does this represent a real difference in manufacturing costs? I can’t say.

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.

Either FC-100 variant is highly recommended.

 

 

 

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