Takahashi FC-100DZ Review

Takahashi pioneered the fluorite doublet apochromat. It’s been making them for fifty years. For decades the sweet spot was the FS-102, a four-inch F8. But when Takahashi replaced the FS-102 with the FC-100DC and FC-100DF they changed the design to appeal to a more modern astronomer.

How so? Well, the new models are optimised for digital imaging (hence the ‘D’), with a faster focal ratio of F7.4 and excellent correction at shorter wavelengths. Meanwhile, simple tubes and focuser keep the price down. They’re smaller too, for easy portability and mounting. But there was a downside - the new FC-100D slightly underperformed the original FC-100 for the critical planetary viewing Tak’ doublets were famed for.

Later, the limited-edition FC-100DL rectified that, but at the price of extending the focal ratio to F9. That didn’t just mean it was slower for imaging, it also added over six inches to the tube and with the same fixed dewshield as the DC and DF it was even less portable than the chunky old FS-102.

The latest and fourth variant, the FC-100DZ on test here has a ‘classic’ focal ratio of F8 (same as the FS-102 and original FC-100), but with a slim tube and sliding dewshield to retain the portability of the F7.4 models. Even more significantly, the DZ uses a new mating element in the objective aimed at achieving FS-102 levels of correction or better. Does it manage it? Read on to find out …

At A Glance

Telescope	Takahashi FC-100DZ
Aperture	100mm
Focal Length	800mm
Focal Ratio	F8
Length	650mm with 2” visual back (595mm without)
Diameter	95mm
Weight	3.5Kg tube (4.6Kg w/ ring and finder)

 Data from Me.

What’s in the Box?

Takahashis typically come beautifully packed in a logoed carton closed with heavy staples, the accessories separately packaged for extra security. Unboxing is a real event:

Design and Build

Even as far back as the early 1970s – when I was a boy and men were walking on the Moon – Takahashi refractors had a look they retain to this day, a high-end version of the classic Japanese refractor. Back then, others – Nikon, Pentax, Swift - were doing much the same thing. Only Takahashi still are.

That signature Tak’ design - a high quality sand-cast focuser with cast silver wheels, an oversized tube with knife edge baffles and a cast lens ring, all threaded together - carries on to this their latest model. It has a slightly retro vibe now, a kind of artisanal chic in the face of mass produced scopes from China. But the latest models are subtly different from those classic designs.

The original FC-100 (see below) and the later FS-102 had a hefty, fully baffled tube with a big focuser. But the newer FC-100D range have a narrower tube and smaller focusers borrowed from the FS-60C and the old Sky-90. As the result, all the FC-100D models are lighter, more transportable and easier to mount, this new FC-100DZ included. You can see this when comparing the original FC-100 with the DZ (both are 100/800):

FC-100DZ (dewshield extended) alongside a classic FC-100 from thirty years before. Both are 100mm F8.

The rare FS-102S looks similar to the DZ, but has wider tube, heftier focuser.

FC-100DL: the first ‘planetary’ FC-100D, it’s much longer than the DZ at F9 and has a fixed dewshield.

Optics

Many will choose the DZ on the basis of its new and special optics, so I’m going to go into some detail here. Skip it if you’re not interested.

The original FC series (including the FC-100) had a lens design called a Steinheil that puts the positive low-dispersion crown (in this case fluorite) element at the back, where most doublet refractors put it at the front in the Fraunhofer design. Takahashi originally did this because fluorite is somewhat fragile and they couldn’t protect it with a coating back then, so they put it out of harm’s way at the back.

I’ll say again here that I don’t believe the Steinheil design is optically better. In fact, once Tak’ could coat the fluorite they soon put it back into the ‘normal’ front position for the later FS series, as the old FS series manual explains.

So you might ask why Tak’ went back to the Steinheil format for the FC-100D, especially since that old FS manual suggests the Steinheil is more costly? One possibility remains durability, another pure marketing.

Whatever the reason, the original F7.4 Steinheil FC-100D lens found in the DC and DF models does slightly underperform the original FC-100 for high power viewing. One reason is the faster F ratio. Another may be that the newer eco-glass flint element just doesn’t perform as well as the old one doped with heavy metals, which is where the FC-100DZ comes in.

For the DZ, Takahashi have apparently used a special exotic glass – presumably one with extra high dispersion - for the flint mating element, an approach pioneered in the FOA-60 (reviewed here).

The spot diagrams, crossings and poly-Strehl graph published by Tak’ (see below) suggest that the F8 DZ outperforms the F9 DL as the result, whilst coming close to the ED triplet TSA-102. Tak’ claims the Strehl stays above 90% across almost the whole visible spectrum, so that spherical aberration remains diffraction limited from blue to red, only dipping a bit in the far violet.

What does that mean in practice? The FC-100DZ objective focuses all colours of light well, unlike many doublets which behave as if poorly figured in the blue and/or red and so give blurry views of Mars and/or bloat white and blue stars in images. Tak’ suggest that blue bloat on images is reduced by about half compared to the F7.4 FC-100D.

So, on paper at least, its special flint element gives the FC-100DZ very high performance for a doublet. No, it’s not quite as theoretically perfect as the FOA-60, but it avoids the very large air gap between the elements (and so heavy cell sensitive to decentring) used in that scope: the DZ lens (still made by Canon/Optron as always) is a simple, rugged air-spaced doublet just like the F7.4 models. Needless to say, coating quality on the DZ is of the very highest for maximum transmissivity.

I’ve compiled a set of spot diagrams for various Takahashi scopes from various sources (not always with best fidelity) to allow direct comparisons, see below. Note that the old FS-102 may still be very slightly better corrected in the red and so maybe better for Mars (the FS series were planetary specialists, it even says so in the manual). But the price is that the FS-102 is less well corrected in the violet for digital imagers, highlighting that ‘D’ tag on the newer models.

FC-100DZ objective has premium coatings, reveals knife edge baffles behind.

2020 FC-100DZ on the left, c. 1992 FC-100 on the right. Note the FC-100 is an ‘F’ lens, so both are fully multi-coated 100m F8 Steinheil fluorite doublets.

FC-100DZ Strehl stays diffraction limited from blue to deep red: this matters.

FC-100DZ crossings improve on the older F9 DL.

Spot diagrams for various Takahashi refractors, compiled by me.

Spot diagrams for the TSA-102 triplet and the old FS-102 doublet showing that the FS-102 is actually better corrected in the red, but much worse in the violet.

Excerpt from the FS-series manual regarding front vs rear fluorite: the FS was Front Surface fluorite.

Tube

The tube is the same as the other models in the new ‘D’ series, but with a sliding dewshield for extra compactness (a feature not seen on a Takahashi 4” since the demise of the triplet TSA-102S). So it’s the usual 95mm O.D. aluminium tube, carefully baffled and blacked within and enamelled in a fetching creamy white without. All DZs will feature the new blue powder coating in place of the classic lime green. Overall fit and finish is of the highest quality.

The long dewshield threads onto a sliding silver ring and locks with a thumbscrew. Internally, the dewshield has machined-in ridge baffles and is painted flat black to help prevent contrast-reducing stray light. The lens cap is a pressed tin item like most Takahashis nowadays, in this case finished in silver to match the lens ring. Owners of older models will miss the cast ‘manhole cover’ (but not its weight), because it was just so easy to fit and remove, whilst the tin cap can be a tight fit.

At ~3.5 Kg bare, the DZ is about 700g heavier than the lightest of the new range, the FC-100DC. But compared with the original FC-100, it’s ~1 Kg lighter and more compact at 95mm vs 114mm diameter and just 650mm long including the (now standard) 2” visual back (see comparison photo above). Unthread the visual back and you get down to 595mm, but you’ll need to unthread the focuser too to bring the FC-100DZ down to a carry-on portable 540mm.

Note that Takahashi’s own quoted length of 770mm likely includes the extension tube and/or 1.25” adapter. For visual use with a 2” diagonal neither are required.

FC-100DZ OTA details.

Focuser

Like the older FC-100DF and DL, the DZ uses the compact focuser from the old Sky-90, avoiding the very small FS-60C derived unit found on the entry-level FC-100DC. It’s a shorter and lighter unit than the focuser fitted to the FS-102, but otherwise it’s just like any other Tak’s focuser: cast and powder coated, with tension adjustment via the big silver knob on top.

The focuser has a reasonable ~63mm of travel and will accommodate most diagonals/eyepieces without the extension. The extension is needed for straight-through viewing or imaging.

The drawtube is 70mm in diameter and the standard 2” visual back unscrews to allow other accessories to thread onto an M67 internal thread.

The big advantage with this focuser compared to the older units is that it’s much shorter (and lighter), but the downside is a short bearing surface for the drawtube. Perhaps that’s why I’ve seen some of these focusers (though only on used scopes) with too much slop and image shift, perhaps caused by lots of use with heavy CCDs or binoviewers. At high powers you should expect a small shift when changing focus direction, but large jerky movements mean a worn focuser. This example, bought new, is free of image shift.

Like all Takahashi focusers, the action is oily smooth and precise. For a relaxed F8 light-cone, no microfocuser is needed to get perfect focus.

Note that the silver cast focuser wheels are just that on the DZ. The entry-level FC-100DC makes do with plastic imitations, perhaps to save cost but also to save a bit of weight (they really are heavy).

Focuser has 63mm travel, 70mm diameter drawtube.

Left: FC-100DC plastic wheel, Right: FC-100DZ cast wheel.

Mounting

The OTA is mounted in the Tak’ standard cast clamshell which directly bolts onto Takahashi mounts via a standard pair of M8 holes, although you can fit a dovetail bar – Takahashi make a slim silver one that works with a Vixen/CG5 shoe. It’s not widely available, but Takahashi do in fact make a dovetail to fit their mounts to make swapping scopes easy.

The light weight and short tube of the DZ mean it will work on just about any small equatorial such as an EQ5, or an alt-az mount like a Vixen Porta. This is a big advantage over Takahashi’s older 4” models, which really need a medium sized mount.

FC-100DZ mounted on a Vixen SX2 using Takahashi’s own dovetail rail.

Accessories

The FC-100DZ includes a 2” visual back as standard, along with a 2” extension tube for imaging or straight-through viewing and a 1.25” adapter with the classic Takahashi twist-lock and silver end cap.

For a finder you have the choice of the excellent 6x30, which has a sharp view and lots of eye relief, or the much bulkier and more costly 7x50 which has less E.R. but can be fitted with a reticle illuminator.

Takahashi sell a microfocuser knob as an accessory, but it’s expensive, fiddly to fit and you run the risk of losing a shim and getting a sloppy focuser. If you must have a microfocuser for imaging, I’d suggest replacing the whole unit or adding an electric one in place of the visual back.

The FC-35 0.66x reducer fits the FC-100DZ and shortens the focal length to 528mm (F5.3). Alternatively, the cheap 1.04x FC/FS Multi Flattener can be fitted to the DZ with the ‘Multi CA Ring 100’ adapter.

Takahashi 6x30 finder and daytime view through it – not your average finder!

In Use – Daytime

The FC-100DZ is obviously too large for spotting, but I find viewing silhouetted branches at 100x plus (114x in this case) a very good way of judging visible false colour. In this case there is effectively none, either in focus or outside. Only the very best, usually triplet, 4” apochromats achieve this, confirming that the new lens design with its exotic mating element achieves outstanding performance for a doublet.

Prime focus daytime images at full frame show vignetting only in the very corners. Zooming right in to silhouetted branches reveals a trace of false colour similar to a fine 4” triplet like the TSA-102. Only a true ortho-apochromat like the FOA-60Q (or a reflector!) does better.

Full frame telephoto with FC-100DZ/Canon EOS 6D Mk2, incl. inset at full resolution.

In Use – Astrophotography

For photography with a DLSR you will need to use the extension tube. The DZ makes for an excellent astrograph out of the box, with a flat field for a doublet, good coverage at full frame and low levels of violet bloat, as you can see from the unprocessed full frame sub below.

The focuser has in I.D. of 67mm to avoid vignetting with larger sensors. Both an expensive reducer and cheaper flattener are available (see accessories section).

I found the focuser accurate enough and free of image shift for imaging with a DSLR, the lock likewise very progressive and free of image shift.

Typical of fluorite doublets, the FC-100DZ produces excellent whole Moon images (much better than say an 8” SCT in my view).

M38: single frame with FC-100DZ/Canon EOS 6D Mk2, 30s at ISO 3200.

Crop of top right corner at full resolution.

Single frame of Comet Neowise, taken in a hurry, with tracking errors due rough polar alignment: Canon EOS 6D Mk2, 20s at ISO 3200.

Crop of Moon with FC-100DZ/Canon EOS 6D Mk2.

In Use – The Night Sky

General Observing Notes

Overall, the FC-100DZ reminds how much I love 4” apochromats and particularly fluorite doublets which give perfect views of everything you set them onto. However, the FC-100DZ will most likely be bought by connoisseurs of the finest lunar and planetary views, so I will spend some extra time here discussing solar system performance.

Cool Down

After a heavy LZOS triplet and a Mewlon reflector, I had forgotten how fast a Takahashi doublet cools – it’s usable more or less straight from a warm room into a frosty night, but gives its best after 20-30 minutes. This is a huge factor for usability as a quick-look scope.

Star Test

Unusually for modern lenses, which typically show some under-correction, this one has a virtually identical diffraction pattern either side of focus. There is almost no false colour in the star test, even on Sirius.

The Moon

A good 4” apochromat makes a wonderful scope for fans of the Moon. Focusing through the bright lunar limb with this example at 160x there is no false colour that I can see and the contrast is intense. Masses of detail is available in good seeing.

Early one frosty January morning, a 24-day Moon hung low in the south, with Copernicus right on the terminator and clear even with 7x bino’s. The DZ quickly started to give wonderful views at 160x – of Montes Carpatus near Copernicus, Sinus Iridium and the Gruithuisen domes. Gassendi south of the plains of Oceanus Procellarum with Kepler and its rays, bright Aristarchus and ghostly Reiner Gamma. Then I turned to another refractor alongside and it was a yellowish mush of turbulence. The DZ gives amazing views in poor seeing.

By contrast, literally, a night of really stable seeing on a waxing gibbous ‘Apollo’ Moon gives views that are breathtakingly good. Detail in the rough dome fields near Copernicus (itself still in the darkness) is extraordinary.

I start finding rilles everywhere. Snaking Rima Hadley near the Apollo 15 landing site is traceable for its whole length, likewise Rima Birt near Rupes Recta; I spot more rilles in Fra Mauro, running down into Bonplan. Rima Hippalus looks like the Grand Canyon.

Then there is the contrast. I see subtle variation in the Maria that I don’t think I’ve noticed before: Mare Nectaris is a filigree of ghostly rays. There are those dark spots in Ptolomaeus and a strange, tiny white spot north of the Rupes Recta that is really a minute crater – Lassell C. Finally, there is the contrast between the limb and black space. It’s perfectly delineated at 160x in the DZ, with white hills jutting into black space and no colour fringe or stray light in between.

The difference between the FC-100DZ and an equally fine smaller refractor is that it’s not just about quick lunar looks because there is so much detail at any given lunation you can spend happy hours with an atlas exploring it and discovering new things.

Venus

Venus at 114x with a 5mm Monocentric showed a perfect gibbous phase with no false colour or much stray light from the objective at all in focus and just the slightest, faintest hint of yellowy green tinging the platinum outside. I am convinced I could see curving streaks, variations in the brightness of the clouds. Few refractors give views of Venus to equal this.

Mars

Just before the 2020 opposition, with Mars at 30 degrees altitude and in good seeing, I popped in a 4mm Zeiss ortho with a Tele Vue Mars bandmate filter. At 200x magnification, the FC-100DZ delivered perfect sharpness and excellent detail for a 4”: a tiny southern polar cap opposite a dark region (Mare Acidalia) with white limb cloud at the north pole. I could easily make out a dark central band with lobes descending towards Mare Acidalia, resolvable in steady moments to the strange ‘eye’ shape of Sabaeus Sinus and Meridiani Sinus with their surrounding lighter areas. I could detect no false colour, in or out of focus: the DZ is just a little better than the FC-100DC in this respect and every bit as good as the F9 DL.

Jupiter

Albeit low in the sky for 2020, Jupiter still showed plenty of detail at 160x with the 5mm setting on a Nagler zoom, revealing variations in the NEB and NEB, in width, colour and tone. Towards the poles, the dark polar hoods hinted at multiple thin belts within them. At 200x, the Galilean moons clearly resolved into tiny discs.

Saturn

Just left of Jupiter, Saturn also gave a great view despite its low altitude: excellent colour and perfect sharpness at 160x with a 5mm monocentric eyepiece (my absolute planetary favourite). Details included the Cassini Division, shadow on the rings, the grey polar hood with hints of belts within and one dark equatorial belt. I could just make out the body of the planet through the rings in places.

Deep Sky

Late one dark and clear January night, with wife and neighbours asleep, I had a great deep sky session with the DZ. The Orion Nebula looked stunning with a 15mm Tele Vue Panoptic, sweeping from the central bright region out into the curving arms; hints of colour too. I easily found nebulosity in other sword areas as well – NGC 1975 above M42 and around the star Nair Al Saif below.

Clusters look really special in a good 4” apochromat like this one, with dazzlingly pin-point stars sparkling across a wide flat field. True to that norm, the DZ gave a magnificent view of the Pleiades at 40x with a 20mm T5 Nagler - lots of blue-misty nebulosity and icily brilliant stars with plenty of spare field width to frame the cluster. The Double Cluster in Perseus looked lingeringly beautiful, with its central twin red stars and the star chain that arcs across to another cluster, Stock 2. The main Beehive stars were all brilliant icy diamonds too, with its three yellowish outer stars in a triangle.

One beautiful summer evening, we got wonderful views of comet Neowise before it set:

The DZ set on comet Neowise, summer 2020.

Summary

The FC-100DZ reimagines Takahashi’s classic F8 fluorite doublets of yesteryear, the FS-102 and the original FC-100. Like those older models, it makes for a near perfect all-rounder, but in a lighter and more compact package and with no compromises for imagers.

Optically, it’s every bit the equal of the FS-102 and slightly superior to the old FC-100. So, this is probably the current FC-100D model you would choose: it combines most of the best features of the DC, DS and (now defunct) DL.

As anticipated, the FC-100DZ is among the very best refractors for solar system viewing, offering razor-sharp and high contrast, low flare views of the Moon and planets, including Mars and Venus (both of which can challenge lesser apochromats). You just don’t need a triplet for visual use now: the DZ is lighter and cools faster, yet performs every bit as well (and perhaps delivers just a tiny bit more contrast).

You might think of the DZ as a planetary specialist; it really isn’t. Field flatness is excellent and views of the deep sky stunning. Meanwhile, F8 is often fast enough for sensitive modern cameras and (unlike an F6 doublet) offers a fairly flat field as standard. With a flattener or reducer you’d have an excellent imaging machine.

If someone were to ask me to recommend an only scope, this would currently be it. It shows you so much more than a 3”, but isn’t that much larger or slower to cool.

The only real downside remains portability. The DZ may be lighter than an F6 triplet and more compact than any of its FC-100D peers, but it still needs the focuser removed to fit in a carry-on bag.

The FC-100DZ gets my very highest recommendation. It’s light, compact, fast-cooling, beautifully made and optically near perfect; great for visual or imaging. It’s not quite carry-on portable, but you can’t have it all.

 

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