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Takahashi FC-100DZ Review

Takahashi pioneered the fluorite doublet apochromat. It’s been making them for fifty years. For decades the sweet spot was a four-inch F8 – a great all-rounder. But then 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 re-introduced the Steinheil lens configuration of the original FC-100 (which preceded the FS-102 and was much loved), but with a faster focal ratio of F7.4 and a design optimised for digital imaging (hence the ‘D’). But now the familiar fluorite doublet had a lighter fixed cell without the collimation screws. What’s more, the whole scope was smaller and lighter and the DC model was cheaper too. It felt like quite a radical change, because there was a downside - the new FC-100D slightly underperformed the old models for critical high-power viewing.

Later, the limited-edition FC-100DL rectified that. But at F9 and with the same fixed dewshield as the DC and DF, it was even less portable than the chunky old FS-102, even more of a specialist planetary scope.

The latest and fourth variant, the FC-100DZ on test here, offers yet another take on the theme of a light and compact 4” fluorite doublet. It splits the difference between the other FC-100D models with a ‘classic’ focal ratio of F8, but uses a sliding dewshield to retain the portability of the F7.4 models.

So what’s the point? After all, the FC-100DC and DF are compact and usefully fast at F7.4 and still work well at high powers, whilst the F9 FC-100DL was optimised for high power lunar and planetary viewing, if that was your thing. The answer is that the DZ advertises similar planetary performance to the DL, but with the compactness of the DC and DF. Does it succeed and if so how? 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

3.9Kg (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.

Both the original FC-100 and the later FS-102 had a hefty, fully baffled tube with a big focuser. But that changed when they replaced the old FS-102 with new FC-100D with its narrower tube and lighter, more compact focusers borrowed from the FS-60C and the old Sky 90. The result is that all the FC-100D models are lighter, more transportable and easier to mount than those older models and this new FC-100DZ carries on that philosophy.

The new DZ looks much like the older DF and DL models, with which it shares a focuser. Compared with the FC-100DL which it effectively replaces, the DZ version is much shorter but not that much lighter due to its sliding dewshield. The DZ looks similar to the FS-102S, but is actually a smaller scope:

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

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

Optics

Most will choose the DZ on the basis of its new and special optics, so I’m going to go into some detail here.

The original FC-100D had a relatively unusual 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 fragile and hygroscopic (it absorbs water and degrades) and they couldn’t protect it with a coating back then, so they put it out of harm’s (and dew’s) way at the back.

I’ve written this before, but I again need to point out here that there is no reason to believe the Steinheil design is better optically. In fact, once Tak’ could coat the fluorite they put it back into the ‘normal’ front position for the later FS series. Below is a note from the old FS series manual that explains this.

So you might ask why Tak’ went back to the Steinheil format for the FC-100D, especially since the old FS manual suggests the Steinheil is more costly? One possibility remains durability. Some Takahashi lenses with ED glass or fluorite at the front have suffered from heavy exposure to dew. Four inch refractors typically get used a lot in my experience – in and out every clear night, regularly dewing and fogging. So perhaps it’s a wisely rugged approach.

Whether due to its design, type of flint mating element or faster F-ratio, the original Steinheil FC-100D lens found in the DC and DF models does slightly underperform the older Fraunhofer FS-102 for high power viewing, as I’ve noted already. But instead of returning to a front surface lens design to create a variant with no-compromise performance, they have taken a more innovative route with the DZ (one pioneered by the FOA-60) - a special exotic glass for the flint mating element claimed to give better correction than the DL despite its shorter focal length.

The spot diagrams and crossings published by Tak’ (see below) suggest that the DZ does outperform the DL, whilst coming close to the ED triplet TSA-102. For the FS-102 the situation seems less clear. From what I can see, the multi-purpose DZ is better at the blue end for digital imaging (it’s still an FC-100D, remember). However, The FS-102 appears unbeatable in the red. This makes sense for Mars given that the FS-series manual stated, “The FS refractors are particularly suited for planetary observation”.

In any case, 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 lens is able to focus properly for all colours of light, avoiding the problem of many doublets which can behave as if poorly figured in the blue and/or red and so give blurry views of Mars and 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 collimation) used in that scope: the DZ lens (still made by Canon/Optron as always) has a foil spaced air gap and a regular cell with no collimation adjustment.

Needless to say, coating quality is of the very highest.

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

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

The FC-100DZ improves slightly on the older F9 DL.

Some Takahashi spot diagrams for comparison.

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

Tube

The tube is basically the same as the other models in the 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. 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).

At 3.9 Kg bare, the DZ is about a kilo heavier than the lightest of the new range, the FC-100DC. But compared with the FS-102 (even the rare sliding dewshield version), it’s ~1.5 Kg lighter and more compact at 95mm vs 114mm diameter and 650mm long including the (now standard) 2” visual back. Unthread the visual back and you get down to 595mm, but you’ll need to unthread the focuser to make the FC-100DZ carry-on portable at just 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 the newer blue shade), 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 virtually 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 slot-in 2” extension tube for imaging or straight-through viewing and a 1.25” adapter that also just slots in and features 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 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).

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. A reducer/flattener is 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.

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.

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 even than the rare FS-102S. Optically, though, it’s every bit the equal of the FS-102. 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, with no compromises on Mars or 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 reducer flattener 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.

In truth, the FC-100DZ is approaching the limits of optical technology – it’s a near perfect 4” apochromat, but in a doublet and without the fragility of front surface fluorite. For me the only real downside remains the usual one – portability. It 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 be carry-on portable.

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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