Takahashi FS-78 Review

It’s difficult to consider Takahashi’s FS series of refractors without discussing fluorite first. There seems to be a lot of misunderstanding (and misspelling – no flour in fluorite), so I’ve included some information on it at the start; if you aren’t interested, or know this stuff already, skip to the review.

First Surface Fluorite (FS)

So what is fluorite and why do people make such a fuss about it? We know fluoride is good for teeth, but why is it good for telescopes? Crystalline calcium fluoride (aka Fluorite) is a naturally occurring crystalline mineral with the simple chemical formula CaF2. For optical use, though, the crystal has to be grown in controlled conditions – natural fluorite has too many flaws, inclusions and staining. So Fluorite is a crystal, which means it’s not a glass (glass has no regular crystal structure). The good thing about fluorite, from an optical standpoint, is that it has a very low dispersion, lower than any glass. Dispersion is the property of an optical material that measures how much it spreads light of different wavelengths to create a spectrum. So, when used as the positive lens in a doublet and paired with a suitable glass for the negative element, fluorite allows you to make a doublet with a very low secondary spectrum, i.e. a good apochromat with minimal chromatic aberration.

Today, glasses containing a high proportion of fluorides (in place of the usual oxides) are available. These are described as “ED” or “SD” glasses; FPL 53 is a common example from the Japanese glass maker Ohara. These glasses have optical properties close to those of fluorite, but crucially they are still not as good.

Takahashi were a pioneer in the use of artificial crystalline fluorite for making apochromatic refractors and there were a number of different product lines incorporating this exotic glass substitute over the years. The high-end triplets carried the “FCT” label and were generally short focal length ‘scopes designed primarily for imaging. Then there were the earlier “FC” series, which had the positive fluorite element at the rear in an unusual Steinheil doublet configuration. Scopes from both of these earlier lines have become something of collectors’ items and so it was surprising a few years ago, when the latest and last line of fluorite refractors from takahashi – the FS series – was becoming regarded as somehow old fashioned and outdated. Now people are starting to realise what I knew at the time – that the FS series was a no-compromise, expensive design aimed at delivering the highest possible contrast and sharpness for visual use.

The FS series was innovative in using a traditional doublet design, the Fraunhofer, which meant putting the positive fluorite at the front (instead of at the rear as it had been for the FC series).  Why innovative? Because fluorite is highly susceptible to weathering and putting it on the outside means using a special coating process to protect it. The advantage here is clear (literally), because fluorite reflects less light back than a glass (anti reflection coatings consist of fluorides) and so an FS doublet lets the maximum amount of light through.

When the FS series was coming to an end, there seemed to be a suggestion that these doublets were a cheap and inferior alternative to a triplet apochromat, but this is simply not true.  Fluorite is expensive and difficult to figure because it’s fragile, what’s more the glass for the mating element is difficult to obtain.

So to conclude this technical piece about the FS doublets, we ask the crucial question: what is so good about these and if they are so good why don’t Takahashi make them anymore? The answer is that for visual use in a small refractor, a fluorite doublet gives as much correction for chromatic aberration as you need, BUT (and here is the thing) a doublet has a slight advantage over a triplet in terms of sharpness, clarity and light throughput. For imaging using CCDs, which are sensitive to a wider range of wavelengths than the eye, an even better corrected triplet may be preferable, but for the very sharpest, most contrasty visual image a doublet has a slight edge. The reason is that a doublet has fewer light-absorbing elements and fewer light-scattering surfaces. Is this difference large? No. Can you see it? I believe so. Doublets have other advantages too: much faster cool-down and lighter weight. So why don’t Takahashi make them anymore? Cost and fashion, it’s as simple as that.

Update: As I write (late 2012), Takahashi have re-introduced a 3” fluorite doublet – the FC-76 with the fluorite at the rear. But as predicted it’s an expensive telescope with a bare OTA starting at around £1700 for the fixed dew-shield version.

Design and Build

Lens

The FS-78 has its Canon-made fluorite doublet mounted in a proper, adjustable, temperature-compensating cell. The coatings appear of high quality (better than the FS-60) and similar to the larger FS doublets, but are not quite as good as those on the FS-102 and FS-128 I owned.

Canon lens in adjustable cell.

Tube

The FS-78 is the smallest in a line of F8 front-surface fluorite apochromats that also included four, five and six inch refractors: the FS-102, the FS-128 and the FS-152 respectively. As such, the FS-78 looks just like its larger cousins (a friend who owns an FS-128, just said “Ahhh”, the way you might about a cute kitten, when he saw it). So, like the others, the FS-78 has a long, glossy white tube with a fixed dewshield and a blue lens ring. To cap it off is a cast “manhole cover” which slides into the dewshield with a perfect fit and which has a 50mm port for solar viewing.

Takahashi started off as a specialist casting firm, so everything on these telescopes is made the old fashioned way from high-quality castings; no CNC here. The castings are finished in the traditional (and to me, beautiful) lime green enamel. As with the larger FS models, the FS-78 is not a compact instrument for its aperture:  at 740mm (almost 30”) it is longer than many 4” refractors and has a 95mm diameter tube; but compared to a triplet it is light at 2.6 kg and well balanced. The OTA is traditional, in that it contains a number of knife-edge baffles in super-matt black (no flocking paper here), which in part explains its bulk.

The FS-78 is not a small scope: lighter but much larger than a Sky-90, which has the same (95mm) tube diameter.

Focuser

The focuser is a heavy duty rack and pinion unit with a single speed and long travel, with a single big tensioning knob on top. The FS-78 does differ in a few ways from the larger scopes in the range, though. For one thing it has a two inch focuser, not the 2.7 inch or 4 inch models found on the bigger scopes.

One sign of cost-cutting, on this the bottom of the FS range, is the silver focuser knobs. These are a Takahashi symbol and are usually heavy anodised metal, but in the FS78 they are cheap plastic imitations (I replaced them with FS-102 knobs). Thankfully, the focuser action is every bit as smooth and precise as on the bigger models and perfectly up to a DSLR or larger eyepiece.

Takahashi sell a 2” adapter (you can see it on the Sky-90 pictured above), but I see little point for a ‘scope of this type; the twist-grip 1.25” adapter is a Tak’ trademark and much better than set-screws.

Mounts

The tube-ring is another typical Takahashi item. It has a very easy-to-use double-hinge design and includes both the standard pair of M6 bolt-holes for Tak’ mounts as well as a central ¼-20 thread. This thread is meant for photo tripods (optimistically I would have thought given the size of the FS-78), but is also useful for attaching plates and clamps.

I mostly use the FS-78 on Takahashi’s own little German equatorial, the P2Z; occasionally atop the AP1200 in my dome. I am reminded of what a pleasure it is to use a driven mount for high powers: you can just relax and gaze without constantly having to shift the image, an important factor in seeing detail.

By way of comparison, I used a TV-76 (another 3” APO) mounted on a TeleVue Panoramic mount – a simple push-pull alt-az mount, a kind of Dobsonian for refractors. At low powers, the Panoramic is ideal for sweeping star fields, but at high powers it’s a pain. The alt-az mount requires constant pushing and jiggling to keep up with the Earth’s rotation; frequently you lose the object altogether and have to scan frantically or swap back to low power to get it back.

Superb views of Jupiter with the FS-78 mounted in my observatory reminded me how important a good mount is for high-power viewing of planets – I had never before seen so much detail with the FS-78.

In Use

In action the FS-78 delivers on the fluorite doublet promise. It cools very quickly and after that it’s all about contrast, contrast, contrast. These telescopes are designed for visual use on the Moon and planets, so I will concentrate on those first.

The FS-78 has a modest aperture considering it’s an F8 fluorite doublet and you would expect minimal false colour; you’d be right. In daytime use there is none in focus and just a trace either side, the same is true on bright white stars at high power.

The star test is good but interestingly, both the FS-128 and two FS-102s I’ve owned had slightly better star tests: they were all effectively perfect whilst the FS-78 shows slight under-correction. This is academic: the FS-78 works superbly in focus, as I’ve said.

Perfect focus on the FS-78 is a precise point, an absolute snap; fortunately the super-precise focuser is up to the task. The only slight down-side was a bit of image shift with the focuser at very high power that you don’t get with a premium CNC focuser like a Feathertouch. This wouldn’t matter for visual use, but could prove annoying for web-cam planetary imaging or if using the scope to precisely align a goto mount.

The Moon

The moon is just pure blacks and whites and greys through the FS-78. One of the things which the FS series do better than any other ‘scopes I have tried is picking out features on the very limb of the moon, where the bright disk meets black space. In many ‘scopes this area is a bit fuzzy from scattered light, but in the FS it is quite possible to see mountains in stark silhouette against space. This telescope surprises in how much detail it shows for such a small aperture and how much magnification it can take without loss of sharpness. A 4mm eyepiece (I tried both a 3-6 Nagler zoom and a 4mm Takahashi Hi-ortho) gives 157x, which seems ideal for Lunar and planetary detail. To my surprise, the FS-78 still gave a very pleasingly sharp, contrast-filled image at 225x with the 2.8mm Hi-ortho, a magnification way over what I would normally use with a three inch scope.

Planets

On Mars, I was genuinely surprised to see detail, despite the fact that the planet was still a month from a particularly unfavourable opposition (2010) and so still very small. Normally I reckon a 4 inch refractor is the minimum for seeing detail on Mars, but the FS-78 faithfully showed me Syrtis Major, the bright region of Hellas and the north polar cap. Incidentally, at the same time I used the Nagler zoom to look at Mars with a Televue 76 (the 3mm setting giving virtually the same magnification as the 4mm setting in the FS-78). The image in the TeleVue was noticeably fuzzier and lacked the contrast of the FS-78. The difference was small, but enough that I wouldn’t have been able to pick out the details in the same way with the TeleVue.

Jupiter is very impressive through the FS-78 and on many nights it will show you as much detail as any ‘scope, period. I recently had the FS-78 mounted atop my TMB 175. With the absolute stability and perfect tracking afforded by the big mount, I pushed the magnification to 180x with a 3.5mm Nagler and was rewarded with a pin-sharp view of Jupiter that included several belts, polar hoods, the GRS and several dark storms, along with a beautifully defined shadow transit. The TMB showed little more and in fact the FS-78’s smaller aperture gave a more stable image in the mediocre seeing.

Deep Sky

On a night of decent seeing, the double double was an easy split at 90x, whilst at 225x each pair was two little hard balls of light with a big dark space in between – an impressive result for a 3” scope. Rigel is very easily split with the FS-78.

I you want to enjoy clusters, bright DSOs and star fields with the FS78, you can. Although it is an F8 (compared to the faster F6.3 of the TV 76), the actual focal length is still just 630mm, so very wide fields are possible. What’s more, the field is flat, the contrast superb with the typical diamonds-on-velvet of a good apo’. The only limitation for use on deep sky is lack of aperture.

Photography

My only experience of imaging with the FS-78 is on the Moon with a DSLR. Unlike many scopes, where you struggle to get a really sharp Lunar images that will stand enlargement, the FS-78 makes this task easy with its absolute sharpness and snappy focus. Lunar images are the best I have taken with a small scope. This sharpness, combined with its flat field and reasonably short F.L. make the FS-78 great for snapping conjunctions like this one (if you enlarge the original image, Jupiter actually shows considerable detail).

An unprocessed prime-focus image of the Moon and Jupiter with FS-78/Nikon 5100.

Cropped, but otherwise unprocessed, prime focus image of the Moon with FS-78.

A field flattener/reducer is available. I suspect that the FS-78 would make an excellent small astrograph for Deep-Sky imaging.

Summary

The FS-78 is a superb, do-everything visual instrument: light weight for grab-and-go and quick to cool, it delivers super-crisp high-contrast views on any object just a few minutes from a warm house. This is important. Triplets don’t do that, they take an hour to cool and are heavier and more cumbersome to boot; for visual use their (in some cases) superior colour correction won’t be noticeable.

It has been said that a 3” scope is too small for planets, but the FS-78 breaks that rule – it will take high powers and shows surprising levels of planetary detail.

The FS-78’s overall performance is undoubtedly a notch above a regular 3” doublet APO (something like a TV-76 or SW Equinox 80).

For astrophotography, it’s more of the same: razor-sharpness, minimal CA. The focuser works well with the weight of a DSLR, but has a bit more image shift than the finest of CNC units (like a Feathertouch).

For visual use, three inch refractors just don’t get any better than this and its only real drawback is large size compared to some. Does this matter? It might. Consider that it’s 10” longer than an AP Traveller (a 4”+ refractor). The FS-78 is not carry-on portable and its length means it’s less useful as a grab-n-go than something like a TV-76 (or even a Sky-90). The new FC-76 is available in a sliding dew-shield version that is much more compact for transport.

Highly recommended. The FS-78 is amongst the very finest small refractors.