Takahashi FS-78 Review
The FS-78 was the smallest in a scaled line of Takahashi refractors that have been out of production for over a decade. Nonetheless, the FS-78 was one of the best 3” refractors ever made, something confirmed in bench tests that suggest it has outstandingly good correction for a doublet. That makes it still relevant today – as a great used buy, but as benchmark for just how well corrected a doublet can be. How was that outstanding correction achieved? It's all about the crown and where you put it …
At A Glance
Data from FS Series manual.
What’s in the Box?
Design and Build
The FS range of refractors included 78mm, 102mm, 128mm and 152mm models, all with F8 fluorite doublets. The odd one out is the FS-60 which is an F6 and the only one still in production.
As well as a shared optical design, the F8 models all had a similar tube and focuser and family ‘look’. Most had fixed dew-shields, all were bulky for their aperture.
The FS-78 objective is 78mm aperture, 630mm FL (F8.1 to be exact). It has superb coatings and a high quality, collimatable, temperature compensating cell. The lenses for the FS series were made by Canon-Optron to Takahashi spec’s, like other Takahashi objectives.
The FS-78 shares the same lens design and cell with its larger and smaller siblings, all of which are ~F8 fluorite doublets with the positive fluorite element on the front (hence the ‘FS’ – Front Surface – moniker). This means the lens is a conventional ‘Fraunhofer’ doublet, like a basic achromat, but with fluorite as the crown.
In recent years, Takahashi have brought back a fluorite doublet at this size, the FC-76D, but it’s rather different with a fluorite-at-the-back Steinheil design, faster f-ratio (F7.5) and optimised for imaging (hence the ‘D’ for digital tag), as well as having a much more compact tube.
Does fluorite offer a genuine advantage over high-fluoride glass like Ohara’s FPL53? Possibly. Fluorite is not a glass, it’s a crystalline mineral and it has optical properties that no glass can quite match, including very high transmissivity and low scatter. The FS manual has this to say:
“… the use of multi-coatings further increases light transmission over any ED glass. This makes any fluorite objective brighter than any comparable glass objective …”
Does putting the fluorite at the front make for a worse optic, as some have claimed? I believe not. Check-out the excerpt from the FS manual below. Independent tests have found the FS-78 to be a true apochromat, whereas ED doublets often aren’t. And it’s not surprising the FS-78 corrects false colour so well, after all the FS-128 and even FS-152 are very well corrected too, even though false colour increases substantially at larger apertures for a given design.
Don’t think of the FS-78 as inferior to a triplet, or ‘old-fashioned’ compared to the Steihnheil FC-76D. Instead think of it as being optimised for visual use, offering razor-sharpness and maximum contrast for the Moon and planets.
The FS-78 looks just like its larger siblings (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 the focuser, lens ring and that dew cap are 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. Below are photos of the FS-78 next to a Takahashi Sky-90 and a Tele Vue TV-76 so you can see what a big OTA the FS-78 is.
Compared to a triplet it is light at 2.6 kg and well balanced, but still 800g heavier than the fixed-dewshield version of the FC-76D. In size and weight terms, the FS-78 is surprisingly similar to the newer FC-100D [sic].
The OTA contains a number of knife-edge baffles in super-matt black (no flocking paper here) to kill stray light and help with contrast, which in part explains its bulk.
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 – typical of the FS series and Takahashis in general. However, the FS-78 has a 2-inch focuser, not the 2.7-inch or 4-inch models found on the bigger scopes.
Fortunately, the action is every bit as creamy smooth and precise as the larger focusers. The only down-side was a bit of image shift at high power. This seems to be a problem with some Takahashi 2-inch focusers (much less so on the larger focusers), but emphatically not all of them. I believe this may be because folks are wont to hang big CCDs and DSLRs off them and eventually the bushings wear. The 2-inch focuser on my FS-60, owned from new, remains perfect.
One sign of cost-cutting, on this the bottom of the FS range, are the focuser knobs. These are a Takahashi trademark (perhaps even literally) and are usually heavy anodised metal, but in the FS78 they are cheap plastic imitations. I replaced them with proper FS-102 knobs.
Takahashi sell a 2” visual back (you can see it below on an FS-60 and on the Sky-90 pictured above).
The tube-ring is another typical Takahashi item. It has a very easy-to-use double-hinge design and is lined in thick snooker-beize-green felt. It has both the standard pair of 35mm-separated M8 bolt-holes for Tak’ mounts as well as a central ¼-20 thread. Takahashi make a narrow plate with matching M8 threads that fits Vixen/CG5 dovetails. The central thread is meant for photo tripods (optimistically I would have thought given the size of the FS-78).
I mostly used 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.
The FS-78 as shown is fitted with the 6x30 finder - one of the very best finders available, with loads of eye relief and a wide sharp field. It’s an expensive option in the UK; but in Japan, Takahashi list it as a point-of-sale upgrade for just Y8000 (about £55) in their catalogue.
Takahashi made both a 1.6x extender and a 0.74x reducer for the FS Series refractors. The FS reducer was a premium product that delivered a 470mm focal length and a 42mm image circle, but it was an expensive option.
Takahashi’s excellent 6x30 finder and the view through it.
In Use – Daytime
The FS-78 is just too big and bulky for use as a daytime spotter, the way you can with a Tele Vue TV-76, but it delivers sparkling high-power terrestrial views. In daytime use there is no false colour in focus and just a trace either side. This confirms bench test results.
In Use – Astrophotography
Before the FC-76D was introduced, the FS-78 was widely used for imaging, but not by me. 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 among 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 (be sure to check out Jupiter - it actually shows the NEB and SEB).
In Use – The Night Sky
General Observing Notes
In action the FS-78 delivers on the fluorite doublet promise. It’s all about contrast, contrast, contrast.
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. the same is true on bright white stars at high power.
Perfect focus on the FS-78 is a precise point, an absolute snap; fortunately the super-precise focuser is up to the task.
Cool down is very rapid, great for a quick-look scope.
The star test is good, but both the FS-128 and two FS-102s I’ve owned had slightly better star tests: they were effectively perfect whilst the FS-78 shows slight under-correction. This is academic: the FS-78 works superbly in focus, as I’ve said.
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 through the FS-78 you 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.
To quote the FS manual again, “The FS refractors are particularly suited for planetary observation. Their ultra high contrast, sharp images will reveal a wealth of detail.”
On Mars, I was surprised to see significant detail, despite the fact that the planet was still a month from an unfavourable opposition (2010) and so still pretty 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 TV-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.
As promised, Jupiter is very impressive through the FS-78 and on many nights reveals 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.
I you want to enjoy clusters, bright DSOs and star fields with the FS-78, 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.
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 was a very easy split for the FS-78.
Optically, the FS-78 remains among the very finest of three inch apochromats – razor sharp, high contrast and effectively false colour free. 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).
There is a problem though – size. It’s a problem the FS-78 shares with others in the FS Series. Bulk and weight are more of an issue for the FS-78, though: mostly people buy refractors of this aperture because they are small and ultraportable, but the FS-78 really isn’t. It’s as large as a current Takahashi four-inch and really needs a medium sized mount. If you want a Takahashi three-inch, the newer FC-76D is much smaller (and the FC-76DCU even splits for airline transport) and perhaps 90% as good.
However, if the portability thing doesn’t bother you, then the FS-78 remains a wonderful small refractor and one of the most false-colour free doublets ever made.
The FS-78 is very highly recommended, but the FS-102 is a better all rounder and not that much bulkier.
Updated by Roger Vine 2018