Takahashi FS-128 Review
A big scope, but more portable than it looks. That big OTA only weighs a few pounds more than most 4” APOs.
My TMB 100/8, reviewed elsewhere on this site, got me interested in larger APOs as perfect for visual use on the planets with good general ability on all other types of astronomy as well.
Back then I couldn’t justify the expense of a 6”, so I started looking for a 5”. The choice wasn’t huge in the first place and having stuck my name on the waiting list I promptly eliminated Astro Physics. My TMB had a great lens and focuser, but fit and finish issues with the APM tube made another TMB easy to eliminate at that time as well (the most recent versions are much improved – see the TMB 175 review). That left Takahashi. Not wanting the extra weight or cool-down times of the TOA (and not needing the faster focal length), I took a deep breath and ordered an FS-128 – back then, my first Takahashi.
Design and Build
The FS-128 is one of a range of F8 (The FS-128 is actually 1040mm F.L., which is F8.1) Takahashi fluorite doublets which were discontinued in favour of slightly faster triplets.
The design is interesting because it uses a front element made of artificial fluorite (which is a crystalline mineral, not a glass). For a full discussion of fluorite, see the FS-78 review and the article on fluorite. Here suffice to say that you should not regard the FS series as inferior to triplets per se. This was an expensive lens design and test-bench investigations by the likes of W. Rohr yield chromatic aberration and poly-strehl values at the level of super triplet super-APOs. Why have the Fluorite in front? Because Fluorite scatters less light than glass. The FS manual makes a telling statement:
“When Takahashi first designed the fluorite apochromat refractor, they realised that the optimum design placed the fluorite element in front. Coating technology of the time did not permit this to be done.”
What does this mean to you?
For the pickiest imager a Takahashi TOA (the most colour free APO of them all) or a TSA, or small LZOS triplet, will be a touch more perfect for CA than an FS doublet; pretty much anything else won’t be. For visual use, the FS-fluorite doublet has a tiny theoretical edge in contrast over any other design.
That means if you own an FS-128 and ‘upgrade’ to a TEC 140, you may be in for a surprise: oil spaced triplets don’t have quite the perfect colour correction of the best air-spaced designs and are little (or no) better than the smaller FS objectives overall. Don’t believe me? Check out these three results for the TOA-130, FS-128 and TEC 140. The right hand image shows CA level.
(copied from http://homepage3.nifty.com/cz_telesco/refracter_test.htm)
As you can see, the CA levels for the FS-128 and TEC 140 are very similar. The FS-102, incidentally, might even be a touch better than the TEC, due to its smaller aperture.
FS-series objectives were made by Canon (along with the fluorite doublets in older Vixens) and would be very expensive to make now, perhaps impossible due to fluorite fabrication costs and the availability of a suitable glass for the mating element. I have seen specs for these that suggest that FS objectives were made to a minimum of about 0.95 Strehl.
In every respect the lens and cell of the FS-128 are high quality. Shining a light obliquely onto the objective and looking up the tube from the eyepiece end shows a very low level of micro-scratching left over from manufacture, less than any of my other refractors. The coatings are startlingly dark, much more so than my TVs or TMB, helped by the front surface fluorite, which scatters less light than glass.
TMB and Tak transmissivity compared under similar illumination: the first surface fluorite undoubtedly helps here. Note those knife-edge baffles.
Apart from the objective design, the rest of the OTA is conventional, if large (1176mm long by 145mm diameter, according to the manual). One advantage of the doublet design is the deceptively (it looks massive) light weight of the OTA at just 7.5 Kg.
Takahashi are famous for good finish and the FS-128 doesn’t disappoint. The finish is different from, say, a Tele Vue though. The whole thing has a spare and workmanlike feel – no mag-wheel focuser knobs or fancy anodising here. The tube is simple, high gloss white enamel with a blue lens cell ring and the focuser in textured pale lime green. It may not be fancy, but this is a beautifully made OTA nonetheless, with flawless paint, blacker-than-black baffling. Lens cell and focuser are threaded on and then secured with inset screws so tiny you struggle to make them out at first glance.
The dew shield is very large and fixed by (very) fine threads – it’s difficult to fit without cross-threading and you wouldn’t want to have to remove it for packing every time you put the scope away.
The cast aluminium, felt-lined ”manhole cover” which protects the lens is a feature of the FS series. In practice it is very easy to use, sliding easily in and out, but not falling if you accidentally tilt the tube downwards. It has a central stop-down port of 50mm diameter (intended for use with Solar projection??).
FS-128 ‘manhole cover’ lens cap.
Overall, then, this is a traditional longish focal length OTA, the result of decades of Takahashi development and a fairly conservative design in all but the lens configuration.
The focuser is typical Takahashi – a large (2.7”), long-travel rack and pinion with a big tensioner knob on top and those signature silver wheels.
The focuser isn’t quite as nice to use as a Feathertouch, but is very smooth, completely stable, without any free play or backlash and comfortably takes several kg of load if required. Unlike the shorter Tak’ focusers I have seen (FS-60, Sky-90), but just like the TSA-102, there is little or no image shift or tilt. You can completely lock the focuser using the knob on top which works very smoothly and progressively and again creates minimal image shift.
The visual back threads on and can easily be swopped for others, or for an endless array of accessories from reducers and extenders to rotators. I had an electric focuser attached to it for use with the webcam, but I never needed to use it visually. People complain about Takahashi’s endless threads, but they have one huge advantage over push-fit: perfect optical alignment and freedom from play.
The massive Tak’ rack and pinion focuser with electric Crayford (accessory) attached. It remains smooth and stable, even with big loads.
A very nice feature is the clamshell which has a massive chunky knob and a double-hinged mechanism that makes it very easy to use, even with cold hands (attention to details like these are what, in part, make Taks special).
The clamshell has two M6 bolt-holes spaced at 35mm apart which fit any Takahashi mount. If you want to attach it to a dovetail a specialist item may be required – Astro-Physics (for example) make beautifully engineered Losmandy D dovetail plates with one or more circular indents for Takahashi clamshells.
You need a good sized mount for
the FS-128, not because it is heavy, but because it is so large and long. It is
too heavy for a Vixen GP and a GPDX struggles. An EM-10 or EM-11 might just
about work. I used the FS-128 with Takahashi’s own EM-200 on the optional long
pier, which was a simply perfect and very stable match.
If you can’t afford the EM200, or want something with proper GOTO and fast slewing, an EQ6 Pro would be a fine choice for mounting the FS-128. One of Takahashi’s own dovetail bars works a treat for this purpose and fits directly to the bottom of the clamshell.
FS-128, EM-200 (and fat-bloke in RAB jacket).
The first thing you notice is how quickly the FS-128 cools: much faster than the smaller TMB 100/8 triplet, for example. After 20 minutes it is usually usable and is completely cooled after less than an hour from a warm house. Interestingly, the cell seems to do a great job of maintaining lens figure during cool-down; the problem is currents in the big tube.
At this price point most people are very concerned with the level of chromatic aberration in an APO, so I thought I’d do a few tests to figure out just how much of it the FS-128 exhibits and how it compares to a super-APO triplet like the TMB 100/8 or Tak’s own TSA-102.
One of the stiffest tests of chromatic aberration does not involve the night sky at all: dark objects, such as distant tree branches, viewed at high magnification against a bright daylight sky, show up chromatic aberration in all but the most perfect APOs. This test produced no chromatic aberration whatsoever in the TMB/TSA at 200x, in or out of focus, just as promised.
The FS-128 showed no in-focus chromatic aberration either at the same magnification, but defocusing did show a trace of violet one side, apple-green the other. On a bright blue-white star at night, such as Rigel, the daylight test results were repeated: no false colour on either scope in focus, a trace out of focus in the FS-128. In use, it must be said, the Tak’ does not exhibit any noticeable chromatic aberration; any apparent always comes down to atmosphere or eyepiece. As I said in above, the FS-series exhibit CA performance in the top-league, just behind the best air-spaced FPL-53 triplets, but ahead of many others.
Performed visually, the FS-128’s star test appears almost perfect. However, for the sake of this review I imaged the diffraction patterns on either side of focus and compared them against the references in Suiter. The result shows up a trace of under-correction, perhaps one sixth wave or better (see below) – on a par with the TMB and significantly better than my Televues’. This result agrees well with published bench-tests on the FS series, which all exceed diffraction-limited ¼ wave by a good margin.
Takahashi (like TEC and AP) may not see fit to supply a test report, but the lens is a fine one nonetheless.
Diffraction patterns on a bright O-B star.
The Moon shows a wealth of detail in the FS-128, a real step-up from a four-inch APO. The contrast is fabulous and for some reason my other APOs seem to give a slightly yellowish tint and softness compared to the Takahashi. Rilles and craterlets and low-contrast ejecta blanket details stand out in stark contrast where you hadn’t seen them before. Last night I could see detail inside the twin Messier craters and individual mountains around the margins of Mare Criseum were standing out in 3D on the terminator. No false colour here and absolute crispness in the details. It’s easy to waste time just cruising over the moon with this level of detail on offer and the perfect tracking and easy hand controls of the EM200 make it a breeze. Lunar observing feels more like exploring with this set up. A wide-field eyepiece like a Nagler gives more of a “Lunar Module window” feel than any other scope I’ve used.
A single, un-processed (NOT stacked) frame of Clavius, taken with the FS-128 and a ToUcam.
Prime-focus image of the Moon with FS-128 and Nikon D40, un-processed.
Unprocessed, single ToUcam frame of Mars, taken a week after opposition – roughly what you might see with the naked eye.
In my opinion, a 4” is the entry level of refractor which can show any significant planetary detail; the 5” shows significantly more and can be really involving. Mars showed quite a lot of detail in the albedo features during the 2005 opposition. Mars is a difficult subject and the very high optical quality and precise baffling of the FS-128 help make it appear as a 3D planet with changing texture and albedo in its surface features, rather than the bright blurry orange ball that an imperfect scope shows.
Many refractors make a mess of Mars because they are highly corrected in the blue and green, but fall off a cliff in the red; not so the FS-128.
Saturn is wonderful with the crepe ring on view and lots of subtle shading in the cloud belts. Jupiter is always a difficult target visually, but the FS-128 gives a lot of cloud-belt detail when the planet is high in the sky and with good seeing.
I tended to use a 5mm eyepiece with the FS-128 for planets (giving 208x) with the little Pentax 5mm SMC ortho’ being my absolute favourite. I also really liked the TV 3-6 zoom and used the 4mm setting (260x) on Mars as it shrunk after the 2005 opposition.
I find using the scope straight through (easier than it might be, due to the tall pier) gives the absolute maximum contrast and detail – noticeably better even than with my Everbrite diagonal. Just 6 elements or less between my eye and what I’m observing. Taks are set up for this with plenty of focus travel and I believe Japanese observers routinely use refractors in this way. I often wonder why other observers miss this trick to eliminate a (probably dusty) optical surface from the light path.
As a final note on using the FS-128 for planets, it is worth pointing out that the theoretical limit of resolution for a 128mm aperture is about 1” of arc, as much detail as most nights’ seeing will allow. So for most occasions, a 5” APO should show the maximum available planetary detail when used visually (web cam imaging can often beat the prevailing seeing, which is why you never get to see images like that through the eyepiece).
Although this is not really my thing, I do like to have the occasional go at a double star in between bouts of lunar and planetary observing. The FS-128 is a fine tool for splitting doubles, given the very tight stellar images, excellent baffling and high contrast. All the usuals are easy splits – the Double Double, Pole Star, Rigel etc.
The FS-128 is a nice compromise between useful aperture and a shortish focal length so that you still get a decent wide (and flat) field, but stars and nebulosity are brighter than with smaller APOs. This, the high contrast and the lack of diffraction spikes makes this my absolutely favourite scope for open clusters and big nebulae. My site may have poor seeing, but it is quite dark, so the Orion Nebula is a stunning mass of whirls and tendrills.
Open clusters are stunning in the FS-128, with that jewels-on-velvet effect. A 26mm Type 5 Nagler will fit the entire double cluster in the FOV. My wife generally takes only a passing interest in my hobby, but even she would occasionally linger over the view of an open cluster seen through the FS-128.
Why do optically fine small refractors like the FS-128 give a level of brilliance to stars that others scopes can’t match? Simply this – they throw more light into the tight inner airy disk and less into the diffraction rings; the better the, optics the stronger the effect.
For some years this was my favourite telescope and one of the few that I have owned with which I can find little to criticise. Personally I preferred the light weight, quick cool-down time and perhaps slightly superior transparency and contrast of the FS-128 to the complete lack of chromatic aberration that the most perfect triplet, such as the newer Takahashi 5” - the TOA130 - would provide. In any case, as I’ve said, the FS-128 gives very little away to other super-APOs in terms of CA (or anything else). The FS-128 just delivers wonderful views and images of anything you choose to set it on.
In common with the rest of the FS-series, the FS-128’s main drawback is its size: this a large OTA and needs a big mount.
Even years after it was discontinued, the FS-128 has few real competitors (certainly not the gold-plated 132 from a certain company). I’ve met those who believe that the SW 120ED has similar optics, but mine – though good – did not match the excellence of the FS-128. In fact, if you want to buy an equivalent now, perhaps the closest would be Takahashi’s own TSA-120.
Very highly recommended, if you come across a good one, but getting rare now.