Takahashi FS128

Introduction

I have been around this hobby since childhood and from the start my primary interest has been in the moon and planets, originally visually and with film and more recently using a web cam. I have owned many telescopes over the years and have, to be honest, found most of them far from ideal in one way or another (perhaps you could say I"m fussy). My current stable includes some small APOs, including TVs 60 and 76, a TMB 100/8 and an 8 inch F8 dob" which I had custom made by Orion Optics specifically for planetary work. The Dob" is significant here as I will use it as a comparison point for performance - it gives the most detailed planetary views I"ve seen (though it is much less convenient and pleasant to use than a driven scope).

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. 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 you-know-who. My TMB has a great lens and focuser, but fit and finish issues with the APM tube made another TMB easy to eliminate as well. 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 FS128 - my first Takahashi.

I have owned the FS128 for about 2 years now and have used it a lot in that time, so feel in a strong position to judge its performance objectively (excuse the pun!). I bought it new from the UK importer, who incidentally reckoned the FS128 to be the “sweet spot" in the FS line.



A big scope, but more portable than it looks. That big OTA only weighs a few pounds more than most 4" APOs.

Overview of the FS128

The FS128 is one of a range of F8 Takahashi fluorite doublets which are now seemingly being 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). Why fluorite? Simply that fluorite has a higher refractive index even than ED glass and allows the design of a doublet with very little chromatic aberration (false colour). Why doesn"t every scope use fluorite? Primarily because larger fluorite blanks are extremely expensive and difficult to work with.

Apart from the objective design, the rest of the OTA is conventional. The dew shield is very large and fixed (but then again the objective has never dewed up). The objective itself is mounted in an adjustable cell on one end of a large and carefully baffled aluminium tube, with a massive 2.7" rack and pinion focuser at the other end. The focuser isn"t as nice to use as a Crayford, but is completely stable, without any free play or backlash and comfortably takes several kg of load if required. The visual back threads on and can easily be swapped for others. I have an electric focuser attached to it for use with the webcam, but I never need to use it visually. 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 cast aluminium, green, felt lined "manhole cover" which protects the lens is a famous Takahashi feature. In practice it is very easy to use, sliding easily in and out, but not falling if you accidentally tilt the tube downwards. One final advantage of the doublet design is the deceptively (it looks massive) light weight of the OTA: about 7kg.

Overall, then, this is a traditional longish focal length APO, the result of decades of Takahashi development and a fairly conservative design in all but the lens configuration.

Fit and Finish

Tak are famous for good finish and the FS128 doesn"t disappoint. The finish is different from, say, a Televue 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 and a focuser which is smooth and with not a trace of play or backlash over its very large range of travel. Lens cell and focuser are anchored with inset screws so tiny you struggle to make them out at first glance. The lens cell is push-pull adjustable and looks of high quality. Full marks on fit and finish then - as good as it gets.



The massive Tak" rack and pinion focuser with electric Crayford (accessory) attached. It remains smooth and stable, even with big loads.

Optical Quality

I don"t want to start a war here, but I"d like to say a couple of things about doublets versus triplets. There is a perception that triplets are always superior to doublets; not so. A doublet has the advantage of fewer light-scattering surfaces and less chance of something being out of alignment, as well as being lighter and quicker to cool.

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 FS128 exhibits and how it compares to a modern triplet - my TMB 100/8. 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 at 200x, in or out of focus, just as promised. In focus, the FS128 showed no 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 FS128. In use, it must be said, the Tak" does not exhibit any noticeable chromatic aberration; any apparent always comes down to atmosphere or eyepiece.

Performed visually, the FS128"s star test appears flawless. However, for the sake of this review I imaged the diffraction patterns 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". Takahashi may not see fit to supply a test report, but the lens is a fine one nonetheless.





Diffraction patterns on a bright blue-white star. Apologies for slight differences in scale and exposure.

There are no other optical defects evident. 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.

Mounting

I started off hoping to use the lightweight (for a 5 inch APO) OTA on my Vixen GP. Don"t try this at home! The Vixen is way too light for such a big OTA and I eventually bought a Takahshi EM200 which works, not surprisingly, perfectly with the FS128. The EM200 is big and very stable, especially with the optional pier. Even so, it is quite portable, taking about 5 minutes to take apart and re-assemble out on my south-facing patio. Incidentally, I can recommend the pier: it"s great not to be tripping over tripod legs and gives superb damping; it is easily one-man liftable, but wouldn"t be much good on any but the firmest of lawns.

First Impressions in Use

The first thing you notice is how quickly the FS128 cools: much faster than the smaller TMB 100/8 and than the 8" dob. 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.

The 6x30 finder is nice to use, with a wide, bright field and loads of eye-relief (great if you wear glasses as I do). The tube is properly baffled inside and is flawlessly finished like the OTA. It has a groove for the mounting screws and is both much easier to adjust and keeps alignment better than others I"ve used (that Tak attention to detail again). My only complaints are the lack of a reticle illuminator (the 7x50 has one) or a right-angled option.

That pier puts the eyepiece at a good height for most altitudes (apart from near the zenith when kneeling is required) and the mount has big friction levers and a lovely fluid movement, so I"m in a relaxed mood when I approach the eyepiece. The focuser is heavier than some, but accurate as I have said and the scope snaps to focus the way an APO should. There is no denying that a two-speed Crayford, like the Feathertouch, makes fine focusing easier, especially for imaging. An easy fix for Taks is to fit an electric unit: the JMI Crayford for SCTs fits in place of the visual back for not much more than the Takahashi dual-speed conversion.

Under The Sky

Note: I don"t think stacked images tell much about a scope as so much is dependent on skill in processing, so I have included a couple of unprocessed, single frames here to give an impression of how the scope performs visually under mediocre seeing.

The Moon



A single frame of Clavius, taken with the FS128 and a ToUcam

The Moon shows a wealth of detail in this scope, a real step-up from a 4 inch. 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 3-d 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.


Planets



Unprocessed, single 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 recent opposition and continues to do so on good nights, even now (late December 2005). Mars is a difficult subject and the very high optical quality and precise baffling of the FS128 help make it appear as a 3-d planet with changing texture and albedo in its surface features, rather than the bright blurry orange ball that an imperfect scope shows.

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 FS128 gives a lot of cloud-belt detail when the planet is high in the sky and with good seeing.

When I"m viewing casually I tend to use a 5mm Nagler on the planets as mount alignment is less critical with the wide FOV. When I"m viewing critically and trying to get the maximum detail, I typically use an orthoscopic (UO or Pentax) or Takhashi"s own LE Plossl, which is a fine planetary eyepiece. I also really like the TV 3-6 zoom and have been using the 4mm setting (260x) on Mars as it shrinks after the 2005 opposition. Incidentally, the 3-6 zoom is every bit as good as the 4mm UO Ortho for planetary contrast and easier to use.

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 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.

On the planets the 8" dob will show significantly more detail than the FS128, but only in fine seeing. However, the subjective quality of the view in the APO seems “nicer", if only because of the lack of diffraction spikes. In practice the extreme lack of comfort and convenience with the dob" make me use the FS128 more if both scopes are out. It"s easy to forget how important a good driven mount is when using high magnifications.

As a final note on using the FS128 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).

Doubles

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 FS128 is a fine tool for splitting doubles, given the very tight stellar images, excellent baffling and high contrast.


DSOs and Star Fields

This scope 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.

For planetary nebulae, for example The Ring, The Crab and The Dumbell, the 8" does a better job simply because it collects so much more light and because its narrow FOV is not an issue. The same is true of many fainter galaxies, of course. Still, nice contrasty casual views are on offer, if you accept the limited aperture.

Open clusters are stunning in the FS128, 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 will linger over the view of an open cluster seen through the FS128.

Overview of FS128 Hots and Nots

Hots	Nots
Contrast	Specific to the FS128:
Versatility	Large for a 5"
Quick Cooldown	Focuser is solid and stable, but stiffer and ultimately less precise than a Crayford.
Optical and mechanical quality	The merest hint of out-of-focus chromatic aberration.
Fit and Finish	For any 5" APO:
Light weight.	Lack of aperture (it"s still only a 5").
Peerless attention to detail.	Expensive, like most APOs.

Summary

This is my most used and favourite telescope and one of the few that I have owned with which I can find little to criticise. Overall, I even prefer it to my TMB, which has a few fit and finish issues with the Vixen-derived tube (though the lens and focuser are of course superb). Personally I would not trade the light weight, quick cool-down time and perhaps slightly superior transparency and contrast of the FS128 for the complete lack of chromatic aberration that a triplet such as the newer Takahashi 5" - the TOA130 - would provide.

Paradoxically, the biggest downside to the FS128 has nothing to do with its design or manufacture: shear lack of aperture. Near perfect optics and alignment and no obstruction means that it does punch above its weight, like any premium APO, but it is no match for larger reflectors on faint DSOs. Visually it does a fine job on The Moon and planets, but for web cam use, again a bigger reflector will reveal more. However, for the use to which I put it - detailed planetary and Lunar viewing with casual general observing in between - I can think of few better telescopes. It has all the advantages of smaller APOs but with more reach and resolving power and few real disadvantages.

I struggle to understand why Takahashi have decided to discontinue this line, apart from reasons of fashion and perception (and the high cost of fluorite blanks). The FS128 may still be available and at a discount whilst stocks last. I highly recommend it.