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Takahashi Sky-90 Review

There’s no getting away from it, when it comes to telescopes at least, size matters. As Scotty said, the laws of physics cannot be broken and so telescopes will always resolve detail in proportion to their aperture, gather light in proportion to its square. But that rule only really applies if you live on the Moon, because here on Earth seeing will prevent you from seeing more detail than a perfect 4”-5” aperture can show on most nights.

So, the sweet-spot in refractor terms comes at a size of about 4” (90-110mm): A 4”-class APO will show real planetary detail, let you find and enjoy the Messier catalogue and let you explore the Moon ‘s rilles and domes and craterlets. Yet a 4” APO can still be a smallish, portable telescope.

But even most 4” APOs fail the carry-on portability test of ~22” in length and are too heavy for a really compact travel mount.

So I would love a 4” APO in a tiny package that I could pick up on its mount and walk out with, that I could put in a small bag and carry off on trips or to friends’ houses or remote sites. Sadly such a telescope doesn’t exist ... or does it?

Made in tiny numbers, there is a semi-mythical telescope that meets my dream criteria. Created by Roland Christen of Astro-Physics in the late 90s, the original Stowaway was a 92mm, F4.9 APO that really was small enough to pick up in one hand and go anywhere with, that could be mounted on a photo tripod.

For some reason, Astro-Physics almost immediately stopped making that original Stowaway in favour of a much longer F7 model, but even that is rare and extortionately expensive used. Then, about a year ago, I saw a new F4.9 come up for sale: it had lain in a bank vault in Italy ever since Roland finished it. OK, so it was about the price of my car, but in the end...

 I didn’t buy it! Of course I didn’t, I’m not that rich (or stupid)!

So is there another scope out there at least similar in size and performance? There is and Takahashi mass produced it for years - the Sky 90.

At A Glance

Telescope

Takahashi Sky 90 Mk II

Aperture

90mm

Focal Length

500mm

Focal Ratio

F5.56

Length

~270mm

Weight

~3 Kg

 Data from Me.

Design and Build

Takahashi has a history of innovation when it comes to refractors and the Sky-90 was designed with a clean sheet. It was quite a departure for Takahashi, which had largely produced slower visual-centric apo’s. They threw some new technology at it too, including a radical lens design (see below) and a new ultra-compact focuser.

The Sky-90 was produced over an extended period and was still listed in a 2009 UK Tak’ catalogue. It was always surprisingly costly. At that time, its price was just 15% less than a TSA-102, for example.

The first thing that really hits you about the Sky-90 (and I’m guessing the Stowaway was the same, but I’m never going to find out) is just how small it really is. This is hard to put into words, so below I’ve included pictures of the Sky-90 alongside some of its contemporaries.

As you can see, at just over a foot long, the Sky-90 is shorter than a TV-76 (itself one of the smallest 3” APOs around) and could serve as a deluxe finder on something like an FS-128. It’s less than half the length of an NP-101, itself a “compact” 4 inch apo’ and the FS-78 dwarfs it.

Sky-90 and TV-76.

Sky-90 and FS-78.

Sky-90 mounted atop one of my all-time favourite scopes, the Takahashi FS-128.

Sky-90 with FS-102S.

Finally, the Sky-90 with a recent FS-60Q.

Optics

In case you were thinking that 90mm doesn’t sound much different from 76mm, I should point out that it offers 18% more resolving power and 40% more light gathering. Remember, the next size up from Tele Vue is just 85mm aperture, so 90mm is a real step up from a 76mm (3”).

The Sky-90s focal length of just 500mm (F5.56) is radically short for a 90mm doublet, given that the word “apochromat” is writ large on the dew-shield. To get the focal length so short, an ordinary doublet wasn’t going to be up to the task, even with a fluorite positive element, so Takahashi have adopted an unusual lens design.

Unlike most air-spaced doublets, where the elements are spaced by the thickness of some pieces of foil, in the Sky-90 the elements are 13mm apart. This configuration allows greater correction of various aberrations than an ordinary doublet, but a heavy, complex cell is required. One potential problem of such a lens design is sensitivity to collimation.

The Sky-90 would probably be even lighter with a conventional lens, as you can see when you unscrew the dew shield revealing a very long cell with little hex collimation screws embedded at various points. Herein lies the main difference between Mark I and Mark II: the newer version (on test here) has more collimating screws set around the large cell than the older version.

So the Sky-90’s lens-cell is different from their other designs, but in other ways the lens looks similar, with superb coatings and that familiar green writing around the edge.

Tube

The Sky 90 has the usual glossy white enamelled tube, in this case with an O.D. of 95mm. That’s the same as the FS-78 and newer FC-76DS and FC-100D, so their clamshells should fit.

Unlike the FS-78 compared here, all Sky-90 models come with a retracting dew-shield that helps it pack small but increase weight a bit (the lightest Taks all come with fixed shields). The shield slides smoothly and clunks solidly into place.

A note on colours. Taks of this era have a lime green powder coat on the focuser and clamshell. The Sky-90 has a white enamelled lens ring, typical for Tak’s with sliding dew-shields of the time (fixed ones were green and later gloss blue, now silver), whereas on more recent Takahashis sliding dew-shield lens rings are likely to be silver.

The Sky-90’s small size does not come at the expense of a long extension tube, or drawtube, or screw-in middle section. Most eyepieces will come to focus with just a 2” diagonal straight in the back of the focuser (though an extension is provided for the EPs over about 40mm and for the straight-through viewing the Japanese love, as well as for imaging).

The Sky-90 is not particularly heavy at 3 kg, although it is heavier than the FS-78, despite being so much smaller; blame that lens cell. It does feel surprisingly weighty and solid when you pick it up, though.

Overall fit and finish is pure Takahashi, i.e. uniformly high quality and well thought out. Outside you get thick paint and quality mechanicals. Inside, despite being so short, the tube crams in three knife-edge baffles and careful blackening. You would expect nice finish on a 90mm scope costing almost £2000 new and you get it. However, the cast FS-series “manhole cover” lens cap has been replaced by a pressed tin one that helps keep weight down.

Focuser

The Sky-90 is fitted with a new focuser (one that has since been rolled out to the recent FC-76DS and FC-100DF). The focuser has a much shorter body than FS-series focusers to reduce OTA length, but is of similar design – single speed with smooth action and a big tension adjustment knob on top. The drawtube is quite wide for stability under load with a heavy DSLR or CCD.

The focuser knobs are reassuringly of anodised metal as on the bigger FS models, not plastic as they are on some older small Tak’s.

Various visual backs and an optional reducer screw straight into the Sky-90 focuser drawtube.

I have seen a number of Tak’ focusers, including this one, that seem to have a bit of image-shift: the image jumps when you change focus direction. On most this is only obvious at high-powers, but is worse on the very short focusers fitted to the Sky-90 and FS-60. I now suspect this is due to having heavy CCDs hung off the back which wears the bushings. New Takahashis I have reviewed – including several with the same focuser - have not suffered from this fault.

Mounting

For mounting, the Sky-90 uses the same clamshell as the FS-78 which (rather optimistically in the FS-78 case) includes a central ¼-20 thread for a photo tripod. Incidentally, the fact that both scopes have the same diameter tube shows either how over-engineered the FS-78 is, or how slim the Sky-90 is, depending on your viewpoint.

The Sky-90 is small and light enough to go on just about any mount. It is shown on Tak’s classic and beautiful P2Z, but worked just as well on a Vixen GP and Vixen Porta.

Accessories

Tak make lots of accessories for the Sky-90, including a reducer and rotator (shown above). The one you will most likely encounter to start with is a finder. The larger 7x50 will fit, but most will choose the superb 6x30 which has an outstandingly sharp image and plenty of eye relief.

As with other fast focal ratio Takahashis, the Sky-90 can be fitted with an ‘Extender-Q’ – a sort of barlow – to increase its focal length, but also to stamp out residual aberrations.

In Use – Daytime

The Sky-90 makes a good daytime spotter: crisp and free from serious false colour. At the Grand Canyon Star Party, someone was using one from the rim to deliver incredible views of rafters on the river deep in the canyon.

Under daytime viewing of dark branches against a bright sky at 100x (the way I generally try to quantify CA), the Sky-90 shows little in-focus false colour and a very modest blur of violet and green on either side.

This well-worn Sky-90 gave stunning views of canyon rafters a mile below.

In Use – Astrophotography

The Sky-90 works well in its intended role as a deep sky astrograph, but it does show some violet bloat on bright O-B stars and the off-axis field curvature is bad enough to mandate a flattener, especially for full frame.

For the Moon at prime focus, the Sky-90’s image scale is really too small (due to the short focal length), but the optics are likely sharp enough to get good results with a planetary camera and barlow. Still, I have included a prime focus snap of the Moon as usual.

Single unprocessed image of Beehive at full frame – lots of off-axis curvature

Crop of waning crescent in a dawn sky with Fuji APS-C through Sky-90.

In Use – The Night Sky

General Observing Notes

So small it looks over-mounted on my P2Z, the size of the Sky-90 makes it easy to push around the sky and the wide field means a 32mm Plossl obviates the need for a finder.  One other bonus with a short tube that most people forget is that the change in eyepiece height between horizon and zenith is small, so it’s easy to find a tripod setting that’s always comfortable on my cramped balcony.

I compared the Sky-90 directly with a TV-76 and more extensively with its contemporary small Tak’, the FS-78. Overall colour correction looks identical to the TV-76 (I compared them side-by-side), but a little more than in the FS-78. Takahashi seem to have done a very good job of keeping CA so well under control in such a (relatively) big fast doublet.

As I said above, there is significant focus image shift on this Sky-90. I’ll repeat that more recent examples of this focuser on a FC-100DL and DZ did not exhibit this problem. To be fair to Takahashi, it looked as if the sealant was broken on the shim-adjustment grub screws atop the focuser, never a good sign - they aren’t user adjustable.

Cool Down

The Sky-90 cools a little more slowly than a 3”, due more to the heavy cell than the increased mass of glass, I suspect. It is still ready to use after 15-20 minutes, not an hour like a 4” triplet.

Star Test

The star test revealed a problem that the Sky-90 is sadly famous for – miscollimation. In this case, it’s not bad and certainly not a show-stopper, but the Airy disk and Fresnel diffraction rings were distorted and unevenly illuminated at high power.

Deep Sky

This is what the Sky-90 was surely designed for, so first up in my mini observing campaign are some easy winter Messier objects - the Orion Nebula, the Double Cluster and the Pleiades. All look crisp, luminous and sharp in the Sky-90. The field is very flat with Naglers or TV Plossls and of course very wide for a given eyepiece. The Orion nebula is brighter and more detailed and extensive than in a 3” refractor and the double cluster just has more sparkle, more vivid star colours and looks more populous.

Naglers 13mm T6 and 17mm T4 work particularly well on these deep sky targets and give you that wide-field space walk.  So it’s a shock when I pop in a Pentax XW40 and see horrid off-axis field curvature and coma beyond about 60% diameter. This is not a Sky-90 problem but a Pentax one, a reminder that Tele Vue eyepieces are tested to F4, whilst Pentaxes (superb though they are in longer focal-length ‘scopes) are not. I had a similar problem using an XW14 in a TV-60.

To test the optics, I try splitting Rigel. Seeing is mediocre and Rigel is hard in a small scope, but eventually I see the companion star nestling in Rigel’s glare.

Planets

Turning to Mars, I swap between the Sky-90 and my FS-78 using the TV 3-6 zoom to obtain the same power of about 160x in each. The two show a similar amount of detail, but the image in the FS-78 is a little crisper if only because the Sky 90’s steep F5 light cone makes the focus sweet spot very tiny indeed and the focuser image shift hinders finding it. However, I feel that the Sky-90 won’t take much more magnification, whilst the FS-78 certainly can.

Overall, planets look better in the FS-78 – crisper and cleaner - but I would like to see a collimated example before saying this is a genuine limitation of the Sky-90.

The Sky-90 shows a small amount of chromatic aberration on Mars at high power, again reducing the image quality compared to the more perfectly corrected FS-78. I have since found out that the Sky-90 – like many short-F doublet APOs - is tuned for imaging excellence in the blue-green, in part explaining its poor performance on Mars.

Summary

It’s hard not to like and want a Sky-90. Travelling with the Sky-90 is going to be far easier than with a Tak’ FS-78 for example, let alone an FS-102. A few alternative 90-100mm APOs seem to come close in terms of size, but in practice they are considerably bulkier, heavier, or require removal of focusers or extension tubes to pack down.

Takahashi adopted quite a radical design to make the Sky-90 work and in some ways it does: it really has CA at APO levels, despite its fast focal ratio and tiny size. However, collimation seems still to be a problem, hardly what you want in a travel scope. Tak’ must know this – they even make a virtue of the Sky-90’s collimatable cell in sales literature.

Push the magnification for doubles or planets and the Sky-90 I tested was just not as sharp as it should be, though I believe this is mainly a collimation/centring issue, not a problem with the optical surfaces.

On the example I tried, there was too much focus shift as well. But that may have been a problem with this particular example, as other examples of this focuser have tested fine.

Still, whilst the Sky-90 is as compact as the Stowaway and of similar overall quality, it falls a bit short at high powers, so maybe it isn’t a true alternative to the mythical Stowaway after all.

Cautiously recommended as a super-portable astrograph, but not as a do-anything, go-anywhere alternative to a Stowaway. And check the collimation and focuser before you buy.

Addendum

I later found an exhaustive suite of tests on the Sky-90 from Wolfgang Rohr. His conclusion, much as my own (much less quantitive) opinion, was that it’s a fine lens that is very sensitive to alignment and centring. Could it be fixed? Maybe. One interesting option would be to fit a Feathertouch focuser and have it professionally collimated. The result just might be a Stowaway competitor, albeit one that would need gentle handling.

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