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. 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.

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 them in favour of a much longer F7 model and 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 have been mass producing it for years- the Sky-90.

Design and Build

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 I’ve included some pictures. As you can see, at just over a foot long, the Sky 90 is shorter than a TV76 (itself one of the smallest 3” APOs around) and could almost serve as a finder on something like an FS102. It’s less than half the length of an NP101, itself a “compact” 4 inch APO and the FS-78 dwarfs it.

Sky-90 and TV-76

Sky-90 and FS-78

Lens

Of course 90mm isn’t quite 4”, but 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 I regard 90mm as in the “four-inch class”.

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 dewshield. 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 Sky90 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 Sky90 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.

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’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). Neither is the Sky-90 heavy at 3 kg, although it is heavier than theFS-78, despite being so much smaller.

Takahashi have designed the Sky-90 from the ground up to be ultra-compact and it has some clever features, apart from the short focal-length. The focuser body is unique to this model and the dew-shield retracts fully over the tube, further reducing length. The slide-in FS-series “manhole cover” lens cap has been replaced by a lightweight push-on one.

Overall fit and finish is pure Takahashi, i.e. uniformly high quality and well thought out with thick paint and quality mechanicals. The coatings are very dark, similar to the bigger FS series and the tube looks well baffled. The focuser knobs are reassuringly of anodised metal as on the bigger FS models, not plastic as they are on the FS-78. You would expect nice finish on a 90mm scope costing almost £2000 new and you get it.

Focuser

The Sky-90 is fitted with a new focuser (one that has since been rolled out to the recent FC-76). 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. Unfortunately, many Tak’ focusers 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.

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

Mounting

For mounting, the Sky-90 uses the same clamshell as the FS-78 which (rather optimistically in the FS78 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.

In Use

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.

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.

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.

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.

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 per-se, but 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 TV60.

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. At high powers, though, I notice a couple of things which concern me. The unsymmetrical Airy disk suggests slight miscollimation; meanwhile, there is significant focus image shift. The former was a known problem with the Mk1 Sky90, but I am surprised to see it in a recent Mk2. The focus shift may be the result of a previous owner fiddling with the adjustment grub screws atop the focuser and may not be typical, but it might be a consequence of the ultra-short focuser body which lacks sufficient bearing surface to prevent movement in the tube. As I said, I saw a similar problem in a recent FS-60C.

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 looks 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 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. There is no other APO on sale now that I know of which packs 90mm aperture into such a tiny, quality package. Let’s be clear, travelling with the Sky-90 is going to be far easier than with an NP101, say. 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 have 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 is just not as sharp as it could 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. On its intended target, deep sky, that may not be an issue.

So 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.

Recommended as a super-portable astrograph, but not as a do-anything, go-anywhere alternative to a Stowaway.

Addendum - 2012

I recently 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 collimation. Could it be fixed? Maybe. One interesting option would be to fit a complete Feathertouch focuser and have it professionally collimated. The result just might be a Stowaway competitor, albeit one that would need gentle handling.