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.
