Four-Inch APO Buyers’ Guide
PLEASE NOTE: This is a legacy page and has been replaced by an updated version, here:
Four Inch APO Buyers Guide 2021
Astro-Physics
Stowaway 92mm F5 EDFS
Astro-Physics
Stowaway 92mm F6.6 EDFS
Astro-Physics
Stowaway 92mm F6.6 – New version (2017-19)
Sky Watcher Esprit
100 Triplet
Tele Vue Genesis /
Renaissance
Lens
Design |
ED
glass (OK4) Triplet |
Aperture |
100mm |
Focal
Length |
800mm |
Focal
Ratio |
F8 |
Length |
Minimum
~ 617mm (25”) |
Tube
diameter |
135mm |
Weight |
4.7
Kg incl rings |
The
100/800 is really a lens, made by the Russian ex-Zeiss contractor LZOS using
its own ED glass (LZOS is a major optical fabricator). The 100/800 lens has
been integrated into various tube assemblies by APM Telescopes. For this reason it is hard to be specific about the OTA design: the
specs given are for the lightest, cheapest version. Most use the Starlight
Instruments Feathertouch focuser – simply the best.
The ultimate model has a very fine German-made CNC tube with particularly
careful baffling and the particularly superb 3.5” rotatable version of the Feathertouch it shares with bigger LZOS refractors, like my
175. But for its weight of 7Kg+ I’d rate this version the purists’ choice of 4”
APOs: best tube, best lens, best focuser, period. Trouble is significantly
larger apertures can be had for a similar size and weight (and cost!).
Portable
versions of the 100/800 typically come in a shortened tube which uses a
draw-tube arrangement for compactness. Personally I’d
rather have a fixed tube, but the benefit is that some versions come close to
airline portability at about 25” when packed-down. These draw-tube designs can
be specified with various focusers, including 2”, 2.5” or 3” FT focusers
according to need and budget.
The
100/800 was my first 4” APO and remains the best I have tried in one single
respect at least: it is the most perfectly colour-free refractor I have tested
– visually zero false colour in or out of focus, any object, any power. Recent
versions are guaranteed 98%+ Strehl with a test-certificate,
which is simply as good as it gets. Like other LZOS lenses, the 100/800 offers
supreme sharpness, a very flat field and the ability to transmit peerless
planetary detail for its size. The only disadvantages are weight (it’s a heavy
lens) and slow cool-down.
APM
telescopes typically have CNC rings with the Astro-Physics hole pattern that is
very flexible and allows attachment to various plates (lighter version come
with a Vixen plate as standard).
Lens
Design |
ED
glass (OK4) Triplet |
Aperture |
105mm |
Focal
Length |
650mm |
Focal
Ratio |
F6.2 |
Length |
Minimum
~ 617mm (25”) |
Tube
diameter |
135mm |
Weight |
6.6
Kg incl rings |
Much
the same comments apply as to the 100/800 – the same kinds of tubes and focuser
options are available. This is a bigger, heavier lens though, with very steep
surfaces. Lens quality isn’t guaranteed to quite the same level as the 100/800,
but still better than 95% Strehl with a test
certificate to prove it. The big advantages of the 105/650 are the faster focal
ratio for imaging that also means it’s carry-on portable in some versions (only
one of a handful of 4” refractors that are). Whether you choose the F8 or F6
LZOS models will depend on what you want mostly to do with it (costs are
similar), planets or imaging, though both will do everything well.
Stock
AP image.
Lens
Design |
ED
glass triplet (oil spaced) |
Aperture |
92mm |
Focal
Length |
450mm |
Focal
Ratio |
F4.9 |
Length |
14”
(356mm) |
Tube
diameter |
3.61”
(91.7mm) |
Weight |
3Kg |
This
is a buyers’ guide and since the chance of you buying an F5 Stowaway is effectively
nil I’ll keep this entry short – like the scope in fact!
Oh
the F5 Stowaway. If ever there was a mythical telescope this is it. This, the
first one AP made in a limited 1999 run, was a 92mm F4.9 and absolutely tiny
with it. But to get a 92mm triplet to the required quality at such a fast ratio
was allegedly a struggle. AP only made a few and they have become the kind of
collector’s item that lives in a bank vault. Ten years ago, one sold for $9000.
For
anyone not familiar with AP gear it is worth pointing out the almost excessive
attention to detail – I have an AP extension tube that’s a micro-baffled work
of art.
True
to that, this original Stowaway is a beautiful little scope. With a flared CNC
tube in white pebble, sliding dew-shield, those signature slim CNC rings and special
2” version of AP’s own black-anodised dual-speed focuser, it has everything
going for it. And it really is tiny: as in Sky-90 size and weight.
Roland’s
own notes on the AP website are all I’ve got to go on, but he reported double
star performance to theoretical resolution and seeing albedo detail on Mars. In
such a small package this is what we all want as a second scope, but no one has
replicated the original Stowaway, not even AP.
“…
this limited production run will become an heirloom for sure.” – Roland
Christen
Stock
AP image.
Lens
Design |
ED
glass triplet (oil spaced) |
Aperture |
92mm |
Focal
Length |
604mm |
Focal
Ratio |
F6.6 |
Length |
19”
(483mm) |
Tube
diameter |
3.61”
(91.7mm) |
Weight |
3Kg |
Whether
or not AP really struggled to make the tiny F5 Stowaway (they say it just
didn’t have enough focal length for many users), they soon switched to an F7
version which had a downsized Traveler lens and so was presumably an oil-spaced
triplet with an ED centre element as well. Interestingly, though, it is not
really much smaller than a Traveler (see next section) – almost the same length
and just a kilo less in weight.
The
F7 Stowaway is another beautiful AP: lots of anodised CNC, pebble coating and
those slim rings. And back when they started producing it the wait list was
supposedly a year or so (bitter laughter follows).
Like
the Traveler and other AP scopes of the era, it had AP’s own focuser but with
Starlight dual-speed pinion. The AP focuser is different in feel from a
Starlight, but not really worse – it is less fluid, but with a slightly more
obvious rack-and-pinion feel, but superbly stable and precise.
This
original F7 Stowaway was produced from 1999 to 2002.
Stock
AP image.
Lens
Design |
ED
glass triplet (air spaced) |
Aperture |
92mm |
Focal
Length |
612mm |
Focal
Ratio |
F6.65 |
Length |
19.5”
(495mm) |
Tube
diameter |
3.62”
(92mm) |
Weight |
3.2Kg |
AP
has a history of rebooting favourite products. They did it with the F8 130 EDT,
for example. Apparently, they were watching the 2017 solar eclipse with their
own Stowaways from Wyoming (just a state east of me in Idaho) and enjoyed using
them so much they decided to do another run … and then another. Deliveries were
between 2018 and 2019. Profiteering you reckon? Uh uh.
Despite two runs, only those on the correct waiting list (I was on the wrong
one) for over twenty years got one, including my fellow reviewer Ed Ting.
I
was really totally wrong about this. Ten years ago, I thought premium small
refractors were a dying breed, that AP would soon be begging for customers. But
as with all things, some people (including me) just love quality. And the real
kicker was the modest price – less than the Baader
Travel Companion (see below), I believe.
This
2nd generation Stowaway looks much the same as the original but in
detail it’s quite different. It’s now an air-spaced objective with the same
aperture (92mm) but a slightly longer focal length.
Like
the rest of us, the Stowaway has put on a little bit since 2002, but probably
less than most. It’s a tad longer and fatter and weight has increased by 200g
as well.
And
now AP has given in, like TEC before them, to using a 2.5” Starlight FeatherTouch focuser in place of their own. I understand why
– it just doesn’t make commercial sense to keep manufacturing something so
readily available off the shelf. Reduces the distinct AP character a bit IMO,
though.
Regarding
that second run, AP says (with that understatement again):
“The initial
response was so positive that that we decided to produce another run of these
scopes in 2019.”
So positive, in fact,
that they have now taken the decision to close the waiting lists to newcomers completely.
I’ve said it before – this is in no way AP’s fault, but is a bit crazy all the
same. Name another company able to produce for only a closed circle of
dedicated fans.
Lens
Design |
ED
glass triplet (oil spaced) |
Aperture |
105mm |
Focal
Length |
610mm |
Focal
Ratio |
F5.8 |
Length |
Minimum
480mm (19”) |
Tube
diameter |
108mm
(4.3”) |
Weight |
4
Kg (5.6 Kg incl rings and plate) |
The
Traveller is legend. Introduced in the early Nineties and discontinued about
ten years later, AP made less than a thousand I’ve heard. Still, that means
that unlike the Stowaway you do have a chance of actually buying one.
No
small APO is made today that matches its combination of features. For though
the Traveller is a true four-inch (just over in fact), its size and weight
belong in the class below – it is 19” long (same as the Stowaway) and weighs
just a kilo more at about 4Kg. Its CNC tube is also slim (just over the bare 4”
for most of its length, flaring towards the objective).
Whilst
being super small and light, the rest of the specs are top-drawer with lens
quality amongst the very best, with minimal false-colour despite very
challenging numbers – 105mm aperture with a 610mm focal length (F5.8). Even the
equivalent LZOS lens adds 40mm to the focal length for the same aperture (and
several kilos more weight for the OTA). In line with its intended use, the
Traveler is reputedly extremely rugged and AP will still service them;
accessories and focuser upgrades are still available too.
The
Traveler is capable of both high powers on planets and stunning wide-angle deep
sky images (with a suitable flattener).
I
eventually owned a Traveler, but I’d had a few false starts over the years.
When you find yourself (as I once did) negotiating for a telescope with an
Italian art dealer who wants paying direct into his offshore account followed
by a hand-over in Paris (yes, honestly), you know things are getting silly. Sadly the Traveller has stopped being an astronomical
instrument and become a collectors’ item, same as the Zeiss APQ.
The
only downside (also an upside, because it means outstanding transmissivity) to
the Traveller is that it has an oil-spaced objective. Oil spacing doesn’t quite
allow the same level of correction as air-spacing and some say you should store
them horizontally if they are to perform properly! There is also (perhaps only
theoretically) a miniscule risk of leaks.
With
a rare and expensive telescope like the Traveler, it’s really important to
understand what you do (and don’t) get for your money. The view is about as good as (but not better
than) the very best of the rest in this guide. Optical quality is better than
any, except perhaps for an unusually perfect LZOS. Build quality – because
everything is made by AP – is at a higher level than almost anything else. Few
other telescopes in this guide are as rugged. None without built-in reducers
are as photographically fast.
But the thing you are really paying for is
that supreme portability: no other refractor larger than 90mm comes close; you
can put the Traveler in its little case and just walk on board, a priceless
convenience for the peripatetic astronomer.
Now you might reasonably ask why no other telescope
in this list combines all these features twenty years on. I just don’t know.
What’s worse is that if AP ever produce another Traveler (and they might, after
all they recently re-booted the smaller Stowaway) you won’t be buying one unless
– as for Eton College - you put your name on the waiting list at birth.
Stock
Baader image.
Lens
Design |
Fluorite
triplet (oil spaced) |
Aperture |
95mm |
Focal
Length |
560mm |
Focal
Ratio |
F5.9 |
Length |
540mm/455mm
(dew-shield ext/retr) |
Tube
diameter |
99mm |
Weight |
3.1
Kg |
One
of the reasons this guide interests me is that it encompasses the ‘travel APO’,
that select breed of 90-110mm APOs short enough to lug onto a plane. Travel
APOs are interesting in part because they have such a chequered history. It’s
over twenty years since you could buy a Traveler from AP (theoretically), but
you still can’t actually go out and buy a travel APO larger than 90mm. But why?
It
seems that travel APOs are hard. From the big Chinese firms there is only the
TS 90/600 and relatives, despite their dominance of the small APO market in
general. LZOS don’t really do a travel APO objective. Canon/Optron
only have the Borg 107 and that may already have been discontinued. If you had
the foresight to go on the Stowaway waiting list before the Millenium
you now have one; otherwise you never will. TEC discontinued the Eclipse after
just a few hundred.
And
then, a couple of years ago, Baader had a go with
this, the Travel Companion …
The
Travel Companion is a 95/560 (F5.9) oil-spaced fluorite triplet (deja vue?) in a very compact OTA
equipped with Baader’s own Steeltrack
focuser and a removable section for compactness and binoviewing.
The whole thing is German made (yes, even the lens), with nods to Zeiss like
the special mineral oil used in the objective. Reviews were overwhelmingly
positive. Like the Traveler and the Eclipse, here was a tiny APO that did
planets as well as deep sky. Excellent! At last you can go out and just buy a
travel APO! Well no …
It’s
not like there’s no market for travel APOs, as Baader
discovered. Despite a steepish asking price of almost E4000, despite the Steeltrack (a decent focuser, but not my favourite), despite
the TEC-like clip-lock tube rings (light but possibly insecure), the 200 they
planned immediately sold out. Many who wanted didn’t get. And they’re not
making any more, at least not any time soon. Presumably, as for TEC and the
rest in the past, they just weren’t making (enough) money on them.
I
hear you ask, ‘why can’t Baader make a profit on a 4”
travel APO costing thousands when Sky-Watcher seem to do fine making an
excellent 4” APO (the ED100 – see below) for hundreds?’ The answer isn’t just
German labour costs either. Travel APOs need very short focal ratios to work
with a 4”+ lens and that means steep curves, tight
tolerances and expensive materials. After all, even F5.9 means a 95m APO with a
focal length that’s just at the carry-on maximum of 560mm. And even F5.9 is
easy to make in a 60mm (Tak’ FS-60) but hard by 95mm.
So,
right now, if you want to buy a travel APO immediately you have just the Borg
90 (get it while you can) or TS 90/600 to choose from. Or keep waiting for Baader to do another run. Does this just seem like AP all
over again?
Lens
Design |
Fluorite
doublet |
Aperture |
90mm |
Focal
Length |
500mm |
Focal
Ratio |
F5.6 |
Length |
~380mm
(15”) without disassembly |
Tube
diameter |
80mm
(3.2”) |
Weight |
1.5
Kg in basic visual config’ shown |
The
Borg 90FL is a very interesting telescope, even though it’s not even quite a
four-inch. Why? I’ll explain …
One
of the smallest and lightest four-inch-class refractors, with a nice fast focal
ratio for wide-field imaging, was Takahashi’s Sky-90 (see below). But the
Sky-90 had a special lens design with a huge air-space and massive cell that
was unfortunately prone to collimation issues – not good in a travel scope.
This latest Borg 90FL replicates the Sky-90s advantageous numbers, but with a
design more suited to travel.
Borgs
have always been among the lightest, most portable and most adaptable of small
refractors. Sadly, past Borgs have been a bit iffy in optical quality terms.
Lately, though, they’ve been using premium Canon/Optron
lenses made in Japan with a fluorite element, just like Takahashi.
The
90FL lens is a Fraunhofer doublet, using crystalline fluorite - just like the
Sky-90. That means the fluorite is at the front, like older Takahashis.
Interestingly, from what I can see from a laser test, the 90FL still has a
larger-than-usual air space, which would allow for better correction of both
false colour and field curvature in this fast lens. But the cell is much
smaller, non-collimatable and hopefully more robust.
Front-surface
fluorite lenses do have a small theoretical advantage over the Steinheil design that puts the mineral at the back –
shallower curves which are easier to make and slightly higher light throughput
since the fluorite is what the light hits first (and fluorite scatters less
light than any glass).
I’d
expected semi-APO levels of chromatic aberration, due to the very fast focal
ratio, but it’s generally very well corrected. The 90FL has similar false
colour to the Sky-90, despite that small air-gap, and less than many 80mm ED
doublets. It also seems pin-sharp at higher powers.
The
tube is Borg’s standard compact 80mm and the standard focuser will be a Borg
helical, with or without a drawtube, but you could spec a Feathertouch
if that’s more your thing. Borg make loads of options too, including a special
tube assembly to turn it into a proper camera lens, a quadruplet 0.72x reducer
is that shortens the focal length to 363mm at F4, and a 1.4x extender for
larger image scales. For urban guerrilla astronomers, all Borgs are now a
stealthy satin black (that shows every finger print).
Crucially, like the Sky-90, the Borg 90FL is
one of the very smallest and lightest APOs in the four-inch class and is easily
carry-on portable: 15” long and with
a weight of 1.5 Kg for a complete OTA minus ring. Alternatively, the objective
unit is easily removed and comes with protective caps; it’s only six inches
long and weighs a kilo. So, if you’re sick of our murky skies and want to head
off to do some imaging on Mt Teide, this could be just the ticket. It’s
expensive though and may soon be discontinued.
A
word of warning for imagers. The 90FL I had came with
their super-reducer and I wasn’t happy with the images so I sent it back. I
later discovered that the reducer had been delivered configured for the 71FL.
For the 90FL a small spacer needed removing. In general, very short F-numbers
like this are very sensitive to reducer spacing.
Borg
make various tube rings and mounting brackets for their 80mm OTAs, but Tak’s 80mm tube rings fit too.
Stock Borg image.
Lens
Design |
Fluorite
doublet |
Aperture |
107mm |
Focal
Length |
600mm |
Focal
Ratio |
F5.6 |
Length |
~500mm
(depends on focuser etc) |
Tube
diameter |
80mm
(3.2”) |
Weight |
1.9Kg
obj. cell, ~3 Kg in basic visual config’ with 80mm tube. |
If
anyone but Canon/Optron offered a 107mm F5.6 doublet,
I’d brace myself for dodgy optics and lots of false colour. But their little Borg
67FL proved me wrong on that. No not a super-APO, but astonishingly good for
it’s crazy-extreme specs. So there’s a good chance the
107FL will be similar and in fact it’s claimed to have lower false colour
on-axis than the 90FL.
In
any case, it will be a front-surface fluorite doublet with a large air gap. The
latter led to problems in the Sky-90 but the 90FL seems better and doubtless
this 107mm will be the same. For more on the optical design, see the entry for
the similar 90FL above.
So,
curiously, it’s not the optics I’m worried about but the mechanical design. In
order to offer the 107FL as a direct OTA upgrade to owners of the 90FL with the
80mm tube set, it has a very unusual telescoping design (see above) and Borg
recommend that you remove it from the OTA to extend or retract it or it might
not work smoothly.
Even
so, I am quite excited about the 107FL, because with the 90FL’s tube set it
would be ridiculously light and portable for such a (in this context) large
aperture. Unfortunately, Borg in general are looking shaky at the moment with
uncertain availability, so maybe get it while you can (travel-scope-itis
strikes again). It is expensive, but well in line with the other 100mm plus
travel APOs described in this guide.
Stock
Pentax image.
Lens
Design |
ED
quadruplet |
Aperture |
100mm |
Focal
Length |
400mm |
Focal
Ratio |
F4 |
Length |
497mm |
Tube
diameter |
115mm |
Weight |
4.2
Kg |
The
SDUF was the most refined (in version II form) of Pentax’s long line of
astrographs. Pentax were one of the first to produce telescopes optimised for
imaging, originally with their own medium-format film cameras, and they have a
great reputation.
The
SDUF uses a four element design, much like an NP101 or
Takahashi FSQ, but delivering an even faster F4 focal ratio. Given the original
Pentax remit of covering a medium format film frame, you can be sure the field
will be flat and well illuminated across the largest CCD chips. What’s more,
this is a very compact and relatively lightweight astrograph that you could use
on a light mount and carry on-board to take to foreign dark skies.
Most
Pentax astrographs have a massive helical focuser, with a big rubber focusing
wheel rather than fiddly knobs, which I particularly like for its ease of use
with cold or gloved hands. The visual back is very flexible too: the whole
thing has been carefully thought-out for imagers.
Pentax
sadly ceased production of their astrographs a few years back and very few come
up used, which probably tells you all you need to know about how good they are.
If you do find a good used SDUF and you’re looking for a wide field imaging
scope you won’t need my encouragement to buy it! Alternatively, it seems Vixen
may be producing a range of similar scopes in future, of which the VSD 100 is
hopefully just the first.
Sky
Watcher 100 ED Equinox
Sky
Watcher 100 ED Pro
Aperture |
100mm |
Lens
design |
ED
glass doublet |
Focal
Length |
900mm |
Focal
Ratio |
F9 |
Length |
Depends
on model |
Tube
diameter |
Depends
on model |
Weight |
Depends
on model |
This
lens is and has been available in various tubes. The Equinox is the premium
offering with a very shiny CNC tube and a quality dual-speed rotating focuser.
The
cheaper ED Pro is lighter, but less compact (it has a fixed dewshield)
and is a more conventional Sky-Watcher OTA, albeit with a (different)
dual-speed Crayford focuser that is smooth and shift-free for visual use, but
doesn’t lock very well for astro-photography.
In
either case, the lens is an ED doublet, apparently using top-quality Schott FPL-53
glass. Optical quality is likely to be good – typically diffraction-limited or
a bit better. As with the TV-102, you shouldn’t expect this to perform quite as
well as a fluorite doublet – its residual chromatic aberration is a notch down
from that level. Nonetheless, my experiences suggest it’s a great all-round
APO, with a flat field and low enough aberrations to give great views on all
types of objects, low or high power, star fields or planets.
The
Equinox 120ED has had some excellent reviews and I liked it a lot. Things
seemed a little less rosy when a well-known tester found that it was really a
113mm scope … even so, it’s a good lens for the price. I’d read that the 100ED
offers no better correction for chromatic aberration than the 120ED, but that’s
not my experience. It may be that older versions used a cheaper ED glass. In
any case, the 100ED in its latest form is a very sharp, low-CA APO with
impressive performance for the price. Don’t tell SW, but the 100ED is a
screaming bargain!
The
Equinox comes with CNC rings that have an Astro-Physics hole pattern (!). The
ED Pro has conventional cast SW rings with ¼-20 threads on the base.
Both
ED Pro and Equinox versions come in a quality hard case and the Pro version may
throw in accessories such as a finder and a reducer to take it down to F7.65.
The reducer didn’t impress me, though. The standard 100ED has quite a flat
field anyway and the reducer added quite a lot of violet bloat on O-A stars for
less than one and a half F-stops.
Stock
Sky-Watcher image
Aperture |
100mm |
Lens
design |
ED
glass triplet |
Focal
Length |
550mm |
Focal
Ratio |
F5.5 |
Length |
~500mm |
Tube
diameter |
105mm |
Weight |
6.3
Kg |
The
esprit 4” APO is a slightly puzzling telescope, at least to me. It’s a very
fast triplet at F5.5, which suggests an ideal travel-to-dark-skies scope. But
in fact, though it is just about carry-on compact, it is heavy and so will need
a biggish mount for imaging, so isn’t ideal as a travel scope.
I’m
not convinced that a very fast triplet is ideal for astrophotography at the
100mm size: compared to quadruplet astrographs the lens will be heavy and have
severe curves, yet it’s not that fast and you’ll absolutely need a
reducer/flattener (though the SW reducer is very inexpensive). Then again, at
just over £1500 it is astonishingly good value.
I
can’t comment on optical quality, but it’s likely to be excellent. I would be
interested to see whether the optics work well visually; if they do it might
make most sense as a dual-purpose scope (most fast quadruplets aren’t great for
visual use). The tube is a traditional aluminium design with a long dew-shield;
build quality looks very good indeed.
The
Esprit range are fitted with heavy duty rotating crayford
focusers that look similar to those found on SW’s other models; I notice some
vendors are also supplying them with (aftermarket?) 3” Feathertouch
focusers.
I
haven’t tried an Esprit yet, but on paper at least I might be inclined to go
for the 120mm model which isn’t that much more expensive and needs a similarly
beefy mount. For really wide-field imaging or as a travel scope, I’d be looking
for something even faster or lighter-weight (or both). Interestingly, SW
themselves soon came out with a quintuplet design with a surprisingly similar
focal ratio and a modest price increase. It’s a shame the triplet had to be so
heavy. At ~4 Kg it might have been a Traveler alternative and I’d might have
bought one.
Aperture |
90mm |
Lens
design |
Fluorite
doublet |
Focal
Length |
560mm |
Focal
Ratio |
F5.6 |
Length |
~300mm
(12”) |
Tube
diameter |
95mm |
Weight |
3.5
Kg |
The
Sky-90 was a hugely popular scope for Tak’, a
departure for a company that had mostly made long-focus refractors. The reason
it was popular was that it combined miniscule size (really, it’s just tiny) for
easy transport with a decent aperture and a fast focal
ratio for imaging. It was, in fact, ideal for many astronomers’ deep sky needs.
The
Sky-90 was a front-surface fluorite doublet made by Canon/Optron,
like Tak’s other FS refractors. But whereas they were
F8, the Sky-90 was F5.6 – very fast for a doublet. Consequently, the Sky-90 had
a special lens design with a huge air-space and massive cell that was designed
to deliver the best possible correction for its fast focal
length. Unfortunately, that cell proved prone to collimation issues – not good
in a travel scope. It also made the Sky-90 surprisingly heavy for its
diminutive size.
The
Sky-90 had a short 95mm diameter tube with a sliding dew-shield. The focuser
was a version of Tak’s classic cast focuser, but with
a short body. When new this was a nice unit, very smooth and precise; but
hanging heavy cameras off it could cause wear and image shift, especially as
extension tubes were often needed due to the radically shortened OTA.
Even
so, the Sky-90 was a handy little scope with excellent basic optical quality.
The small amount of miscollimation on the one I
reviewed was only noticeable at image scales greater than most imagers use
anyway. And whilst other reviewers and testers have also noted miscollimation, I know of Sky-90s that are absolutely fine.
Today
you might consider the Borg 90FL instead – it’s essentially an updated version
of the same scope, but with a more conventional cell design and a much lighter
OTA. Alternatively, test thoroughly before you buy.
Aperture |
102mm |
Lens
design |
Fluorite
doublet (fluorite at front) |
Focal
Length |
820mm |
Focal
Ratio |
F8.1 |
Length |
930mm
(37”) |
Tube
diameter |
114mm |
Weight |
5.3Kg |
The
FS-102 may have been out of production for pushing a decade, but still come up
for sale used and are an excellent choice. Most are the fixed dew-shield
version (the FS-102N) which makes for a physically large scope, at almost a
metre long (but quite light-weight at about 5.3Kg). A few later ones had a
sliding dew-shield and so will be usefully more compact but slightly heavier.
As shown above, it’s on a Tak’ EM-200 mount, a
perfect if overkill match that once gave me a rock-steady view with the FS-102
in a howling gale.
The
FS-102 has an F8.1 (820mm F.L.) doublet with crystalline fluorite as the front
element. My experience, confirmed by independent tests, is that this superb,
expensive and uncompromising design gives correction as good as most triplets but
with quicker cool-down and (perhaps) a tiny contrast and brightness advantage.
I’ll say it yet again – don’t think the FS-102 is inferior to the more modern
FC-100 (see below for discussion).
I
had the opportunity to compare an FS-102 with my AP Traveler at the Grand
Canyon Star Party. The Tak’ was every bit as good for
visual use, except of course that it’s about twice the size and much slower for
imaging.
Like
other Takahashis, the FS-102’s Canon-made (Japan)
lens is of very high quality – typically better than 1/6th PV (95% Strehl). The focuser is Tak’s
own, but is smooth, stable and virtually image-shift free. Overall quality is
of course top-notch.
The
FS-102, like other Taks, has a clamshell with two M8
holes at 35mm spacing.
NOTE
2019: Takahashi have just announced a new F8 4”, the FC-100DZ. It’s smaller and
lighter than the FS-102 but promises to be every bit as good optically, so
don’t pay crazy money for an FS-102.
Lens
Design |
ED
glass triplet (air spaced) |
Aperture |
102mm |
Focal
Length |
816mm |
Focal
Ratio |
F8 |
Length |
890mm/790mm
(minimum ~ 650mm, 25”) |
Tube
diameter |
114mm |
Weight |
5.4Kg |
The
TSA-102 replaced the FS-102. As with the previous scope, there are two
versions: the TSA-102N with fixed dew-shield and the TSA-102S with a sliding
one. The fixed version is about 4” shorter than the FS-102N, but the TSA-102S
is much shorter still. Tak’s own claims are
conservative: my TSA-102S will pack down
to about 25” with the visual back removed and so is almost airline portable
(same as the NP101 in fact). Weight is modest at a claimed 5.4Kg, but it still
feels like quite a big and heavy scope compared to say a Traveller.
‘TSA’
stands for Triplet Super Apochromat, which is right because the TSA-102 has
simply one of the best lenses in astro-world. The
Japanese 102mm F8 (816mm F.L.) triplet is colour-free in any circumstances at
any power, almost (but perhaps not quite) as perfectly free from chromatic
aberration as the TMB 100/800. It doesn’t stop there though, because my TSA had
a perfect start test and cooled quickly for a triplet (probably due to careful
design of the cell). Contrast is superb and the TSA-102 takes
high-magnifications better than almost any other small refractor I have
encountered.
The
focuser is the home-grown 2.7” r&p unit from the
FS-102, but again is very smooth, stable and virtually image-shift free. Build
quality is again of the very highest.
The
TSA-102, like other Taks, has a clamshell with two M8
holes at 35mm spacing.
Stock
Takahashi image.
Lens
Design |
Quadruplet
Petzval with 2 ED elements |
Aperture |
106mm |
Focal
Length |
530mm |
Focal
Ratio |
F5 |
Length |
580mm
(23”) |
Tube
diameter |
125mm |
Weight |
7Kg |
Like
the VSD100, the FSQ is a dedicated astrograph. On the face of it, it’s a very
compact scope at just 23” long, but it is both chunky and heavy, with a massive
dual-speed, rotating focuser.
The
optical system, like the NP101, is a four-element Petzval. However, this one is
even more sophisticated because it uses large air-spaces and two ED elements to
achieve near perfect correction across a very wide field and an even faster
F-ratio than the NP101 to boot, at F5.
I
have heard from people who have been underwhelmed by FSQ’s visual performance,
but perhaps that’s not surprising given its astrographic
design. If you do want to use it for high-power planetary viewing, Takahashi
sell a dedicated extender, tuned for the FSQ optical system, that sharpens it
up. Collimation problems are also not unheard of (like most Petzvals
it seems). A final thing to look for is signs of degradation of the front
element which is soft ED glass and may be prone to damage if regularly left wet
from dew.
The
FSQ needs a bigger tube ring than other Tak 4”
refractors and the weight of the FSQ takes it beyond most small GEMs,
especially for imaging - you’d need an EQ6 (or better still Tak’s
own superb-but-expensive EM200).
Lens
Design |
Fluorite
doublet (Steinheil) |
Aperture |
100mm |
Focal
Length |
740mm |
Focal
Ratio |
F7.4 |
Length |
815mm
(FC-100DC) / 785mm (FC-100DF) |
Tube
diameter |
95mm |
Weight |
2.8Kg
(FC-100DC) / 3.5Kg (FC-100DF) |
To
describe Takahashi’s re-booted FC-100 I’ll have to delve into the optical
design of Tak’s refractors for a few paragraphs.
We’ve been here before, so please skip this bit if
you’re bored.
Before
the FS-102 (see above), Takahashi made a slightly different fluorite doublet four inch APO – the now legendary original FC-100. Takahashi re-introduced the FC-100 in 2013, but
it’s actually a different design again. Confused? Read on …
Both
the original FC-100 and the FS-102 that followed it were F8 air-spaced doublets
made by Optron in Japan. However, their optical
design was different. The FS-102 had its fluorite element at the front, making
it a Fraunhofer type doublet (like most refractors). The original FC-100 was a Steinheil doublet with the fluorite positive convex element
at the back. Urban myth would have it that the Steinheil
lens of the FC-100 was better, but I doubt it. Here’s why:
·
Steinheil lenses in general have steeper curves (more
difficult to make) and are “seldom used unless necessary” (Rutten and Van Venrooij).
·
In the FS-102,
the front-surface fluorite confers a slight advantage in transmissivity because
the fluorite is the first thing the incoming light hits and fluorite transmits
more light than glass … any glass.
The only real advantage of the Steinheil, as far as I can tell, is that putting the
fragile fluorite at the back makes for a more robust optic.
But
for whatever reason, Takahashi have chosen to re-introduce the FC-100. However,
the FC-100 is now a slightly faster F7.4 (good news for astrophotographers).
The objective is still made by Canon/Optron in Japan,
but it’s also likely to be a bit different behind the scenes. For one thing,
the new lens will have a coated fluorite element for better transmissivity
whereas the original FC-100 had an uncoated fluorite element. For another, it
uses eco-glass for the flint element (the original’s
was doubtless laced with heavy metals, like Lanthanum).
Thankfully
that’s the optical theory out of the way. Mechanically, the new FC-100D is
pretty conventional Takahashi, which is a good thing. There are two versions.
The FC-100DC (shown here) uses a version of the small focuser from the FS-60
and has a fixed dew shield. It’s very compact and weighs just 2.8Kg. For purely
visual use, this is probably the one you’d go for because it’s cheaper and
lighter. If you remove the focuser and dew-shield it’s easily carry-on sized at
47cm/18.5” long.
The
more expensive version is the FC-100DF aimed at imagers (but just as good for
visual as the DC). This one uses the larger focuser from the Sky-90 that should
cope better with heavy cameras, gives more in-focus travel and a slightly
shorter packed length. The FC-100DF weighs a bit more though, at 3.5Kg.
Neither
are carry-on portable unless you unscrew the dew-shield and or focuser.
The
FC-100DC I saw was another highly desirable Takahashi refractor: compact and
beautiful. Build quality and cosmetics seem typical Takahashi: lime green
castings, beautiful white tube and with the recent silver (rather than green or
blue) lens ring. The focuser was smooth and gave minimal image shift. The lens
sits in a proper temperature-compensated cell and the coatings were as good as
you will ever (hardly) see.
I
have used an FC-100 (the DC version) quite a lot and the optical performance was
excellent – much like the older Tak’ fluorite
doublets – with outstanding contrast and sharpness. A tad more chromatic
aberration than the FS-102, due to the shorter focal length and eco-glass flint?
I struggled to convince myself so: the FC-100DC was very close to the FC-100DL
at high powers on the Moon and planets.
The
FC-100D is another near-perfect small APO – great on the Moon and planets, but
capable of stunning photography with a reducer or flattener.
The
FC-100D has a 95mm diameter clamshell with two Tak-standard
M8 holes at 35mm spacing, like the Sky-90 and old FS-78 (so clams for those
scopes should also fit).
The
light weight and short tube of the FC-100DC in particular mean it will work on
almost any small mount, equatorial or alt-az. Vixen dovetail bars that fit Tak’ clamshells are available. A huge range of dedicated
accessories, such as reducers and extenders are marketed for the FC-100.
Lens
Design |
Fluorite
doublet (Steinheil) |
Aperture |
100mm |
Focal
Length |
900mm |
Focal
Ratio |
F9 |
Length |
940mm
(782.5 w/o extensions??) |
Tube
diameter |
95mm |
Weight |
3.8Kg |
This
is where your author gets a bit over-excited. You see no-one makes scopes for
planetary observers anymore … and then Tak’ go and do
just that, bless their little fluorite blanks.
The
FC-100DL is a version of the newer fluorite-doublet FC-100 (see above) with a
longer focal length of F9. What difference do those extra 1.5 F-stops make?
They allow (so Tak’ claim) near super-APO (i.e. TSA
102) levels of false colour correction whilst still being a light, quick-cool
doublet. Does this matter? For critical planetary observing, especially on Mars
(many doublets are poorly corrected in the red) it might.
The
FC-100DL turns out to be startlingly long (near a metre) with its fixed dew
shield, but still quite light at 3.8 Kg.
In other respects, it’s like the DF, with its
larger, heavier longer-travel focuser. The other radical (sarcasm alert) change
was black enamel for the focuser for the first run (changing to the standard
blue-green for the second).
Planetary
observers in the North are in for a rough time over the next few years. Mars is
about to have two close oppositions, but it’s barely over the horizon; Jupiter
and Saturn aren’t great either. So if you’re looking for a lightweight (so easy
to mount) planetary refractor with super-sharp optics that you can shove in the
back of the car and drive to Southern Europe (where Mars makes it to a more
reasonable 20-30 degrees altitude), this may be your scope, but check out the
new FC-100DZ below.
Lens
Design |
Fluorite
doublet (Steinheil) with SD mating element |
Aperture |
100mm |
Focal
Length |
800mm |
Focal
Ratio |
F8 |
Length |
770mm
(595mm w/o visual back) |
Tube
diameter |
95mm |
Weight |
3.9Kg
(4.6Kg w/ ring and finder) |
When
Takahashi introduced the FC-100DL, I thought that was it for FC-100D variants.
I was wrong. The original F8 FC-100, first released in 1981, now commands high
prices as a collectors’ piece and perhaps that’s why Tak’
decided to re-launch it as this, the F8 version of the FC-100D, the FC-100DZ.
You won’t hear any complaints from me, because the F8 FS-102 is one of
my all time favourite
telescopes – F8 is a great focal ratio for a 4” refractor because it’s a
perfect compromise between portability, imaging speed and performance at high
powers for planetary and Lunar.
Talking
of the FS-102, as I have explained at length above, there are differences in
lens configurations here. The FS-102 had a Fraunhofer lens arrangement that put
the fluorite at the front: great for optical performance, but (theoretically)
exposed to knocks and damp which soft-and-sensitive fluorite doesn’t like.
Meanwhile, the old FC-100 F8 had the fluorite at the back in a Steinheil arrangement like the modern FC-100D variants, but
its fluorite was uncoated and so transmitted a bit less light.
This
new FC-100DZ puts its fluorite at the back too, but critically then mates it
with a modern special-dispersion glass of some type up front for superior
optical performance. If you’re thinking that sounds familiar, you’re right –
the FOA-60 also uses a special dispersion mating element. In fact, I had rather
hoped that Tak might bring out an FOA-100, but the
FC-100DZ will have to do.
Now
you might ask, ‘what’s the point’, given that the FC-100DC and DF are nice and
fast and compact at F7.4 and still work well at high powers, whilst the F9
FC-100DL is (or rather ‘was’ – it was a limited edition) optimised for high
power lunar and planetary. Well the answer seems to be that the DZ is taking
over the role of the DL. The special mating element purportedly allows it better
correction than the DL despite its shorter focal length. The spot diagrams
and crossings published by Tak do seem to bear that
thesis out. Crucially, the Strehl stays above 90%
across the whole visible spectrum, avoiding the problems of many doublets which
are blurry on Mars at the red end and bloat hot stars in the blue and violet
for imaging.
Lens
aside, the DZ has the 95mm tube common to all the FC-100D variants and the larger
focuser like both the DL and DF. But in addition to the new F8 lens it also
has a sliding dew-shield unlike any other FC-100D, meaning it packs shorter
without unscrewing bits. It’s about 23.5” (595mm) long with the shield
retracted and the visual back removed, whilst without the focuser it makes it
down to a carry-on portable ~21.5” (543mm to be exact).
Weight,
meanwhile, is slightly more than the DL at 3.9 Kg (4.6 Kg with the ring and
finder) because the shorter tube is lighter, but the sliding dew-shield
heavier! Are you following all this?
So
should you buy it? In reality, I found the FC-100DC (the original F7.4 FC-100D)
actually very good for the Moon and planets (though I didn’t try it on Mars
which often tests the long-wavelength Strehl of
doublets). The DZ is also quite a lot more expensive than the DC, though (about
£500 more here in Blighty), and a bit more than the DF too (probably due to the
sliding shield which Tak’ always charges extra for).
But
if you like your Moon and planets just as crisp and false-colour-free as can
be, then the DZ is a fair bit more portable than the other FC-100D models with
their fixed dew-shields. And I am guessing the DZ will outperform the DC/DF on
Mars next year (2020). Then again, for imaging F8 is noticeably faster than F9,
whilst the DZ has just half the violet bloat of the DC/DF. So, all in all, yes,
the DZ is probably worth it.
The
DZ was released on my birthday (and just coincidentally on the 50th
anniversary of Apollo 11), so of course I bought one. It must be karma …
So
far, my the DZ appears very impressive – effectively false colour free and very
similar to the FS-102.
Stock
image of Yuri with his Eclipse.
Lens
Design |
Fluorite
triplet (oil spaced) |
Aperture |
110mm |
Focal
Length |
616mm |
Focal
Ratio |
F5.6 |
Length |
Minimum
~ 482mm (19”) |
Tube
diameter |
113mm |
Weight |
4.5Kg
incl ring |
The
littlest TEC comes closest to the Traveller in terms of its size, weight and
performance. It is the same length, a little heavier at 4.5Kg and a bit
chunkier with wider tube and bigger dew-shield. Like the Traveller, optical
quality is supposedly high and the numbers similarly extreme (110mm
aperture/616mm F.L. giving F5.6). Make no mistake, F5.6 is fast for a 110mm
triplet and this is achieved by using crystalline fluorite (not ED glass) as
the central element. Again, though, it’s an oil-spaced design with the possible
disadvantages mentioned for the Traveller.
The
110FL has TEC’s own large 2-speed rotatable focuser (not a Feathertouch)
and a clamshell with a rather unhelpful configuration of holes that may mean
you need to buy a dovetail from TEC.
Like
the Traveller once was, the 110FL was sensibly priced new from TEC at $4500,
but that’s theoretical because the production runs are irregular and sell-out
quickly. I did find a single new one in an Italian dealer for about £4700.
When I contacted TEC to buy an Eclipse direct,
they didn’t feel the need to reply and I believe it has since been
discontinued.
Lens
Design |
Petzval:
ED doublet objective + doublet Petzval lens (reducer-flattener). |
Aperture |
101mm |
Focal
Length |
540mm |
Focal
Ratio |
F5.4 |
Length |
650mm
(26”) |
Tube
diameter |
101mm
(4”) |
Weight |
~4.5Kg |
The
NP101 is Tele Vue’s current offering in the 4” refractor market. They have been
offering something similar for many years, but the NP101 is still different
from most of the scopes in this roundup. The reason? It’s a Petzval – a four
element design that effectively has a built-in reducer-flattener.
Unfortunately, the Petzval design doesn’t reduce the OTA length as much as the
focal length, so the NP101 is still just above the strict carry-on threshold at
26”. Unlike most of the compact competition, though, it doesn’t require
mandatory extensions or screw-in sections. What’s more the NP101 is light (at
around 4.5Kg) and slim.
The
Petzval design creates a very fast (F5.4) optical system with a natively flat
field – great for star-fields and astrophotography. But the NP101’s killer
feature is that it’s far from being a specialist astrograph and can take high
powers and give sharp Solar System views too. What’s more, its colour
correction is at super-APO levels; the NP101 is effectively free from false
colour.
Optical
fabrication quality is excellent, but perhaps not quite at the level of the finest in this roundup, though you’d need
a perfect night and very high powers to see any difference (and I concede that
any small test-bench inferiority in Strehl or
whatever may not be noticeable in use).
Build
quality is first rate and these days the NP101 gets a quality focuser with a Feathertouch 2-speed pinion. It also comes with a very good
hard case (which on recent versions is compact too). If you opt for the imaging
system (i.s.) version, you get a bigger focuser with
a tilt feature and larger rear element to give full coverage on a big CCD chip.
As
with other Tele Vues, the clamshell fits straight on
a Tele Vue Gibraltar (or even the smaller Panoramic) alt-azimuth mount and TV
sell a Vixen dovetail for an easy fit on most small GEMs (German Equatorial
Mounts – EQ5s etc).
Lens
Design |
ED
glass doublet (air spaced) |
Aperture |
102mm |
Focal
Length |
880mm |
Focal
Ratio |
F8.6 |
Length |
787mm
(32”) |
Tube
diameter |
102mm
(4”) |
Weight |
4.5Kg |
Tele
Vue have dropped the 102. Why? Honestly, I suspect, because it just can’t
compete with other slow ED doublet APOs from China on price. In most ways the
TV-102 is just a scaled up TV-85: cream pebble finish
tube; ‘mag’ wheels, slim clamshell; baffles-less, flocked interior; sliding
dew-shield with screw-on metal cap. Recent versions may have a dual-speed
focuser as standard and will certainly have a more rugged focuser with dual
clamps, in comparison to older models that have a slimmer chrome draw-tube and
single clamps. In all versions the long focal length means that the TV-102 is
quite a long scope for a 4” refractor, yet very slim and also light for the
class at just 4.5 Kg.
The
lens is a 102mm aperture, 880mm F.L. (F8.6) air-spaced ED doublet. Now you
might think that in comparison with the FS-102 “ED F8.6 doublet vs fluorite F8
doublet – there’s not going to be much in it.” Unfortunately
nothing could be further from the truth. Whilst the FS-102 is effectively
colour free, with one of the lowest residual chromatic aberration indices of
them all, the TV-102 is described by optical-testing guru Wolfgang Rohr as “not
quite an APO” – it has noticeable false colour in the star test. If critical
imaging or high-magnification planetary viewing are your thing, this will make a difference.
If
it sounds as if I’m damning the TV-102, far from it. In many ways it is a
typical Tele Vue – high build quality with optical quality better than
diffraction limited. It’s just not a premium fluorite doublet.
Owning
the TV-102 will be easy - the design is simple and rugged and Tele Vue make
numerous accessories. The clamshell fits straight on a Tele Vue Gibraltar
alt-azimuth mount and TV sell a Vixen dovetail for an easy fit on most small
GEMs.
This
Genesis (top) is an F5 model, the Renaissance (bottom) F5.5.
Lens
Design |
Petzval:
achro doublet objective + doublet Petzval lens
(reducer-flattener). |
Aperture |
100mm |
Focal
Length |
500-550mm
depending on model |
Focal
Ratio |
F5-5.5 |
Length |
640-710mm |
Tube
diameter |
102mm
(4”) |
Weight |
~4.5Kg |
The
Genesis was Tele Vue’s forerunner to its current flat-field Petzval designs
(see discussion of NP101 above). Like the NP101, the Genesis had a long focal
length doublet up front with a field flattener/reducer at the back. The
resulting focal length varied from model to model at between F5 and F5.5 – much
the same as the NP101. Like the NP101, the field is very flat – great for both wide field viewing and imaging. And
though it was clearly intended for low powers and wide fields, the optical
quality was quite good enough for higher powers too. The focuser was a
typically super-smooth Tele Vue single speed and the OTA very similar to modern
TVs. So why pay big bucks for an NP101 when you can get a nice old Genesis for
much less?
The
big difference is that in the Genesis the front doublet was an ~F11 achromat,
whereas the NP101’s is an ED apochromat. This means that the Genesis (even
‘SDF’ versions that contained fluorite in the field flattener) has a lot more false colour than the NP101 - the Genesis is
effectively a semi-APO.
‘So,
how much false colour?’ you ask. My experience is that the F5 version has a lot,
though less than most (i.e. F5-F8) achromats. The F5.5 version seems quite a
lot better: false colour only becomes noticeable visually above 100x, so it
gives very decent views of the Moon and planets, works well as a terrestrial
scope and could be used for imaging too (perhaps with a bit of post-processing
or a semi-APO filter to remove the inevitable violet bloat).
The
only other thing to consider is that you will need some very short eyepieces
for high powers (4-6mm) and flat field eyepieces at lower powers to get the
most from that super-flat field. If you’re thinking Tele Vue eyepieces, then of
course that’s the point - eyepieces like Naglers or Panoptics (Ethos if you can get one). Incidentally, early
versions shipped with the TV 26mm Plossl, which was a
really gorgeous EP.
All
in all, I really like the Genesis.
Back in 1999, Ed Ting wrote in his review that it had a more friendly, relaxed
feel than other scopes and I completely agree. There is something singularly
lovely about a scope intended for the useless and unscientific activity of
sweeping the Milky Way on a balmy August evening, surrounded by late summer
flowers and the smell of cut grass. And if that’s your thing, there’s honestly
nothing better. Thanks Al’. It will be a sad day when every scope on sale is a
soullessly efficient imaging machine.
The
Renaissance? Early examples are just a Genesis made of brass – brass tube,
focuser drawtube, knobs and even set screws. My daughter thinks all that
polished golden metal is a bit tacky; personally, I love it. Later, the
Renaissance became a TV-102 under the skin and lost some of its charm.
Both
Renaissance and Genesis have standard Tele Vue features, including clamshells
that fit straight on a Tele Vue Gibraltar or Panoramic alt-azimuth mount,
whilst TV sell a Vixen dovetail for an easy fit on most small GEMs.
Stock
Image.
Lens
Design |
FPL-53
Triplet |
Aperture |
90mm |
Focal
Length |
600mm |
Focal
Ratio |
F6.7 |
Length |
450mm
(18”) w/o disassembly |
Tube
diameter |
90mm |
Weight |
~4
Kg |
This
is another very interesting 90mm APO, this time from the German company Teleskop Service (TS). Optically it’s an FPL-53 (Ohara’s
best ED glass) triplet made in China or Taiwan with a focal ratio of F6.7. That
may not sound all that exciting, but what makes this telescope worth
considering is its outstanding optical quality. A renowned bench-tester found
several examples to be completely free of false colour and of very high Strehl. This means the 90/600 is that rare thing – a scope
that will do double duty as either a high power
planetary scope or a fast astrograph when fitted with the Riccardi-designed
F4.9 reducer.
If
the 90/600 has noteworthy optics, the mechanical side is innovative too. Latest
versions have a tube in four threaded sections for easy transport, or to
provide loads of in-focus for imaging or binoviewing.
Combine that with a 2.5” dual-speed CNC focuser and the 90/600 is a tempting
travel scope, available either from TS, APM or other resellers.
(Note
that previous versions of this scope had single-section carbon tubes and/or
other focusers.)
The
only issue? It’s that ole devil called weight. The 90/600 is the same size and
weight as an AP Traveler.
Lens
Design |
ED
glass doublet (air spaced) |
Aperture |
102mm |
Focal
Length |
650mm |
Focal
Ratio |
F6.5 |
Length |
675mm
(27”), minimum 550mm (22”) |
Tube
diameter |
115mm
(4.5”) |
Weight |
3.3Kg
without rings |
The
Vixen ED102SS is quite a rare scope – the longer focal length ones are much
more common. I’ve included it here because I was impressed with it – both in
optical terms and because it is one of the lightest and most compact 4” APOs
available this side of a Traveller. The
OTA is about 27” long, but remove that push-fit dewshield
(no cross-threading risk like a Tak’s) and you’ve got
a genuine carry-on portable 4” at just 22” long, which is a rarity. What’s
more, despite being quite bulky (the 115mm tube is larger diameter than most
Vixen/Synta 4” refractors), the Vixen is very light (among
the lightest in this Buyers’ Guide) at about 3.3Kg without rings.
At
the time when the 102SS was in production (over a decade ago), most Vixen APOs
were F8-9 fluorite doublets that compared well with Takahashis
in optical terms. The 102SS was a departure - for a start it had a fast (F6.5)
doublet and that doublet contained ED glass not fluorite. Such a fast focal ratio in a 4” ED doublet might make for chromatic
aberration, but the one I saw was well corrected and took high powers to give
very sharp, detailed and colour-free views of Jupiter.
The
Mechanicals are typical Vixen. The 2.5” rack-and-pinion focuser is larger than
similar-looking 2” units on other four-inch Vixen/Synta
refractors, designed to be stable with a heavier camera. It is accurate and
free from play, but not as smooth as the best. The light weight and standard
Vixen dovetail plate make the 102SS an ideal fit on an EQ5 or GP, both of which
handle it fine.
At
the time these were available new I read some reports of poor optical quality,
so be careful to test before you buy, but the one I tried was excellent. If you
can find one and it tests well, snap it up!
Lens
Design |
Fluorite
Steinheil doublet (air spaced) |
Aperture |
102mm |
Focal
Length |
902mm |
Focal
Ratio |
F9 |
Length |
970mm
(38”) |
Tube
diameter |
115mm
(4.5”) |
Weight |
3.6Kg
without rings |
In
everything but its lens the 102FL is just like any other Vixen of the time and
is pretty much identical to contemporary Vixen achromats. That means a large,
but lightweight baffled tube with a fixed dewshield
and a simple 2” rack and pinion focuser in a cast body that has widely been
copied by Synta (Sky-Watcher and others). Vixen
quality from this period was decent, but not in the Takahashi league in my
opinion. However, the FL series were expensive new and are highly regarded now.
The reason is the lens.
The
Vixen 102 FL was one of the early doublet apochromats
and was discontinued a decade ago. Does ‘early’ mean ‘worse’? Not in this case.
For the 102FL uses crystalline fluorite in a very high
quality doublet made by Canon-Optron in Japan.
A lot like the Takahashi FS-102, then? Well, yes, to a point. However, it’s not
the same lens. In fact the Vixen lens is like the
predecessor to the FS-102 – the FC-100. The difference? Most obviously the
Vixen is an F9, not an F8 like the FS-102, but behind the scenes the Vixen uses
another lens design altogether: it is a Steinheil
doublet which puts the fluorite positive convex element at the back (‘normal’
doublets have the positive lens at the front). So
which is better? Well urban myth would have it that the Steinheil
is better, but I doubt it. Here’s why:
1)
Steinheil lenses in general have steeper curves (more
difficult to make) and are “seldom used unless necessary” (Rutten and Van Venrooij).
2)
The Fluorite
element is uncoated – not so good for transmission.
3)
In the FS-102,
the front-surface fluorite confers a further advantage in transmissivity
because the fluorite is the first thing the incoming light hits and fluorite
transmits more light than glass … any glass.
I’ll
leave the subject with a quote from the Takahashi manual:
“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.”
Whichever
design is really better, the Vixen FL lenses are excellent and a cut above a
run-of-the-mill ED doublet. I have tried the 102 only briefly, but have tested
some of the smaller FL models and found them to be among the finest small
refractors. In particular you can expect a high-Strehl
objective that is better than many triplets for chromatic aberration and is a
particularly good choice for the Moon and planets.
Why
did Vixen and others drop fluorite doublets if they were so good? Essentially
because the glass for the negative element contained the heavy element
Lanthanum – not the stuff of ‘Eco glass’.
The
Vixen 102FL comes with a Vixen dovetail and was originally sold on the GP (or
GPDX) mount, which handle the long but light-weight tube well. The optical
quality of these was uniformly high, so buy with confidence (though obviously
caveat emptor). If you are looking for a planetary scope on a budget, this
would be a good choice. Apparently they also make
great solar scopes (see photo above).
Lens
Design |
Five
element air spaced |
Aperture |
100mm |
Focal
Length |
380mm |
Focal
Ratio |
F3.8 |
Length |
497mm |
Tube
diameter |
115mm |
Weight |
4.5
Kg |
Theoretically
this latest Vixen 4” APO is a telescope, but more than any other ‘telescope’
listed here, it’s really a dedicated camera lens. Yes
you could put an eyepiece in the back, but even with the 1.58x extender it’s
likely to be a lacklustre experience. In any case, if you are prepared to pay
around £5000 for a 4” APO, throwing in an extra 10% for an SW 100ED for visual
use shouldn’t break the bank, so why bother?
If
my introduction sounds like I’m down on the VSD100, far from it. In fact it’s a very exciting telescope (sorry, lens). A
super-fast (for a telescope) F3.8 focal ratio and the ability to throw its wide
field flat across the widest chips, makes the VSD100 unique at present. I
haven’t tried one, but the build quality on show at Astrofest
was made-in-Japan impressive and (very) Pentax-like, with a massive and
super-precise helical focuser that felt like it could handle the heaviest
cameras with ease. The fact that it’s carry-on portable, even in its supplied
hard case, makes the VSD100 even more desirable: easy to transport to really
dark skies.
So
what about the technicalities? Well, the VSD100 continues the astrographic arms race by adding an extra lens compared to
established Petzval designs like the Takahashi FSQ. From the published
diagrams, it seems to have the same wide-spaced doublet up front that the FSQs
(and Sky90) employ to obtain the best possible correction. The key difference
is that extra element, placed in the corrector at the back, which allows
tighter stellar images across the whole field.
Is
it a Pentax? There was talk before its release that the VSD100 was just a
re-badged Pentax 100 SDUF, but it isn’t. The SDUF had four lens elements giving
F4, the VSD five elements giving F3.8; the optical design looks generally
different too, with the VSD 100 having a much larger air-space in the
objective. Vixen make a big deal about specially annealing the glasses,
checking everything on the latest Zygo interferometer
and so forth, so I guess they must have taken the Pentax telescope works over
when Pentax ceased production, but have since updated the design.
So
should you buy one? The VSD100 seems expensive when compared against the other
4” APOs in this buyers’ guide, but as I said it’s not
really a telescope. So if instead we compare it
against similarly specc’d camera lenses, then the value looks good. Take the closely
comparable Canon EF
400mm F4 USM lens as an example. The optical spec’ is almost identical and
though the Canon lens adds photo-specific features like image stabilisation, it
retails at a whopping £7000! The Vixen certainly looks beautifully Japanese
made and a lot of care seems to have gone into its design and manufacture, so
if wide-field imaging is your things it’s probably worth it.
A brief warning for any wealthy newcomers to
astronomy reading this: dedicated astrographs are usually very compromised for
visual use. So please don’t buy this thinking it’s a luxury visual refractor - you
will be disappointed!
Finally,
is it worth paying for a dedicated fast astrograph like the VSD100, when you
could achieve something similar with a cheaper, slower scope plus a reducer?
Honestly, I am not sure. Reducers have improved. A built-in reducer is
certainly convenient, but a quality bolt-on (from Takahashi, Borg, AP etc) can
do a really fine job without compromising the scope for visual use. Meanwhile,
though fast reducers are typically expensive, some companies make much cheaper
flatteners that can still work well.
Stock
image.
Lens
Design |
ED
glass triplet |
Aperture |
102mm |
Focal
Length |
770mm |
Focal
Ratio |
F7 |
Length |
625mm
(25”) |
Tube
diameter |
115mm
(4.5”) |
Weight |
6Kg |
The
FLT110 is certainly a beautiful thing to behold, with superb mechanical
engineering and that white-gold finish. There are various models, but all have
large two-speed imaging focusers and quality CNC tubes. However, the triplet
and all that CNC do make for quite a heavy OTA.
Early
FLT110s have a premium lens made by TEC (the Telescope Engineering Company),
which gives them the best of both worlds – superb CNC tube and focuser with one
of the finest APO lenses. These FLT110s are a very desirable 4” APO.
Unfortunately,
later models with WO’s own lenses are a very different animal. Independent
tests suggest many of the larger WO triplets come in various fruit flavours –
mostly citrus – and the FLT110 can be similarly lemony. Wolfgang Rohr tested
three FLT132s in a row with severe overcorrection and the FLT110 didn’t fare
much better. Are they all like that? Presumably not, but one disadvantage of
running ScopeViews by obtaining scopes for test as a
private buyer is the risk of paying (my) good money for a real dud. That risk
is just too high with the larger WO triplets, so I can’t give you my own
experiences of them.
So
which one is best? As usual, no single answer.
·
If you want the
very best lens, perhaps for critical use on the planets– highest optical
quality, sharpest at high powers, zero false colour etc – it’s the (discontinued)
triplet TSA-102 or TMB 100/800. In terms of available scopes, the new FC-100DZ may
run them very close (so does the FS-102, but it’s getting rarer on the used
market now).
·
For sheer value,
the Sky Watcher 100ED wins hand down, at least in its latest versions. It’s not
very portable, but is light, easy to mount and performs astonishingly well for
its price. It’s an easy Best Buy for the
rest of us who don’t want to spend thousands on a small APO.
·
If you want the
most portable, it’s got to be the AP Traveller. Long out of production at the
time of writing, it will be hard to find and expensive. Borg’s larger FL models
may be worth a look for full carry-on portability.
·
The new Takahashi
FC-100 models (DC/DF) are compact, lightweight and available (and the DC is
carry-on portable with shield and focuser removed). The new DZ may be the
sweet-spot with ultimate planetary performance at F8, but it’s expensive. All
will cool much quicker than a triplet for grab-and-go.
·
The best native
astrograph may well now be the Vixen VSD100, with its massive helical focuser
and innovative optical system that delivers such a flat, wide field across the
biggest chips, but the Vixen probably won’t be all that good in a multi-purpose
role: it’s a dedicated astrograph. Otherwise, the FSQ-106 is highly rated by
serious imagers.
·
If you want a
dual-purpose visual/imaging scope with no accessory-swapping required, then the
Tele Vue NP101 remains a good all-rounder: it comes close to the best in
optical terms, has an excellent focuser in the ‘i.s.’
version, is fairly light-weight and comes with a standard fast, flat field for
imagers. Unfortunately, it isn’t carry-on portable and is expensive now.
Buy
a Traveller for ultimate portability or an FSQ-106 for imaging. On a sensible
budget, an FC-100DC is small and light and does everything well. On a tighter
budget, the Sky Watcher 100ED is unbeatable value.