Four-Inch APO Buyers’ Guide
The
clue is in the URL. This website represents my opinion, my view, on a range of astro-related gear and subjects. But I can
state one thing as simple fact - if I personally had to own just one telescope
it would be an APO in the four-inch class. A four-inch APO can be both highly
portable and capable, attributes
larger and smaller refractors struggle to combine. The following is a guide to
some of the more commonly encountered and interesting examples, past and
present.
I
have expanded the list to include refractors between 90mm and 110mm, rather than
strictly 4”.
Please
note that I have tried many, but not all, of the scopes listed here. Also, some
of the photos are many years old and not great quality, whilst others are
generic marketing images.
Astro-Physics Stowaway 92mm F5 EDFS
Astro-Physics Stowaway 92mm F6.6 EDFS
Astro-Physics Stowaway 92mm F6.6 – New version
(2017-22)
Astro-Physics Starfire 110
GTX
Sky Watcher Esprit 100 Triplet
Tele Vue Genesis (non-SDF versions)
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.
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) and in recent years they’ve done it with the Stowaway. 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; four in all so far. Deliveries were
between 2018 and early 2022.
This
new 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.
AP
has given in, like TEC before them, to using a 2.5” Starlight FeatherTouch focuser in place of their own. It reduces the
distinct AP character a bit, but it’s the perfect focuser for the job, including
a rotator and a two-stage visual back to accommodate really big sensors. It’s
sufficiently heavy duty for bigger cameras or Bino’s, but doesn’t add too much
weight.
The
2nd generation F6.6 Stowaway is slightly larger than the original,
but it’s still just tiny nestled into its standard Peli case – almost identical
in size and weight to a TV-85, but with a useful 7mm extra aperture, a better
focuser and a much more perfectly corrected objective.
Attention
to detail is the usual AP near-obsessive, with clever features to make it
light, easy to use and deliver the very highest contrast.
Standard
equipment is generous, including that Peli case with custom dividers,
super-thin CNC rings, an AP Vixen dovetail plate and all the visual back
adapters. With that in mind the $3700 (£3700 once you’ve paid shipping and
duties) isn’t even that expensive.
For
that most recent (fourth) run in 2021, AP changed its frustrating wait-list
strategy and had a draw instead (I’ve posted an article on the process here).
And for the first time in my life, I got lucky in a draw! My new Stowaway
arrived before Christmas. First impressions are everything I’d hoped and more,
with correction and quality in the very highest league.
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 (~5.4 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.
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 for the OTA. 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.
Update: AP have announced a new Starfire 110
GTX to start deliveries in 2023. Initially, it will be via a draw, like the
last Stowaway release. Read my own experience of that here.
Stock AP image.
Lens Design |
ED glass (FPL55?) triplet (air spaced) |
Aperture |
110mm |
Focal Length |
660mm |
Focal Ratio |
F6 |
Length |
Minimum 482mm (19”) |
Tube diameter |
145mm (5.7”) |
Weight |
~5.5 Kg |
I’ve been doing this a while, but I still felt
irrationally exuberant when I read that AP were (finally) replacing the
Traveler with a new model. The reason is simple: the Traveler is one of my
top-five all-time favourites because it does everything well, is ridiculously
small (see above) and perfect for travel. Even twenty years on nothing has quite
replicated it. The 110 GTX promises more of the same (if a less evocative
name).
The 110 GTX is (unsurprisingly) 110mm aperture vs
105mm for the Traveler, but it’s slightly slower, at F6 (vs F5.8 for the old
scope). The lens is still an ED triplet, but like the more recent Stowaway it’s
a more conventional air-spaced design.
The centre element is described as ‘the newest
Super-ED glass’ which likely means Ohara’s FPL-55. AP say the ‘hard’ mating
elements are ‘carefully-selected’ to give superb correction without a wide
airspace and its ‘sensitivity to decentring’ or ‘putting the sensitive ED glass
up front’. The result is a lens light and rugged, but not only…
Unusually, the 110 GTX has been designed for both imaging
and visual: it covers a full-sized sensor with pin-point stars and keeps volet
bloating to a minimum, but is still corrected to a Strehl of 90% across the
whole visual spectrum and a wide field, even without the (optional) F5 reducer.
AP say it will perform like a long focal length apochromat at high power.
In a major difference from the Stowaway (that uses a
modified 3rd party focuser – a 2.5” Feathertouch),
the 110 GTX has an AP focuser. But why? I suspect no 3rd party could
supply off-the-shelf with the required spec’s. The focuser has a 3.5” drawtube
for big chips and heavy cameras (or bino’s), but appears much shorter (and
perhaps lighter) than a 3.5” FT. AP have long made their own focusers, so no
worries about quality – it’ll be superb.
The OTA looks more like the Stowaway than the Traveler
and lacks that scope’s tapering tube. The finish is AP’s long-standard textured
powder coat. The short focuser and a sliding dew-shield allows the OTA to be
super-compact: a bit heavier than the Traveler it’s (remarkably) the same 19” length
and so is easily carry-on portable. Internally, it will have AP’s usual
multiple baffles for supreme contrast.
The CNC rings are super-slim like the Stowaway’s and
help keep the weight down.
Overall, the 110 GTX is going to have unmatched
capabilities, like the Traveler before it. The reason is that AP make
everything in-house, permitting a level of integration that off-the-shelf
components can’t replicate. The combination of compactness, ruggedness,
do-everything lens and premium focuser mean you likely want one (me too). But
will we get one?
At least the multi-decade wait list is over: like the
recent Stowaway production run, it’s going to be a draw. AP will send
instructions to anyone who signed up, after which you’ll have to act fast to
enter. For reference you can read about the Stowaway draw and order process here.
I was lucky to get a Stowaway in the draw and I will
try again, but I’m realistic about getting lucky twice.
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.
From
the big Chinese firms there is only the TS 90/600 and relatives. LZOS don’t
really do a travel APO objective. Canon/Optron only
have the Borg 90s and 107 and they may be hard to find. TEC discontinued the
Eclipse after just a few hundred. The LOMO 95 is as rare (rarer actually) than
the proverbial rocking horse dung.
It’s
over twenty years since you could buy a Traveler from AP (theoretically), but it’s
still amazingly difficult to just go out and buy a travel APO. But why? It
seems that travel APOs are hard.
And
then, a few 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 (still waiting in early 2022). 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. 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 or TS 90/600 to choose from – if you can still find one. Hope AP will do a
fifth run of the Stowaway? 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’ |
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 to
achieve good correction, the Sky-90 employed a lens with a big air-space that
was prone to centring/collimation issues – not good in a travel scope.
The
90FL lens is a Fraunhofer doublet, using crystalline fluorite, just like the
Sky-90. But unlike the Sky-90, it uses a small air gap and so has a more
conventional lens cell - more rugged for travel and lighter too.
I’d
expected semi-APO levels of chromatic aberration, due to the fast focal ratio
and small air-gap, 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.
Some
Borgs with ED lenses were a bit iffy in optical quality terms. But the 90FL’s
objective is made by Canon/Optron in Japan, just like
a Takahashi. So optical quality seems outstanding.
So
the 90FL has a great lens, but not only. The Borg 90FL is also one of the
very smallest and lightest APOs in the four-inch class and is easily carry-on
portable. At 15” long and just 1.5 Kg minus ring, it’s the same size but half
the weight of a Sky-90. Alternatively, the objective unit is easily removed and
comes with protective caps; it’s only six inches long and weighs a kilo.
Typical
for Borg you could spec the OTA with one of several helical focusers, or a Feathertouch if that’s more your thing.
Borgs
are super adaptable and the 90FL has various options to change its optical
characteristics. You could fit a basic flattener, a quadruplet 0.72x reducer that
shortens the focal length to 363mm at F4, or even a 1.4x extender for more
image scale.
Borg
make various tube rings and mounting brackets for their 80mm OTAs, but Tak’s
80mm tube rings fit too.
The
only downside to the 90FL is the usual one: it’s expensive.
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 enthusiastic 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. It is expensive, but well in line with the other 100mm plus travel
APOs described in this guide: depending on which focuser you spec’d, an
imported one might start from ~£3000.
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 was the premium
offering with a very shiny CNC tube and a quality dual-speed rotating focuser,
but it’s discontinued.
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 imaging.
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. But you shouldn’t expect this to perform
quite as well as a premium fluorite doublet – its residual chromatic aberration
is a notch down from that level.
Still,
the 100ED is 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 came with CNC rings. 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 a lot of violet bloat on O-A stars for less
than one and a half F-stops.
The
100ED is still significantly cheaper than a premium 4”, but its price has gone
up in recent years and its not the screaming bargain
it once was.
Stock
Sky-Watcher image
Aperture |
100mm |
Lens design |
ED glass triplet |
Focal Length |
550mm |
Focal Ratio |
F5.5 |
Length |
470mm |
Tube diameter |
105mm |
Weight |
6.3 Kg |
Esprit
is Sky-Watcher’s range of premium astrographs from 80mm to 150mm. The Esprit
100ED is a fast (F5.5) triplet with a heavy duty focuser.
The
triplet lens design means less violet bloat in your images than a doublet and potentially
better coverage on larger chips than a quadruplet (because Petzvals
like the FSQ range vignette more than other designs). That fast focal ratio
means you will need the flattener for imaging, but fortunately being a
Sky-Watcher it’s much more affordable than many. The flattener gives a flat
field across a 40mm image circle, so should be good for larger sensors.
Like
most Sky-Watchers, the lens is fabricated with premium glasses, including
FPL-53 for the centre element.
The
short focal length and sliding dew-shield means it’s a very short scope, which sound
ideal for travel. But in fact, though it is carry-on compact, the Esprit 100ED
is heavy and so will need a biggish mount for imaging, so might not be the
ideal travel-to-dark-skies scope it seems.
I
can’t comment on optical quality, but it’s likely to be excellent. I would be
interested to see whether the imaging-centric optics also work well visually at
high powers. The tube is a traditional aluminium design with a long dew-shield;
build quality looks very good indeed.
The
Esprit 100ED is fitted with a hefty dual-speed Crayford focuser that boasts a
3” drawtube to support larger cameras and a capstan-wheel style rotator to
adjust camera angle.
As
usual with SW, standard equipment is generous, with a carrying case, rings, Losmandy D-plate (this is a heavy OTA, remember), 9x50
finder and diagonal. Value at under £2000 is excellent.
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 for its popularity was a
combination of miniscule size (really, it’s just tiny) for easy transport with
a decent aperture and a fast focal ratio for imaging.
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/centring issues – not good in a travel scope, but a known
downside of large air gaps. 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 slightly-off centring 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 alignment problems, 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.
2020 FC-100DZ (dewshield retracted) and FC-100 Classic from 1992.
Aperture |
100mm |
Lens design |
Fluorite doublet (fluorite at back) |
Focal Length |
800mm |
Focal Ratio |
F8 |
Length |
800mm (31.5”) w/o visual back |
Tube diameter |
114mm |
Weight |
4.2Kg (w/o ring and finder) |
This
was the ‘original’ Takahashi F8 100mm fluorite doublet, produced from the 1980s
into the early 1990s, when it was replaced by the FS-102 (below).
The
OTA design is very similar to the FS-102’s – a large, fully baffled tube 114mm
in diameter, with a fixed dew-shield, a heavy-duty 2.7” focuser and a lens in a
collimatable cell.
However,
that lens has an unusual design, one that Takahashi have reverted to for their recent
FC-100D and FC-76D models.
Both
the original FC-100 and the FS-102 that followed it were F8 air-spaced doublets
(made by Optron in Japan as always). However, their
optical design was different. The FS-102 had its fluorite crown 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. Takahashi did this because at the time fluorite couldn’t be coated and
was vulnerable (fluorite is quite soft), so putting it at the back, out of the
way of lens wipes and dew, made sense. That fluorite crown was partnered with a
conventional flint containing heavy metals that wouldn’t be used for
environmental reasons today.
So
most FC-100 Classics have an uncoated fluorite element at the back, which means
they transmit a bit less light than later models. However, the FC-100 pictured
dates from the last years of production, when some lenses with fully coated
fluorite were fitted. These later lenses have a red capital ‘F’ engraved in the
lens ring.
The
original FC-100 was (and is) highly regarded and considered outstanding for
high power planetary use. Whilst I can’t speak for the earlier models, the
F-lens is certainly an excellent optic. Not only does it have a near-perfect
star test, but very low false colour levels. It does give excellent high-power
views, including of Mars. Compared to the very latest FC-100DZ (pictured) which
uses some sort of special dispersion flint, the classic FC-100 has a trace more
false colour, but otherwise gives it a strong run for its money, including
outstanding lunar, planetary and double-star performance. In fact, the main area
it lags the newer models is for imaging – it’s off-axis field is less well
corrected.
So
the original FC-100 does make a superb used buy if you like visual astronomy
and especially for the Moon and planets.
Note
that mounting is via the same clamshell as the FS-102 with its 35mm spaced M6
holes.
I’ll
say here that the FC-100 and all subsequent F8-F9 fluorite doublets from Tak’
are better corrected than any ED glass doublets I have tested of similar
aperture and focal length, suggesting fluorite does allow for a better
corrected doublet.
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. Details below.
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 in Tak’s refractor line-up, before they introduced
the FC-100D. It’s discontinued, though the similar TSA-120 remains in
production.
As
with the FS-102, 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: the 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 – lighter than
a modern SW Esprit 100ED, but still like a big and heavy compared to an AP
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 102mm F8 (816mm F.L.) triplet,
made by Canon/Optron, is visually colour-free in any
circumstances at any power. It achieves this with an centre element of premium
ED glass and an air-spaced design.
My
TSA had a perfect star test and cooled quickly for a triplet (probably due to
careful design of the cell). Contrast is superb and visual high-power
performance outstanding. Interestingly, though, Tak’s published spot diagrams
suggest that the TSA isn’t quite as well corrected in the red as the FS-102.
Why? Because it’s a more recent design that’s tuned for low levels of violet
bloat in images.
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.
FSQ-106
with FSQ-85.
Lens Design |
Quadruplet Petzval with 2 ED elements |
Aperture |
106mm |
Focal Length |
530mm |
Focal Ratio |
F5 |
Length |
580mm (23”) |
Tube diameter |
125mm |
Weight |
7Kg |
The
FSQ-106 is a quadruplet with an advanced design that Takahashi have refined
over a number of iterations, primarily as an imaging machine. Similar to its
smaller sibling the FSQ-85 I(see above), it’s optical spec’s and imaging
performance are even better (at a price in terms of both cost and weight).
The
optical system, like Tele Vue’s 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. The FSQ-106ED replaced an
earlier Petzval quadruplet, the FSQ-106N which had a fluorite crown.
The
FSQ is clearly designed for AP, but If you do want to use it for high-power
viewing, Takahashi sell a dedicated 1.6x extender, tuned for the FSQ optical
system, that sharpens it up.
I
have heard from people who have been underwhelmed by FSQ’s visual performance. However,
a recent session viewing Mars at high power using a friend’s FSQ with the 1.6
extender proved the FSQ-106 is in fact excellent for visual use too.
The
FSQ-106 a very compact scope, but it is both chunky and heavy, with a massive
dual-speed, rotating focuser. The FSQ needs a bigger tube ring (125mm) than
other Tak 4” refractors and the weight takes it beyond most small GEMs,
especially for imaging - you’d need an EQ6 or larger.
By
all accounts recent versions produce stunning images and a reducer is available
to make it even faster, but the FSQ-106 is an expensive option now.
Collimation
problems are not unheard of (like most Petzvals it
seems) as is degradation of the soft ED glass front element if regularly left
wet from dew. So shine a bright torch in the front and do a quick star test
before buying used.
According
to Tak’, the latest FSQ-106ED boasts the following improvements:
1)
Better
correction for false colour and distortion
2)
20-30%
lower Vignetting thanks to a larger third lens and altered optical
configuration
3)
Improved
high-magnification visual performance – as good as a TOA with the 1.6Q (F8) extender
they claim (wow!)
4)
More
back focus for easier use of normal visual accessories
5)
A
new focuser clamp to reduce image shift
6)
A
new micro-focuser with less slip
7)
A
new (optional) 4-element F3.6 reducer (again, wow!)
Update
2023: The FSQ-106ED has been replaced by the FSQ-106EDP. The main difference
seems to be that they’ve ditched the rotating focuser for a downstream
camera-angle-adjuster on the end of the drawtube. I guess this was because
rotating focusers – though super convenient in terms of positioning the knobs -
can introduce image shift.
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) |
Compared
with the expensive, heavy, sophisticated FSQ-106, the FC-100D is Takahashi Zen:
a simple, light, fairly cheap, fluorite doublet that does everything.
Twenty
years and more ago, Tak’ replaced the original F8 FC-100 with the FS-102 which
had a different lens design with the fluorite element at the front but in a
similar large tube with a hefty focuser. The current FC-100D reboots the basic
lens configuration of the original, but with a new (faster) design tuned for
the digital imager (hence the ‘D’) in a lighter tube.
There
have been four different FC-100D variants so far, but the first were the
FC-100DC and DF which share an F7.4 100mm, air-spaced doublet objective with
the fluorite element at the back in a Steinheil configuration like the original
FC-100. Still made by Canon/Optron in Japan, the new
lens has a coated fluorite element for better transmissivity and employs
eco-glass for the flint element. The lens sits in a proper
temperature-compensated cell for quick cool-down (an important feature unless
you store your scope outside).
Both
versions have light-weight, 95mm diameter tubes with fixed dew-shields. The
basic FC-100DC has a smaller focuser derived from the FS-60C that makes it one
of the lightest 4” apochromats at just 2.8 Kg. Unthread the dew-shield and
focuser (not hard to do, the threads are quite coarse) and the DC is easily
carry-on portable.
The
more expensive FC-100DF is aimed at imagers and has a larger focuser (from the
Sky-90) that should cope better with heavy cameras and has more travel. The
FC-100DF weighs a bit more, though, at 3.5Kg.
Build
quality and finish seem typical Takahashi: blue powder-coated castings,
beautiful white tube and with the recent silver (rather than green or blue)
lens ring. The focusers are smooth and gave minimal image shift unless abused.
Optical
performance is truly excellent – much like the older Tak’ fluorite doublets –
with outstanding contrast and sharpness. Surprisingly, the FC-100DC is really
very close to both the FS-102 (and the recent FC-100DL/DZ) at high powers on
the Moon and planets. On a recent comparison evening, it gave the best
planetary views out of a range of quality scopes, with zero false colour on
Mars and superb sharpness and contrast on Jupiter.
For
imaging you can get an F5.4 reducer shared with the FC-76, but Tak’ also make a
spacer (‘CA’) ring to fit their attractively priced multi-flattener to the
FC-100D giving a 44mm image circle.
The
FC-100D is another near-perfect small APO from Tak’ – great on the Moon and
planets, but capable of producing good images with a reducer or flattener. It’s
light weight and something of a bargain in Tak’s range.
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).
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-100D (see above) with a
longer focal length of F9. This allows (so Tak’ claim) near super-APO (i.e. TSA-102)
levels of false colour correction whilst still being a light, quick-cooling 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, the FC-100DL is like the DF,
with its larger, heavier longer-travel focuser in a 95mm diameter tube with a
fixed dew-shield. The other radical (sarcasm alert) change was black enamel for
the focuser and a broader clamshell to support the longer OTA.
The
FC-100DL was only made in two limited runs, but if you want an FC-100D that’s
oriented towards visual use on the Moon and planets, don’t despair: Takahashi
replaced the FC-100DL with yet another variant, the FC-100DZ…
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 FC-100, first released in 1981 was F8 and perhaps
that’s why Tak’ launched an F8 version of the FC-100D - the FC-100DZ.
The
FC-100DZ puts its fluorite at the back like the other FC-100D variants, but
critically then mates it with a modern special-dispersion glass flint 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.
What’s
the point? Well, the answer seems to be that the DZ takes over the role of the
DL as the planetary specialist in the range. In fact, the special mating
element purportedly gives even better correction than the DL despite its
shorter focal length. 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 from DF. But in addition to the new F8 lens it also has a sliding
dew-shield unlike any other FC-100D, meaning it packs down shorter: the DZ
is 23.5” (595mm) long with the shield retracted and the visual back removed, or
just ~21.5” (543mm to be exact) without the focuser.
Weight,
at 3.9 Kg (4.6 Kg with the ring and finder), is slightly more than the other
FC-100D variants thanks to that sliding dew-shield.
So
should you buy one? I found the basic F7.4 FC-100D already very good for the
Moon and planets, whilst the DZ is heavier and more expensive. But if you like
your Moon and planets just as crisp and false-colour-free as can be, then the
DZ is you scope. Meanwhile, the DZ is usefully faster than the F9 DL, but supposedly
has less violet bloat than the DC/DF for imaging.
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 …
Update:
The DZ impresses - it is effectively false colour free, just a little better
corrected than the original FC-100 and perhaps even better than the FS-102.
Everything else is top-drawer too.
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 |
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. The black finish may be satin powder coat or a gloss black
on older models.
Recent
versions may have a dual-speed focuser as standard, in comparison to older
models that have a single-speed, chrome draw-tube focuser. 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 an FS-102/FC-100 “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 Tak’ F8 fluorite doublets are
effectively false-colour free, the TV-102 is described by optical-testing guru
Wolfgang Rohr as “not quite an APO” – it suffers noticeable false colour. 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 and isn’t as well
corrected as one.
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.
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.
This
Genesis is an early F5 model.
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 current 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. 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 these original Genesis models the front
doublet was an ~F11 achromat, whereas the NP101’s is an ED apochromat. The
Genesis lens does have a large air gap – a solution that Tele Vue often employ
to improve correction. But the original Genesis still has a lot more false
colour than the NP101 – it’s is a semi-APO at best.
Another
difference is the size of the rear Petzval doublet – the NP101’s is larger for
less vignetting when imaging.
I
need to clear up some (including my own) confusion here. The later 101mm Genesis
SDF model – produced from 1993 to 2001 - does have much better correction than
the earlier models, but that’s because it has an SD glass element in the
objective doublet as well as a fluorite element in the reducer (hence SDF): see
separate entry below.
The
only other thing to consider with any Genesis or Renaissance 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).
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.
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.
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
Renaissance? Early examples are just a Genesis (see above) 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.
The
F5.5 Renaissance you see here was produced in 1986 for Comet Halley’s
anticlimactic return. As noted, it has a slightly longer focal length than the
early Genesis models that means its doublet was more like an F12 than F11 and
correction is slightly better, though it’s still a semi-APO at best.
The
Renaissance came in an early version of TV’s hard case with red velvet lining
and a matching diagonal with a brass thumb screw, along with a long-defunct
26mm version of TV’s Plössl, an eyepiece that I really like.
The
version you see had an early type of Panoramic mount that has a metal stretcher
and is a bit wobbly. Best to replace it with one of the brass-and-walnut
Gibraltar mounts that TV used to make. The ring has the usual three ¼-20
threaded holes and will go straight on any TV mount.
Later,
the Renaissance became a TV-102 (again, see above) under the skin, losing the
flattener and some of its charm.
Stock image.
Lens Design |
SD doublet objective + doublet Petzval lens with fluorite. |
Aperture |
101mm |
Focal Length |
540mm |
Focal Ratio |
F5.4 |
Length |
84cm |
Tube diameter |
102mm (4”) |
Weight |
~5.5Kg |
I’ve
covered the original Genesis above, but the later SDF version needs a section
of its own. Why? Because it’s a very different scope and to clear up some
confusion around these older Tele Vue Petzvals.
The
Genesis SDF model – produced from 1993 to 2001 – has a slightly larger aperture
than the original at 101mm. But more importantly, the doublet up front, which
determines the level of false colour in these scopes, is no longer an achromat
– it contains SD (Special Dispersion) glass, hence the SDF name. So the SDF is
a doublet apochromat, a forerunner of the NP101, but likely with an older type
of SD (as opposed to ED in the NP101) fluoro-crown
glass.
Now
we know what the ‘SD’ in SDF stands for, what about the ‘F’? The SDF has a
fluorite element in the reducer, like some older Genesis models. However, the
fluorite has little effect on false colour, per Roland Christen and contrary to
popular opinion!
The
upshot is that the SDF has much better correction for false colour (and
probably spherochromatism as well) than the earlier Genesis and Renaissance
models, but slightly worse than the essentially-perfect NP101.
I
believe that the SDF also has a larger Petzval lens and so is better for
imaging than the original Genesis (all Petzvals
suffer vignetting, but less so if the rear doublet is larger).
The
SDF has another big (literally) difference from the NP101 and earlier Genesis:
it’s substantially longer and heavier too, but otherwise it’s classic TV with
the mag-wheels focuser and glossy anodising that I always preferred to the
later satin.
Given
its size and weight the SDF needs the bigger Gibraltar mount or a medium
equatorial.
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 + mounting hardware |
This
is another 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 and much heavier than a TV-85.
Update
2022: the 90/600 has been discontinued in favour of an F6 90mm that ditches the
segmented tube.
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 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 likely won’t give a great view.
If
it sounds like I’m down on the VSD100, far from it. 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 (at least without an extra
reducer).
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. 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.
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
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 different too: the
VSD 100 has a much larger air-space in the objective.
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 spec’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!
A brief warning for any wealthy newcomers to
astronomy reading this: dedicated astrographs can be compromised for visual
use. So please don’t buy this thinking it’s a luxury visual refractor - you may
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. A built-in reducer is convenient and avoids the
spacing issues that become a real pain at very fast f numbers, 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.
2023
Update: It looks as if the VSD has been discontinued (puzzlingly, because the
similar FSQ-106 is on long backorder).
FL-102S, ED-102S (top).
Lens Design |
ED glass doublet (air spaced) |
Aperture |
102mm |
Focal Length |
920mm |
Focal Ratio |
F9 |
Length |
940mm (37”) w/o EP Holder |
Tube diameter |
115mm (4.5”) |
Weight |
~3.5Kg incl. rings and plate |
The ED-102S hails back to the same green ‘Hammerite’ era as the more famous FL models (see below) and it looks very similar to the FL-102S with very similar specs. In detail, though, it’s very different.
The objective is the same 102mm aperture and is an air-spaced doublet too, but is slightly slower at the full F9 (920mm vs 900mm for the FL) and sits in a basic cell without collimation screws or even (from what I can see) a temperature compensation ring. But the main difference is that it substitutes the expensive fluorite crown element at the back of the FL-102S’ objective with an ED glass crown at the front. Which ED glass? No idea.
In front of that ED objective, the dew-shield is shorter than the FL-102S’ and slides rather than screws off. Other visible differences include a grey lens ring rather than the FL’s green and a different dew cap.
At the other end, the small visual-oriented focuser of the FL-102S is replaced with a larger unit that looks more like modern Vixen and Sky-Watcher focusers and has a wider drawtube better suited to imaging, but with the same black plastic knobs as other Vixens of the time (if you buy one, please don’t paint them silver – it won’t make it into a Tak’!)
Another difference is the finder mount: The FL has an unusual one that hinges on a single bolt, the ED a standard dovetail.
Build quality is otherwise very similar to the FL. This is a large (115mm diameter) but very light OTA that lacks the premium finish of a Takahashi (the focuser attaches with screws, for example) but is perfectly serviceable and usefully lighter to mount – about 2 Kg lighter than a contemporary FC-100 or FS-102.
Like other Vixens, the ED-102S came with thin formed split rings (no Tak’ clamshell here) and a Vixen plate.
Performance should be good, with false colour just a small notch higher than the fluorite model.
The ED-102S would originally have been much cheaper than the FL, but was still available as a set on a GP (or possibly GPDX) mount.
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 ED-102SS is quite a rare scope. 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 ED-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 ED-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 ED-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 |
900mm (920mm) |
Focal Ratio |
F8.8 (F9) |
Length |
940mm (37”) w/o EP Holder |
Tube diameter |
115mm (4.5”) |
Weight |
3.75Kg incl. rings and plate |
In
everything but its lens the FL-102S is just like any other Vixen of the time
and is pretty much identical to contemporary Vixen achromats. That means a
large, but unusually lightweight tube with a fixed dewshield and a single-speed,
55mm-drawtube rack and pinion focuser in a cast body secured by screws.
Vixen
mechanical 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 FL-102S was an early doublet apochromat, discontinued more than a decade
ago. Does ‘early’ mean ‘worse’? Not in this case. The FL-102S uses a crystalline
fluorite rear element in a Japanese-made doublet of outstanding quality.
Contemporary
with the original FC-100, it’s not the same lens. The difference? Most
obviously the Vixen is an F8.8, not an F8 like the FC-100. It is a Steinheil
doublet like the FC-100, but was designed by a famous Japanese optical guru and
likely uses a different mating element. I’ve read that the FL-102S lens has an
even better reputation that the FC-100 in its homeland.
In
my experience, this reputation is justified. Visually, only a modern super-ED
triplet betters the FL-102S’ correction for chromatic aberrations and the high
fabrication quality means the FL-102S takes high powers to give peerless (for a
4”) views of the Moon and planets.
A
reader (Alex Shen) with an FL-102S and a TSA-102 (a super-ED triplet, see
above) has compared them extensively and reports similar sharpness and contrast
with only a trace more false colour over 300x from the FL-102S.
Why
did Vixen and others drop fluorite doublets if they were so good? Essentially
because the glass for the negative element contained heavy metals – not the
stuff of ‘Eco glass’. But in performance terms, the FL-102S is the equal of any
in all but its uncoated fluorite element.
The
Vixen FL-102S comes with a Vixen dovetail and was originally sold on the SP, 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 classic planetary scope, this would be
a good choice.
Vixen
later produced an F9 version with an updated focuser like the ED-102S’ above,
with a finder dovetail and a wider drawtube. I’m reliably informed that optical
performance is similar.
Note:
the F9 FL-102S was sold under the Orion brand in the US, whilst it seems the
F8.8 version was similarly re-branded to a Celestron.
Stock image.
Lens Design |
ED glass doublet (air spaced) |
Aperture |
103mm |
Focal Length |
795mm |
Focal Ratio |
F7.7 |
Length |
810mm |
Tube diameter |
115mm |
Weight |
3.6 Kg (5.4 Kg with rings, accs). |
The SD103S is Vixen’s current Everyman’s 4” refractor
– a basic ED doublet, in a lightweight fixed dew-shield tube, that’s suited to
both imagers and observers. Think of it as Vixen’s Takahashi FC-100DC.
The SD103S is closely related to the earlier F9
ED-102S, but has a slightly larger and faster objective at 103mm and F7.7.
What’s less obvious is that Vixen use ‘SD’ to mean premium FPL-53 ED glass, so
the SD103S is very well corrected according to Vixen’s own colour-line plots.
However, the ED-102S had a basic cell without
collimation screws and seemingly without a temperature compensation ring and
the SD103S objective looks similar.
The focuser is single-speed and looks a lot like a
Sky-Watcher’s, but has a wide drawtube for stability and a flexible visual back
to take various accessories.
The SD103S may be aimed at the successful FC-100DC’s
market, but it is a bit longer, fatter and heavier to go with its extra
aperture.
Despite its similarity to Synta’s
refractors, the SD103S is made in Japan, like a Takahashi. Overall build
quality is high.
For imaging, Vixen offer a flattener with a 44mm image
circle and a separate F6.1 (0.79x) reducer which can be used separately or
combined to give exceptionally small spot sizes, even at full frame. It’s an
approach previously adopted by Astro Physics, for example.
In many markets, the SD103S is almost as expensive as
the FC-100DC bare OTA, but tot up all the freebies you get from Vixen and value
looks good: the SD103S comes with rings, a plate, a carry-handle, Vixen’s
flip-mirror diagonal and an illuminated 7x50 finder.
Stock image.
Lens Design |
Four element air spaced |
Aperture |
103mm |
Focal Length |
825mm |
Focal Ratio |
F8 |
Length |
670mm |
Tube diameter |
115mm |
Weight |
6.4 Kg |
Now that the VSD100 has been discontinued, Vixen
describes the AX103S as its top-of-the-range imaging scope; but what’s so
special about it? It looks like another version of Vixen’s venerable 103mm ED
doublet refractor (and the ED102 before it), but it’s not. The AX103S is a
quadruplet. So like a Tak’ FSQ or Tele Vue Petzval then? Nope, try again.
Those Petzvals have a long-f
ED doublet up front with a reducer-flattener doublet at the back. Instead, the
AX103S has an air-spaced triplet objective with an ED centre element (like most
triplet APOs, such as the TSA-102 – see above). Note that Vixen describes the
centre element as ‘SD’, but this seems to refer to full-fat FPL-53.
So the AX103S’ objective is like other triplet ED
super-APOs. The unusual feature is the addition of a singlet flattener (not
reducer) in the focuser drawtube, like old Pentax imaging refractors.
This design might offer even better correction than a
Petzval because an F8 ED triplet should offer more scope for correcting violet
bloat than an ED doublet (even the F10 objective in an FSQ-106). It’s also
possible that vignetting is less of a problem. On the downside, having the
flattener in the focuser isn’t ideal - it could lead to miscollimation
if the drawtube sags in its bearings. And in this case the focuser looks more
like a standard cast Vixen item than the heavy-duty one in an FSQ or Tele Vue
NP.
The other elephant in the room for imaging is the F8
focal ratio. However, Vixen sells a dedicated reducer that drops it to a more
imaging-friendly F5.6.
The AX103S is much longer than an FSQ-106, but a bit
lighter to mount. It is also much cheaper than the Takahashi. I’ve seen
images that suggest outstanding correction and it could be better than the FSQ
for high-power visual use too.
Mounting the AX103S should be easy. The rings are a common
115mm diameter and are included, as is a Vixen plate. The AX103S should go
perfectly on a medium mount like a SkyWatcher HEQ5
(or Vixen’ own SX2) just as it comes. It also ships with a nice 7x50 finder and
dual speed focuser pinion.
Worth a look if you’re in the market for a premium
100mm imager, then? I reckon so…
Lens Design |
Oil spaced, ED glass triplet |
Aperture |
110mm |
Focal Length |
715mm |
Focal Ratio |
F6.5 |
Length |
575mm (553mm w/ visual back unthreaded) |
Tube diameter |
115mm (4.5”) |
Weight |
5.9Kg w/o rings |
It’s confusing. There’s TEC’s own ‘Eclipse’ 110
(above) and the widely available William Optics FLT110 F7 (below). What then is
this? Kinda both and neither. But read on, cos it’s a potentially interesting
member of that desirable-but-rare intersection of 4” apochromats and carry-on-portable
telescopes.
The tube and focuser are typical of William Optics
(i.e. Chinese) fare from twenty years back. So we have a heavy duty CNC-made
Crayford focuser with a 3” drawtube, a rotator and a flexible back threaded
onto a nicely finished tube with a sliding dew-shield and that questionable
gold lens ring. So far, so standard.
But also threaded onto that tube is a 110mm
oil-spaced triplet, made by the Telescope Engineering Company (TEC) – a
high-end bespoke maker of apochromats, based in the US and comparable with
Astro Physics.
So this is a budget Eclipse, then? Nope. The
objective is F6.5 (vs F5.6 for the Eclipse) and the centre element is ED glass
rather than the Eclipse’s fluorite. Furthermore, the focuser isn’t as good as
the Eclipse’s and the OTA is both heavier and longer.
Still, the TEC-lensed FLT110 is both
photographically fast and optically fine, is just about carry-on portable,
boasts a hefty and imaging-friendly focuser. It is one of the largest-aperture
carry-on scopes in this list and is much more available and likely cheaper than
the TEC Eclipse (or the AP Traveler for that matter).
It comes with CNC rings and a case (that
unfortunately isn’t carry-on sized).
The management summary is that the TEC-lenses
FLT-110 is very desirable – whether you’re after an imaging machine or a high-performance
portable visual scope.
Stock
image.
Lens Design |
ED glass triplet |
Aperture |
110mm |
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, see above. These are a very desirable scope,
with excellent optics.
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 triplet TSA-102 (discontinued), TMB 100/800 or
Stowaway. The doublet FS-102 isn’t far off and neither is the available-new
FC-100DZ.
· The new Takahashi FC-100 models (DC/DF) are highly
recommendable: compact and lightweight (the DC is carry-on portable with shield
and focuser removed) with superb do-anything Canon fluorite doublets that run
the DL/DZ versions close, even at high power. Both will cool more quickly than
a triplet for grab-and-go.
· For best buy value, it’s a tie between the Sky
Watcher 100ED and Takahashi’s easy-to-love FC-100DC.
· If you want the most portable, it’s got to be
the AP Stowaway or Traveler. Both are difficult to find. Borg’s larger FL
models easily fit in a carry-on bag. Takahashi’s FC-100DC is carry-on portable
with shield and focuser removed.
· The FSQ-106 remains highly rated by serious
imagers and may be the best dedicated imaging machine, but it’s pricey and
heavy.
· If you want a dual-purpose visual/imaging
scope with no accessory-swapping required, then the Tele Vue NP101 remains a
good all-rounder: optically close to the best, it has an excellent focuser, 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 Stowaway or 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. For the best
corrected lens in a 4”, a TMB 100/800 or Tak’ FC-100DZ (or Stowaway again) would
be your choice.