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 most, 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.

APM/LZOS 100/800

APM/LZOS 105/650

Astro-Physics Stowaway 92mm F5 EDFS

Astro-Physics Stowaway 92mm F6.6 EDFS

Astro-Physics Stowaway 92mm F6.6 – New version (2017-24)

Astro-Physics Traveller

Astro-Physics Starfire 110 GTX

Baader Travel Companion

Sky Watcher Esprit 100 Triplet

Takahashi Sky-90

Takahashi FC-100 Classic

Takahashi FS-102N

Takahashi TSA-102S

Takahashi FSQ-106

Takahashi FC-100DC / DF

Tele Vue Genesis (non-SDF versions)

Tele Vue Renaissance

Tele Vue Genesis SDF

TS Photoline APO 90/600

William Optics FLT110 TEC

William Optics FLT110

Summary

APM/LZOS 100/800

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, but the same objective may also be found with a cheaper (but still good) Chinese FT rip-off focuser.

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 Feather Touch 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!).

The most 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. Mine had the smallest 2” Crayford, which was ideal for visual use.

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 4” refractor I have tested (just – see TSA-102 below) – 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).

Right now (late 2024) this lens no longer seems available new and I suspect we won’t be seeing any more.

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APM/LZOS 105/650

TMB 105/650 with a Takahashi Sky-90. Image Credit w/ thanks: Richard.

Imaging version by Officina Stellare with motorized 3.5” focuser.

Lens Design	ED glass (OK4) Triplet
Aperture	105mm
Focal Length	650mm
Focal Ratio	F6.2
Length	Minimum ~18”
Tube diameter	135mm
Weight	5-10Kg, depending on tube

This is a sister lens to the 100/800, i.e. it’s a Russian-made (by LZOS) air-spaced triplet with an OK4 ED centre element.

Much the same comments apply as to the 100/800 – the same kinds of tubes and focuser options from APM are available. The lightweight APM tube you see above – with a 2” Feather Touch focuser on a drawtube - is the most compact option and easily carry-on portable (only one of a handful of >4” refractors that are). At the other extreme, you can get imaging versions (including the Officina Stellare version above) with the big Starlight FT 3.5” focuser that are much bigger and heavier.

This is a heavy lens, 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. Despite its relatively fast focal ratio, the 105/650 has a reputation for excellent correction (after all, even the 123mm F6 LZOS lens is well corrected).

The same comments about availability apply as for the 100/800.

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Astro-Physics Stowaway 92mm F5 EDFS

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 as short as the scope!

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

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Astro-Physics Stowaway 92mm F6.6 EDFS

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.

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Astro-Physics Stowaway 92mm F6.6 – New version (2017-24)

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 2^nd 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.

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Astro-Physics Traveller

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.

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Astro-Physics Starfire 110 GTX

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 3^rd party focuser – a 2.5” Feathertouch), the 110 GTX has an AP focuser. But why? I suspect no 3^rd 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.

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Baader Travel Companion

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. But who might sell you such a thing? Let’s recap …

From the premium brands your options are limited and all the following contenders are long discontinued. The LZOS 105/650 in the lightweight tube only. The Sky-90 is getting rarer, has its problems, but still isn’t cheap. TEC gave up with the desirable the Eclipse 110 after just a few hundred. The LOMO 95 is as rare (rarer actually) than the proverbial rocking horse dung. It’s been almost a quarter of a century since you could buy a Traveler from AP.

If you want to buy new, only the Borg 90FL and 107FL are still available, but may be hard to find. The new AP 110 GTX is theoretically available (good luck in the draw), but it’s not cheap. 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 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. At last you can go out and just buy a travel APO! Excellent! Well no …

It’s not like there’s no market for travel APOs, as Baader discovered. Despite a steepish asking price of around 4500 Euros, despite the Steeltrack (they can upgrade you to a Feather Touch for a thousand Euros more), 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. The Travel Companion’s 560mm focal length is at the carry-on limit, so F5.9 is about the maximum for a 95mm travel scope. And though a well corrected 60mm F5.9 is easy enough (Tak’ FS-60), by 95mm it means steep curves in the glass and expensive materials like fluorite.

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. Or keep waiting for Baader to do another run (they’ll add you to the waitlist). Does this just seem like AP all over again?

Update Oct 2024: Baader tweeted that it’s going to be ‘available’ again from next year …

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Borg 90FL

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.

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Borg 107FL

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.

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LOMO 95/650

LOMO 95/650 with Moonlite focuser next to a Sky-90 for scale. Image credit w/ thanks: Richard.

Lens Design	ED triplet
Aperture	95mm
Focal Length	650mm
Focal Ratio	F6.8
Length	~500mm (depends on focuser etc)
Tube diameter	Varies
Weight	~3 Kg

LOMO? Who? You might not be familiar with the acronym if you’re an astronomer, but probably will be if you’re a photographer. That’s because LOMO made a basic little 35mm camera that was widely used across the former USSR and still has a following today (Google ‘Lomography’). So LOMO was and is a Russian optics maker like LZOS, founded in Leningrad a century ago, that makes military, research and consumer optics.

If you want an LZOS-quality objective in this size range, this is it: LZOS didn’t make anything below 100mm AFAIK. So, like LZOS, a LOMO telescope basically meant getting your lens fabricated by the Russian version of a NASA subcontractor. So they’re good then? Oh yes.

The specs describe it as a ‘Super Apochromat Triplet’ and if I understand the specs correctly it is well corrected across the whole visual spectrum. The 96/650 in the image above was certified at 1/10^th wave PV which is as good as it gets (1/4 PV is ‘diffraction limited’ and what you can expect from most good objectives/mirrors) and the included interferogram looks perfect.

Trouble is – unlike LZOS - LOMO didn’t make many or for very long. Legend has it that they made less than a hundred 95/650 objectives (more of the smaller 80/480). So this lens is very rare now. APM even claim they never retailed them (they did, cos I remember agonising over their website deciding whether to buy one circa 2007).

What I’m getting around to saying is that I’ve never tried one and only seen one (above) – in a very basic tube from a US maker I’d never heard of. But I’m pretty sure there are APM-tubed examples out there, probably branded ‘TMB’. If the smaller 80mm models are anything to go by it should be an outstanding optic, but probably a fairly heavy one.

If anyone out there has more info to share, please get in touch!

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Pentax 100 SDUF

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.

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Sky-Watcher 100ED

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.

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Sky Watcher Esprit 100 Triplet

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.

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Takahashi Sky-90

Sky90

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.

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Takahashi FC-100 Classic

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, their optical design was different. The FS-102 was a Fraunhofer doublet with its fluorite crown element at the front, but the original FC-100 was a Steinheil with the fluorite positive convex element at the back. Takahashi did this because at the time fluorite couldn’t be coated and was potentially fragile, so putting it at the back made sense. That fluorite crown was partnered with a 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 for high power planetary viewing. Whilst I can’t speak for the earlier models, the F-lens is certainly an excellent optic. It has a near-perfect star test and very little false colour. 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 runs it very close for visual. The main area it lags the newer FC-100D models is for imaging.

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 M8 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.

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Takahashi FS-102N

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 over 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 an 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 for visual, with quicker cool-down and (perhaps) a tiny contrast and brightness advantage. Compared with the F7.4 FC-100D that replaced it, the FS-102 has a small advantage for visual at high powers, but the FC-100D is better for imaging.

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 1/6^th 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: since 2019, Takahashi have produced the F8 FC-100DZ. It’s smaller and lighter than the FS-102 but is every bit as good optically, so don’t pay crazy money for an FS-102. Details below.

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Takahashi TSA-102S

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.

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Takahashi FSQ-106

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.

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Takahashi FC-100DC / DF

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).

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Takahashi FC-100DL

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

The FC-100DL was 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 …

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Takahashi 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 50^th 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.

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TEC 110FL (Eclipse)

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.

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Tele Vue NP-101

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).

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Tele Vue TV-102

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.

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Tele Vue Genesis (non-SDF versions)

Early F5 Genesis.

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.

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Tele Vue Renaissance

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.

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Tele Vue Genesis SDF

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.

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TS Photoline APO 90/600

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.

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Vixen VSD100

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? I originally got this one wrong because I guessed not! But in the blurb for the replacement VSD90SS, Vixen refer to ‘the parent Pentax 100SDUFII’, so yes they seem to have bought the IP and maybe the tooling from Pentax.

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.

2024 Update: The VSD has been discontinued and replaced by the VSD90SS with a conventional focuser and a more modest F5.5 – see below. This is probably because the VSD100’s rear element group wasn’t wide enough for good coverage on larger sensors.

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Vixen VSD90SS

Stock image.

Lens Design	Five elements air spaced
Aperture	90mm
Focal Length	495mm
Focal Ratio	F5.5
Length	597mm (403mm dewshield stowed)
Tube diameter	115mm
Weight	4.3 Kg

The VSD100 (above) has been discontinued in favour of something smaller and seemingly less radical but equally expensive: this, the VSD90SS. Still, the 90SS has very impressive specs beyond the headline 90/495 (F5.5) and is probably the best small imaging APO right now. Read on to find out why.

The five-element Petzval optical train is one of the most advanced I’ve come across. It contains two premium ED (called SD by Vixen, but probably FPL-53 or even FPL-55) crown elements, one ED flint (like a Tak’ FOA-60 for example) and another element of special Lanthanum glass!

All five elements feature very large air gaps for even better correction, despite which Vixen say this scope works for high power visual use too. This is noteworthy because most camera lenses are built this way but absolutely wouldn’t. That’s because big air gaps make it difficult to achieve really high overall optical quality – lots of camera lenses suffer from decentring.

All that optical complexity aims to produce the best axial star images possible for imaging – minimal astigmatism, false colour and vignetting (i.e. brightness reduction in the outer field) across a wide image circle for big sensors. All these things are difficult with a Petzval and the 90SS has a large rear lens group to achieve it (much larger than the VSD100). Apparently they had to ditch the Pentax-derived helical from the VSD100 for a conventional rack-and-pinion focuser to squeeze it in!

If their published specs are true, the VS90SS achieves an almost-miraculous 90% illumination across a 60mm image circle (i.e. medium format, never mind full frame which is 44mm), although this drops when using the 0.79x reducer. Meanwhile, centre field it’s supposed to have a poly-chromatic Strehl of 96.7% - a very high value that promises minimal violet bloat and super-sharp planetary views. Wow!

I’m starting to understand why the 90SS is so expensive. As above, compare the closely comparable Canon EF 400mm F4 USM lens. The optical spec’ is almost identical and though the Canon lens adds photo-specific features like image stabilisation, it retails at a whopping £7000 (and certainly won’t be usable for viewing the planets as well!)

Mechanically, it looks good too. The VSD90SS has focuser designed for heavy loads with a lever lock and is compatible with 3^rd party motor focusers (though the micro-focus is still an accessory). Meanwhile, it is small and quite light, so should be easy to track and guide accurately on a medium mount. The dew-shield packs down over the tube for easy storage and transport.

I started off cynical, but now I want to try one of these things with medium format!

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Vixen ED-102S

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.

Note: a kind reader emailed in with a pic of his ED103S, which actually looks like an early version of the SD103S, with its 103/795 optics.

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Vixen ED-102SS

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!

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Vixen FL-102S

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.

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Vixen SD103S

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.

Note: Vixen once made an ED103S, packaged on the Sphinx mount, which actually looks like an early version of the SD103S and with the same 103/795 optical spec, not the 102/920 of the ED102.

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Vixen AX103S

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…

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William Optics FLT110 TEC

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. It looks like a big Megrez 90. 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 WO.

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 just an Eclipse objective in a cheapo tube, 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. Still, the TEC-lensed FLT110 is both photographically fast and optically fine, boasts a hefty and imaging-friendly focuser and may just about be carry-on portable, though you’d have to get a very snug-fitting soft case for it to be so.

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.

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William Optics FLT110

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.

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Summary

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. Fluorite doublets - FS-102, FC-100DL and more recent FC-100DZ - run them very close, are lighter and cool faster.

· The Takahashi FC-100DC/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.

· 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 would be your choice.