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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 powerful, attributes larger and smaller refractors struggle to combine. The following is a guide to some of the more commonly encountered examples.

For this 2017 update, I have (rather arbitrarily) included more 90mm scopes.

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.

Table of Contents

 

APM/LZOS 100/800

APM/LZOS 105/650

Astro-Physics Traveller

Borg 90FL

Pentax 100 SDUF

Sky-Watcher 100ED

Sky Watcher Esprit 100 Triplet

Takahashi Sky-90

Takahashi FS-102N

Takahashi TSA-102S

Takahashi FSQ-106

Takahashi FC-100DC / DF

Takahashi FC-100DL

TEC 110FL (Eclipse)

Tele Vue NP-101

Tele Vue TV-102

Tele Vue Genesis / Renaissance

TS Photoline APO 90/600

Vixen ED102SS

Vixen 102FL

Vixen VSD100

William Optics FLT110

Summary

 

APM/LZOS 100/800

TMB1008

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).

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APM/LZOS 105/650

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.

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Astro-Physics Traveller

Traveler

Lens Design

ED glass triplet (oil spaced)

Aperture

105mm

Focal Length

610mm

Focal Ratio

F5.8

Length

Minimum 480mm (19”)

Tube diameter

108mm (4.3”)

Weight

4 Kg (5.6 Kg incl rings and plate)

 

The Traveller is legend. Introduced in the early Nineties and discontinued about ten years later, AP made less than a thousand I’ve heard. 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 and weighs about 4Kg. Its CNC tube is also slim (just over the bare 4” for most of its length, flaring towards the objective). Whilst being super small and light, the rest of the specs are top-drawer with lens quality amongst the very best, with zero 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 important to understand what you do (and don’t) get for your money. The view is as good as (but not better than) the very best telescopes 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.

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Borg 90FL

Borg90FL

 

Lens Design

Fluorite doublet

Aperture

90mm

Focal Length

500mm

Focal Ratio

F5.6

Length

~380mm (15”) without disassembly

Tube diameter

80mm (3.2”)

Weight

1.5 Kg in basic visual config’ shown

 

The Borg 90FL is a very interesting telescope, even though it’s not even quite a four-inch. Why? I’ll explain …

One of the smallest and lightest four-inch-class refractors, with a nice fast focal ratio for wide-field imaging, was Takahashi’s Sky-90. But the Sky-90 had a special lens design with a huge air-space and massive cell that was unfortunately prone to collimation issues – not good in a travel scope. This latest Borg 90FL replicates the Sky-90s advantageous numbers, but with a design more suited to travel.

Borgs have always been among the lightest, most portable and most adaptable of small refractors. Sadly, past Borgs have been a bit iffy in optical quality terms. Lately, though, they’ve been using premium Canon/Optron lenses made in Japan with a fluorite element, just like Takahashi.

The 90FL lens is a Fraunhofer doublet, using crystalline fluorite - just like the Sky-90 but with a smaller air-space. That means the fluorite is at the front, like older Takahashis. Personally, I liked the front-surface fluorite lenses and they do have a small theoretical advantage over the Steinheil design that puts the mineral at the back – shallower curves which are easier to make and slightly higher light throughput since the fluorite is what the light hits first (and fluorite scatters less light than any glass). Interestingly, from what I can see from a laser test, the 90FL still has a larger-than-usual air space, which would allow for better correction of both false colour and field curvature in this fast lens.

I’d expected semi-APO levels of chromatic aberration, due to the very fast focal ratio, but my early tests suggest it’s very well corrected. The 90FL has similar false colour to the Sky-90, despite that small air-gap, and less than many 80mm ED doublets. It also seems pin-sharp at higher powers.

The tube is Borg’s standard compact 80mm and the standard focuser will be a Borg helical, with or without a drawtube, but you could spec a Feathertouch if that’s more your thing. Borg make loads of options too, including a special tube assembly to turn it into a proper camera lens, a quadruplet 0.72x reducer is that shortens the focal length to 363mm at F4, and a 1.4x extender for larger image scales. For urban guerrilla astronomers, all Borgs are now a stealthy satin black (that shows every finger print).

Crucially, like the Sky-90, the Borg 90FL is one of the very smallest and lightest APOs in the four-inch class and is easily carry-on portable: 15” long and with a weight of 1.5 Kg for a complete OTA minus ring. Alternatively, the objective unit is easily removed and comes with protective caps; it’s only six inches long and weighs a kilo. So, if you’re sick of our murky skies and want to head off to do some imaging on Mt Teide, this could be just the ticket.

Borg make various tube rings and mounting brackets for their 80mm OTAs, but Tak’s 80mm tube rings fit too.

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Pentax 100 SDUF

Pentax official photo

 

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 is the premium offering with a very shiny CNC tube and a quality dual-speed rotating focuser.

The cheaper ED Pro is lighter, but less compact (it has a fixed dewshield) and is a more conventional Sky-Watcher OTA, albeit with a (different) dual-speed Crayford focuser that is smooth and shift-free for visual use, but doesn’t lock very well for astro-photography.

In either case, the lens is an ED doublet, apparently using top-quality Schott FPL-53 glass. Optical quality is likely to be good – typically diffraction-limited or a bit better. As with the TV-102, you shouldn’t expect this to perform quite as well as a fluorite doublet – its residual chromatic aberration is a notch down from that level. Nonetheless, my experiences suggest it’s a great all-round APO, with a flat field and low enough aberrations to give great views on all types of objects, low or high power, star fields or planets.

The Equinox 120ED has had some excellent reviews and I liked it a lot. Things seemed a little less rosy when a well-known tester found that it was really a 113mm scope … even so, it’s a good lens for the price. I’d read that the 100ED offers no better correction for chromatic aberration than the 120ED, but that’s not my experience. It may be that older versions used a cheaper ED glass. In any case, the 100ED in its latest form is a very sharp, low-CA APO with impressive performance for the price. Don’t tell SW, but the 100ED is a screaming bargain!

The Equinox comes with CNC rings that have an Astro-Physics hole pattern (!). The ED Pro has conventional cast SW rings with ¼-20 threads on the base.

Both ED Pro and Equinox versions come in a quality hard case and the Pro version may throw in accessories such as a finder and a reducer to take it down to F7.65. The reducer didn’t impress me, though. The standard 100ED has quite a flat field anyway and the reducer added quite a lot of violet bloat on O-A stars for less than one and a half F-stops.

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Sky Watcher Esprit 100 Triplet

http://ca.skywatcher.com/upfiles/670x435xen_telescope_caty01358550651.jpg.pagespeed.ic.0nO2xx3JXH.jpg

Official SkyWatcher image

Aperture

100mm

Lens design

ED glass triplet

Focal Length

550mm

Focal Ratio

F5.5

Length

~500mm

Tube diameter

105mm

Weight

6.3 Kg

 

The esprit 4” APO is a slightly puzzling telescope, at least to me. It’s a very fast triplet at F5.5, which suggests an ideal travel-to-dark-skies scope. But in fact, though it is just about carry-on compact, it is heavy and so will need a biggish mount for imaging, so isn’t ideal as a travel scope.

I’m not convinced that a very fast triplet is ideal for astrophotography at the 100mm size: compared to quadruplet astrographs the lens will be heavy and have severe curves, yet it’s not that fast and you’ll absolutely need a reducer/flattener (though the SW reducer is very inexpensive). Then again, at just over £1500 it is astonishingly good value.

I can’t comment on optical quality, but it’s likely to be excellent. I would be interested to see whether the optics work well visually; if they do it might make most sense as a dual-purpose scope (most fast quadruplets aren’t great for visual use). The tube is a traditional aluminium design with a long dew-shield; build quality looks very good indeed.

The Esprit range are fitted with heavy duty rotating crayford focusers that look similar to those found on SW’s other models; I notice some vendors are also supplying them with (aftermarket?) 3” Feathertouch focusers.

I haven’t tried an Esprit yet, but on paper at least I might be inclined to go for the 120mm model which isn’t that much more expensive and needs a similarly beefy mount. For really wide-field imaging or as a travel scope, I’d be looking for something even faster or lighter-weight (or both). Interestingly, SW themselves soon came out with a quintuplet design with a surprisingly similar focal ratio and a modest price increase. It’s a shame the triplet had to be so heavy – at ~4 Kg I’d probably have bought one.

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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 it was popular was that it combined miniscule size for easy transport with a decent aperture and a fast focal ratio for imaging. It was, in fact, ideal for many astronomers’ deep sky needs.

The Sky-90 was a front-surface fluorite doublet made by Canon/Optron, like Tak’s other FS refractors. But whereas they were F8, the Sky-90 was F5.6 – very fast for a doublet. Consequently, the Sky-90 had a special lens design with a huge air-space and massive cell that was designed to deliver the best possible correction for its fast focal length. Unfortunately, that cell proved prone to collimation issues – not good in a travel scope. It also made the Sky-90 surprisingly heavy for its diminutive size.

The Sky-90 had a short 95mm diameter tube with a sliding dew-shield. The focuser was a version of Tak’s classic cast focuser, but with a short body. When new this was a nice unit, very smooth and precise; but hanging heavy cameras off it could cause wear and image shift, especially as extension tubes were often needed due to the radically shortened OTA.

Even so, the Sky-90 was a handy little scope and a small amount of miscollimation was only noticeable at image scales greater than most imagers use anyway. Today you might consider the Borg 90FL instead – it’s essentially an updated version of the same scope, but with a more conventyional cell design and a much lighter OTA.

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Takahashi FS-102N

FS-102_2

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).

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 M6 holes at 35mm spacing.

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Takahashi TSA-102S

TSA102

Lens Design

ED glass triplet (air spaced)

Aperture

102mm

Focal Length

816mm

Focal Ratio

F8

Length

890mm/790mm (minimum ~ 650mm, 25”)

Tube diameter

114mm

Weight

5.4Kg

 

The TSA-102 replaced the FS-102. As with the previous scope, there are two versions: the TSA-102N with fixed dew-shield and the TSA-102S with a sliding one. The fixed version is about 4” shorter than the FS-102N, but the TSA-102S is much shorter still. Tak’s own claims are conservative: my TSA-102S will pack down to about 25” with the visual back removed and so is almost airline portable (same as the NP101 in fact). Weight is modest at a claimed 5.4Kg, but it still feels like quite a big and heavy scope compared to say a Traveller.

‘TSA’ stands for Triplet Super Apochromat, which is right because the TSA-102 has simply one of the best lenses in astro-world. The Japanese 102mm F8 (816mm F.L.) triplet is colour-free in any circumstances at any power, almost (but perhaps not quite) as perfectly free from chromatic aberration as the TMB 100/800. It doesn’t stop there though, because my TSA has a perfect start test and cools quickly for a triplet (probably due to careful design of the cell). Contrast is superb and the TSA-102 takes high-magnifications better than almost any other small refractor I have encountered.

The focuser is the home-grown 2.7” r&p unit from the FS-102, but again is very smooth, stable and virtually image-shift free. Build quality is again of the very highest.

The TSA-102, like other Taks, has a clamshell with two M6 holes at 35mm spacing.

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Takahashi FSQ-106

Stock image

Lens Design

Quadruplet Petzval with 2 ED elements

Aperture

106mm

Focal Length

530mm

Focal Ratio

F5

Length

580mm (23”)

Tube diameter

125mm

Weight

7Kg

 

Like the VSD100, the FSQ is a dedicated astrograph. On the face of it, it’s a very compact scope at just 23” long, but it is both chunky and heavy, with a massive dual-speed, rotating focuser.

The optical system, like the NP101, is a four-element Petzval. However, this one is even more sophisticated because it uses large air-spaces and two ED elements to achieve near perfect correction across a very wide field and an even faster F-ratio than the NP101 to boot, at F5.

I have heard from people who have been underwhelmed by FSQ’s visual performance, but perhaps that’s not surprising given its astrographic design. If you do want to use it for high-power planetary viewing, Takahashi sell a dedicated extender, tuned for the FSQ optical system, that sharpens it up. Collimation problems are also not unheard of (like most Petzvals it seems). A final thing to look for is signs of degradation of the front element which is soft ED glass and may be prone to damage if regularly left wet from dew.

The FSQ needs a bigger tube ring than other Tak 4” refractors and the weight of the FSQ takes it beyond most small GEMs, especially for imaging - you’d need an EQ6 (or better still Tak’s own superb-but-expensive EM200).

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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)

 

To describe Takahashi’s re-booted FC-100 I’ll have to delve into the optical design of Tak’s refractors for a few paragraphs. We’ve been here before, so please skip this bit if you’re bored.

Before the FS-102 (see above), Takahashi made a slightly different fluorite doublet four inch APO – the now legendary original FC-100. Takahashi re-introduced the FC-100 in 2013, but it’s actually a different design again. Confused? Read on …

Both the original FC-100 and the FS-102 that followed it were F8 air-spaced doublets made by Optron in Japan. However, their optical design was different. The FS-102 had its fluorite element at the front, making it a Fraunhofer type doublet (like most refractors). The original FC-100 was a Steinheil doublet with the fluorite positive convex element at the back. Urban myth would have it that the Steinheil lens of the FC-100 was better, but I doubt it. Here’s why:

·        Steinheil lenses in general have steeper curves (more difficult to make) and are “seldom used unless necessary” (Rutten and Van Venrooij).

·        In the FS-102, the front-surface fluorite confers a slight advantage in transmissivity because the fluorite is the first thing the incoming light hits and fluorite transmits more light than glass … any glass.

The only real advantage of the Steinheil, as far as I can tell, is that putting the fragile fluorite at the back makes for a more robust optic.

But for whatever reason, Takahashi have chosen to re-introduce the FC-100. However, the FC-100 is now a slightly faster F7.4 (good news for astrophotographers). The objective is still made by Canon/Optron in Japan, but it’s also likely to be a bit different behind the scenes. For one thing, the new lens will have a coated fluorite element for better transmissivity whereas the original FC-100 had an uncoated fluorite element. For another, it uses eco-glass for the flint element (the original’s was doubtless laced with heavy metals, like Lanthanum).

Thankfully that’s the optical theory out of the way. Mechanically, the new FC-100 is pretty conventional Takahashi, which is a good thing. There are two versions. The FC-100DC (shown here) uses a version of the small focuser from the FS-60 and has a fixed dew shield. It’s very compact and weighs just 2.8Kg. For purely visual use, this is probably the one you’d go for because it’s cheaper and lighter. I am also informed that if you remove the focuser and dew-shield it’s carry-on sized.

The more expensive version is the FC-100DF aimed at imagers (but just as good for visual as the DC). This one has a sliding dew shield to give a slightly shorter packed length and uses the larger focuser from the Sky-90 that should cope better with heavy cameras. The FC-100DF weighs a bit more though, at 3.5Kg.

But is the DF carry-on portable? Takahashi are terrible at quoting lengths for their telescopes – they almost always include the extension tube for straight-through viewing. From my own advanced calculations (measuring online pics with a ruler), I reckon the FC-100DF is just about airline portable when packed down, at about ~23”. If you have one and care to measure it, I’d love to hear from you!

The FC-100 I saw was another highly desirable Takahashi refractor: compact and beautiful. Build quality and cosmetics seem typical Takahashi: lime green castings, beautiful white tube and with the recent silver (rather than green or blue) lens ring. The focuser was smooth and gave minimal image shift. The lens sits in a proper temperature-compensated cell and the coatings were as good as you will ever (hardly) see.

I have used an FC-100 (the DC version) only briefly, but I have it on very good authority that optical performance is excellent – much like the older Tak’ fluorite doublets – with outstanding contrast and sharpness albeit with a tad more chromatic aberration than the FS-100, due to the shorter focal length and eco-glass flint. It’s another near-perfect small APO – great on the Moon and planets, but capable of stunning photography with a reducer or flattener.

The FC-100 has a 95mm diameter clamshell with two Tak-standard M6 holes at 35mm spacing, like the Sky-90 and old FS-78 (so clams for those scopes should also fit).

The light weight and short tube of the FC-100DC in particular mean it will work on almost any small mount, equatorial or alt-az. Vixen dovetail bars that fit Tak’ clamshells are available. A huge range of dedicated accessories, such as reducers and extenders are marketed for the FC-100.

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Takahashi FC-100DL

Stock Tak’ image

 

Lens Design

Fluorite doublet (Steinheil)

Aperture

100mm

Focal Length

900mm

Focal Ratio

F9

Length

940mm (782.5 w/o extensions??)

Tube diameter

95mm

Weight

3.8Kg

 

This is where your author gets a bit over-excited. You see no-one makes scopes for planetary observers anymore … and then Tak’ go and do just that, bless their little fluorite blanks.

The FC-100DL is a version of the newer fluorite-doublet FC-100 (see above) with a longer focal length of F9. What difference do those extra 1.5 F-stops make? They allow (so Tak’ claim) near super-APO (i.e. TSA 102) levels of false colour correction whilst still being a light, quick-cool doublet. Does this matter? For critical planetary observing, especially on Mars (many doublets are poorly corrected in the red) it might.

So the FC-100DL turns out to be long (near a metre) but nice and light at just 3.8 Kg. In other respects it’s like a cross between the other FC-100 versions – with the larger focuser of the DF but a fixed (I think!) dew-shield like the DC. The other radical (sarcasm alert) change is black enamel for the focuser.

Planetary observers in the North are in for a rough time over the next few years. Mars is about to have two close oppositions, but it’s barely over the horizon; Jupiter and Saturn aren’t great either. So if you’re looking for a lightweight (so easy to mount) planetary refractor with super-sharp optics that you can shove in the back of the car and drive to Southern Europe (where Mars makes it to a more reasonable 20-30 degrees altitude), this may be your scope.

Do I want one? Do you even have to ask?

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TEC 110FL (Eclipse)

Stock image

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 very 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 is 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. I almost paid their premium too … almost …

You can judge the healthy state of TEC’s order book from the fact that when I contacted them to ask when I could buy an Eclipse direct, they didn’t feel the need to reply.

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Tele Vue NP-101

Np101

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

 

Tele Vue have dropped the 102 recently. Why? Honestly, I suspect, because it just can’t compete with other slow ED doublet APOs from China on price. In most ways the TV-102 is just a scaled up TV-85: cream pebble finish tube; ‘mag’ wheels, slim clamshell; baffles-less, flocked interior; sliding dew-shield with screw-on metal cap. Recent versions may have a dual-speed focuser as standard and will certainly have a more rugged focuser with dual clamps, in comparison to older models that have a slimmer chrome draw-tube and single clamps. In all versions the long focal length means that the TV-102 is quite a long scope for a 4” refractor, yet very slim and also light for the class at just 4.5 Kg.

The lens is a 102mm aperture, 880mm F.L. (F8.6) air-spaced ED doublet. Now you might think that in comparison with the FS-102 “ED F8.6 doublet vs fluorite F8 doublet – there’s not going to be much in it.” Unfortunately nothing could be further from the truth. Whilst the FS-102 is effectively colour free, with one of the lowest residual chromatic aberration indices of them all, the TV-102 is described by optical-testing guru Wolfgang Rohr as “not quite an APO” – it has noticeable false colour in the star test. If critical imaging or high-magnification planetary viewing are your thing, this will make a difference.

If it sounds as if I’m damning the TV-102, far from it. In many ways it is a typical Tele Vue – high build quality with optical quality better than diffraction limited. It’s just not a premium fluorite doublet.

Owning the TV-102 will be easy - the design is simple and rugged and Tele Vue make numerous accessories. The clamshell fits straight on a Tele Vue Gibraltar alt-azimuth mount and TV sell a Vixen dovetail for an easy fit on most small GEMs.

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Tele Vue Genesis / Renaissance

Genesis2

Renaissance

This Genesis (top) is an F5 model, the Renaissance (bottom) F5.5.

Lens Design

Petzval: achro doublet objective + doublet Petzval lens (reducer-flattener).

Aperture

100mm

Focal Length

500-550mm depending on model

Focal Ratio

F5-5.5

Length

640-710mm

Tube diameter

102mm (4”)

Weight

~4.5Kg

 

The Genesis was Tele Vue’s forerunner to its current flat-field Petzval designs (see discussion of NP101 above). Like the NP101, the Genesis had a long focal length doublet up front with a field flattener/reducer at the back. The resulting focal length varied from model to model at between F5 and F5.5 – much the same as the NP101. Like the NP101, the field is very flat – great for both wide field viewing and imaging. And though it was clearly intended for low powers and wide fields, the optical quality was quite good enough for higher powers too. The focuser was a typically super-smooth Tele Vue single speed and the OTA very similar to modern TVs. So why pay big bucks for an NP101 when you can get a nice old Genesis for much less?

The big difference is that in the Genesis the front doublet was an ~F11 achromat, whereas the NP101’s is an ED apochromat. This means that the Genesis (even ‘SDF’ versions that contained fluorite in the field flattener) has a lot more false colour than the NP101 - the Genesis is effectively a semi-APO.

‘So, how much false colour?’ you ask. My experience is that the F5 version has a lot, though less than most (i.e. F5-F8) achromats. The F5.5 version seems quite a lot better: false colour only becomes noticeable visually above 100x, so it gives very decent views of the Moon and planets, works well as a terrestrial scope and could be used for imaging too (perhaps with a bit of post-processing or a semi-APO filter to remove the inevitable violet bloat).

The only other thing to consider is that you will need some very short eyepieces for high powers (4-6mm) and flat field eyepieces at lower powers to get the most from that super-flat field. If you’re thinking Tele Vue eyepieces, then of course that’s the point - eyepieces like Naglers or Panoptics (Ethos if you can get one). Incidentally, early versions shipped with the TV 26mm Plossl, which was a really gorgeous EP.

All in all, I really like the Genesis. Back in 1999, Ed Ting wrote in his review that it had a more friendly, relaxed feel than other scopes and I completely agree. There is something singularly lovely about a scope intended for the useless and unscientific activity of sweeping the Milky Way on a balmy August evening, surrounded by late summer flowers and the smell of cut grass. And if that’s your thing, there’s honestly nothing better. Thanks Al’. It will be a sad day when every scope on sale is a soullessly efficient imaging machine.

The Renaissance? Early examples are just a Genesis made of brass – brass tube, focuser drawtube, knobs and even set screws. My daughter thinks all that polished golden metal is a bit tacky; personally, I love it. Later, the Renaissance became a TV-102 under the skin and lost some of its charm.

Both Renaissance and Genesis have standard Tele Vue features, including clamshells that fit straight on a Tele Vue Gibraltar or Panoramic alt-azimuth mount, whilst TV sell a Vixen dovetail for an easy fit on most small GEMs.

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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

 

This is another very interesting 90mm APO, this time from the German company Teleskop Service (TS). Optically it’s an FPL-53 (Ohara’s best ED glass) triplet made in China or Taiwan with a focal ratio of F6.7. That may not sound all that exciting, but what makes this telescope worth considering is its outstanding optical quality. A renowned bench-tester found several examples to be completely free of false colour and of very high Strehl. This means the 90/600 is that rare thing – a scope that will do double duty as either a high power planetary scope or a fast astrograph when fitted with the Riccardi-designed F4.9 reducer.

If the 90/600 has noteworthy optics, the mechanical side is innovative too. Latest versions have a tube in four threaded sections for easy transport, or to provide loads of in-focus for imaging or binoviewing. Combine that with a 2.5” dual-speed CNC focuser and the 90/600 is a tempting travel scope, available either from TS, APM or other resellers (I’d love to review one – hint, hint, Wolfi or Marcus).

(Note that previous versions of this scope had single-section carbon tubes and/or other focusers.)

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Vixen ED102SS

Lens Design

ED glass doublet (air spaced)

Aperture

102mm

Focal Length

650mm

Focal Ratio

F6.5

Length

675mm (27”), minimum 550mm (22”)

Tube diameter

115mm (4.5”)

Weight

3.3Kg without rings

 

The Vixen ED102SS is quite a rare scope – the longer focal length ones are much more common. I’ve included it here because I was impressed with it – both in optical terms and because it is one of the lightest and most compact 4” APOs available this side of a Traveller.  The OTA is about 27” long, but remove that push-fit dewshield (no cross-threading risk like a Tak’s) and you’ve got a genuine carry-on portable 4” at just 22” long, which is a rarity. What’s more, despite being quite bulky (the 115mm tube is larger diameter than most Vixen/Synta 4” refractors), the Vixen is very light (the lightest in this Buyers’ Guide) at about 3.3Kg without rings.

At the time when the 102SS was in production (about a decade ago), most Vixen APOs were F8-9 fluorite doublets that compared well with Takahashis in optical terms. The 102SS was a departure - for a start it had a fast (F6.5) doublet and that doublet contained ED glass not fluorite. Such a fast focal ratio in a 4” ED doublet might make for chromatic aberration, but the one I saw was well corrected and took high powers to give very sharp, detailed and colour-free views of Jupiter.

The Mechanicals are typical Vixen. The 2.5” rack-and-pinion focuser is larger than similar-looking 2” units on other four-inch Vixen/Synta refractors, designed to be stable with a heavier camera. It is accurate and free from play, but not as smooth as the best. The light weight and standard Vixen dovetail plate make the 102SS an ideal fit on an EQ5 or GP, both of which handle it fine.

At the time these were available new I read some reports of poor optical quality, so be careful to test before you buy, but the one I tried was excellent. If you can find one and it tests well, snap it up!

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Vixen 102FL

Lens Design

Fluorite Steinheil doublet (air spaced)

Aperture

102mm

Focal Length

902mm

Focal Ratio

F9

Length

970mm (38”)

Tube diameter

115mm (4.5”)

Weight

3.6Kg without rings

 

In everything but its lens the 102FL is just like any other Vixen of the time and is pretty much identical to contemporary Vixen achromats. That means a large, but lightweight baffled tube with a fixed dewshield and a simple 2” rack and pinion focuser in a cast body that has widely been copied by Synta (Sky-Watcher and others). Vixen quality from this period was high, but not in the Takahashi league in my opinion. However, the FL series were expensive new and are highly regarded now. The reason is the lens.

The Vixen 102 FL was one of the early doublet apochromats and was discontinued a decade ago. Does ‘early’ mean ‘worse’? Not in this case. For the 102FL uses crystalline fluorite in a very high quality doublet made by Canon-Optron in Japan. A lot like the Takahashi FS-102, then? Well, yes, to a point. However, it’s not the same lens. In fact the Vixen lens is like the predecessor to the FS-102 – the FC-100. The difference? Most obviously the Vixen is an F9, not an F8 like the FS-102, but behind the scenes the Vixen uses another lens design altogether: it is a Steinheil doublet which puts the fluorite positive convex element at the back (‘normal’ doublets have the positive lens at the front). So which is better? Well urban myth would have it that the Steinheil is better, but I doubt it. Here’s why:

1)     Steinheil lenses in general have steeper curves (more difficult to make) and are “seldom used unless necessary” (Rutten and Van Venrooij).

2)     The Fluorite element is uncoated – not so good for transmission.

3)     In the FS-102, the front-surface fluorite confers a further advantage in transmissivity because the fluorite is the first thing the incoming light hits and fluorite transmits more light than glass … any glass.

I’ll leave the subject with a quote from the Takahashi manual:

“When Takahashi first designed the fluorite apochromat refractor, they realised that the optimum design placed the fluorite element in front. Coating technology of the time did not permit this to be done.”

Whichever design is really better, the Vixen FL lenses are excellent and a cut above a run-of-the-mill ED doublet. I have tried the 102 only briefly, but have tested some of the smaller FL models and found them to be among the finest small refractors. In particular you can expect a high-Strehl objective that is better than many triplets for chromatic aberration and is a particularly good choice for the Moon and planets.

Why did Vixen and others drop fluorite doublets if they were so good? Essentially because the glass for the negative element contained the heavy element Lanthanum – not the stuff of ‘Eco glass’.

The Vixen 102FL comes with a Vixen dovetail and was originally sold on the GP (or GPDX) mount, which handle the long but light-weight tube well. The optical quality of these was uniformly high, so buy with confidence (though obviously caveat emptor). If you are looking for a planetary scope on a budget, this would be a good choice. Apparently they also make great solar scopes (see photo above).

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Vixen VSD100

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 latest Vixen 4” APO is a telescope, but more than any other ‘telescope’ listed here, it’s really a dedicated camera lens. Yes you could put an eyepiece in the back, but even with the 1.58x extender it’s likely to be a lacklustre experience. In any case, if you are prepared to pay around £5000 for a 4” APO, throwing in an extra 10% for an SW 100ED for visual use shouldn’t break the bank, so why bother?

If my introduction sounds like I’m down on the VSD100, far from it. In fact it’s a very exciting telescope (sorry, lens). A super-fast (for a telescope) F3.8 focal ratio and the ability to throw its wide field flat across the widest chips, makes the VSD100 unique at present. I haven’t tried one, but the build quality on show at Astrofest was made-in-Japan impressive and (very) Pentax-like, with a massive and super-precise helical focuser that felt like it could handle the heaviest cameras with ease. The fact that it’s carry-on portable, even in its supplied hard case, makes the VSD100 even more desirable: easy to transport to really dark skies.

So what about the technicalities? Well, the VSD100 continues the astrographic arms race by adding an extra lens compared to established Petzval designs like the Takahashi FSQ. From the published diagrams, it seems to have the same wide-spaced doublet up front that the FSQs (and Sky90) employ to obtain the best possible correction. The key difference is that extra element, placed in the corrector at the back, which allows tighter stellar images across the whole field.

Is it a Pentax? There was talk before its release that the VSD100 was just a re-badged Pentax 100 SDUF, but it isn’t. The SDUF had four lens elements giving F4, the VSD five elements giving F3.8; the optical design looks generally different too, with the VSD 100 having a much larger air-space in the objective. Vixen make a big deal about specially annealing the glasses, checking everything on the latest Zygo interferometer and so forth, so I guess they must have taken the Pentax telescope works over when Pentax ceased production, but have since updated the design.

So should you buy one? The VSD100 seems expensive when compared against the other 4” APOs in this buyers’ guide, but as I said it’s not really a telescope. So if instead we compare it against similarly specc’d camera lenses, then the value looks good. Take the closely comparable Canon  EF 400mm F4 USM lens as an example. The optical spec’ is almost identical and though the Canon lens adds photo-specific features like image stabilisation, it retails at a whopping £7000! The Vixen certainly looks beautifully Japanese made and a lot of care seems to have gone into its design and manufacture, so if wide-field imaging is your things it’s probably worth it.

A brief warning for any wealthy newcomers to astronomy reading this: dedicated astrographs are usually very compromised for visual use. So please don’t buy this thinking it’s a luxury visual refractor - you will be disappointed!

Finally, is it worth paying for a dedicated fast astrograph like the VSD100, when you could achieve something similar with a cheaper, slower scope plus a reducer? Honestly, I am not sure. Reducers have improved. A built-in reducer is certainly convenient, but a quality bolt-on (from Takahashi, Borg, AP etc) can do a really fine job without compromising the scope for visual use. Meanwhile, though fast reducers are typically expensive, some companies make much cheaper flatteners that can still work well.

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William Optics FLT110

Stock image

Lens Design

ED glass triplet

Aperture

102mm

Focal Length

770mm

Focal Ratio

F7

Length

625mm (25”)

Tube diameter

115mm (4.5”)

Weight

6Kg

 

The FLT110 is certainly a beautiful thing to behold, with superb mechanical engineering and that white-gold finish. There are various models, but all have large two-speed imaging focusers and quality CNC tubes. However, the triplet and all that CNC do make for quite a heavy OTA.

Early FLT110s have a premium lens made by TEC (the Telescope Engineering Company), which gives them the best of both worlds – superb CNC tube and focuser with one of the finest APO lenses. These FLT110s are a very desirable 4” APO.

Unfortunately, later models with WOs 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 TSA-102 or the TMB 100/800. In terms of available scopes, the new FC-100DL may run them very close (so does the FS-102, but it’s getting rarer on the used market now).

·        For sheer value, the Sky Watcher 100ED wins hand down, at least in its latest versions. It’s not very portable, but is light, easy to mount and performs astonishingly well for its price. It’s an easy Best Buy for the rest of us who don’t want to spend thousands on a small APO.

·        If you want the most portable, it’s got to be the AP Traveller or TEC 110. Both are out of production at the time of writing and will be hard to find and expensive. The new Takahashi FC-100 models (DC/DF) are compact, lightweight and available (and the DF may be carry-on portable). If you’re willing to compromise on aperture, the new Borg 90FL looks very interesting.

·        The best native astrograph may well now be the new Vixen VSD100, with its massive helical focuser and innovative optical system that delivers such a flat, wide field across the biggest chips, but the Vixen probably won’t be all that good in a multi-purpose role: it’s a dedicated astrograph. Otherwise, the FSQ-106 is very highly rated by serious imagers.

·        If you want a dual-purpose visual/imaging scope with no accessory-swapping required, then the Tele Vue NP101 remains a good all-rounder: it comes close to the best in optical terms, has an excellent focuser in the ‘i.s.’ version, is light-weight and comes with a standard fast, flat field for imagers. Unfortunately it isn’t definitively carry-on portable.

Buy a Traveller for ultimate portability, a VSD100 or FSQ-106 for imaging, but on a sensible budget go for an FC-100D, or an old FS-102 if you can find one. On a tighter budget, the Sky Watcher 100ED is unbeatable value. As an all-rounder, the NP-101 still makes a wonderful ‘only telescope’.

 

 

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