William Optics FLT-123 ( APM LZOS 123/738 ) Review

Many of my favourites scopes are four-inch refractors, because they are portable but capable. All things being equal, though, I would still prefer a five-inch APO. In average-good seeing, a five-inch aperture will often resolve the maximum available detail on the Moon and planets (~1 arcsec), whilst giving bright views and images of the deep sky. The problem of course is that all things are not equal: most five-inch refractors are too heavy to be grab-and go-portable, too big to carry on a plane. Bulk and weight increase rapidly with increasing aperture, as does focal ratio (and so length) to combat increasing levels of false colour.

Some of the few five-inch-class APOs to be really portable are based around one of the newest in LZOS’ range of air-spaced triplet lenses, the 123mm F6. Though the particular combination reviewed here - in a William Optics tube with a 3” Feathertouch focuser - is discontinued and rare, the LZOS 123/738 lens is available in various other OTAs (mainly from APM). So, in a way, the relevance of this review may be the lens more than the rest of the OTA. That’s why I’ve linked this review to both the telescope and the lens.

At A Glance


WO FLT-123



Focal Length


Focal Ratio



24” (21” focuser unplugged)




~7.5 Kg


Design and Build

The William Optics FLT-123 takes the best components from different sources and integrates them – Russian lens, far-eastern tube, American focuser. It’s an approach taken by many of the best telescope makers (even Takahashi don’t actually make their lenses, Canon do), but one that often creates a larger-than-necessary OTA. That’s not the case here, though – this is a surprisingly small and compact telescope for its aperture: check out the title photo.

The whole telescope has a well-integrated look and feel, too: everything threads together with no bulky adapters or protruding screws.

Three potentially cabin-baggage size refractors: FLT-123, AP Traveler and Takahashi FC-76DCU. Only the Traveler would go on board without some disassembly.


I’ve owned and reviewed a number of LZOS lenses over the years and found them to be the very best of the best in terms of fabrication quality, flatness of field and correction for chromatic aberration. So, who exactly are ‘LZOS’ anyway?

The acronym LZOS stands for “Lytkarino Zavod Optychisovo Sticklo”, which roughly translates to Lytkarino Optical Glass Works. LZOS was set up in Soviet times to make high-end optics for military and research purposes; it still does. LZOS also make and test some of the very largest professional telescopes. LZOS aren’t a little backstreet optical shop, they are an enormous, cutting-edge manufacturing facility that also developed the laser holography method of verifying huge professional mirrors.

As a sideline, LZOS makes refractor lenses from the ground up (they used to make the lenses for Zeiss telescopes). LZOS lenses are most frequently encountered in APM telescopes, but they have cropped up in other brands too, including Stellarvue and William Optics (the LZOS lensed version of the FLT-110 being the most common).

Part of the LZOS advantage is that they make the whole thing, including melting the glass. This means that they can tailor crowns and flints to implement a specific design very precisely – they are not dependent on existing glasses from the likes of Schott and Ohara, though they do have a catalogue of standard glass types. This also means they can make bigger lenses too, since off-the-shelf blanks often don’t exceed six inches (which is why the larger lenses from other companies like TEC and Takahashi use fluorite, which is available in larger sizes). LZOS can apparently make APOs up to 20” – now that’s what I’d do with a million or two spare!


This LZOS objective, like most of their others, is an air-spaced triplet, made from an OK4 crown sandwiched between two flints. OK4 is LZOS’ proprietary high-fluoride near-equivalent to Ohara’s commonly used FPL-53 ED glass. The glass is set into a substantial cell.

The aperture is (unsurprisingly) 123mm and the focal length 738mm (i.e. F6). This is an ambitiously short focal length for a lens of this size and gives it the potential to create a highly portable telescope, as well as a fast astrograph. The thick glass elements and hefty cell mean that it’s a heavy objective, however, at 2.8 Kg.

The question is whether this short focal length allows good correction for false colour (chromatic aberration), which is what apochromatic refractors are all about. By comparison, a conventional achromatic doublet would need to be F15 to achieve good correction at this aperture, whilst Takahashi’s outstanding fluorite-doublet FS-128 was F8. Tele Vue’s N127 is faster at F5, but is a quadruplet design that actually ends up substantially larger and heavier than this FLT-123 and still isn’t perfectly false colour free.

Note that the original specification for LZOS lenses is ‘better than 95% Strehl’, whilst this one is 98.8% Strehl – an unusually high value (esp. for an F6 of this aperture). You might have to pay extra to get this quality now.

The supplied LZOS test certificate showing 0.988 Strehl.


Some William Optics telescopes have had dodgy optics in the past, but you could never fault the quality of their tubes. In this case, the tube is just as good as it gets this side of Astro Physics and has much in common with other high-end Chinese OTAs from the likes of Skywatcher.

The lens and focuser thread-on, as does the dew-shield which slides smoothly and has a machined-in end-stop to protect the lens. The finish is a very attractive cream powder coat, though whether you like the goldy-lookin’ dew-shield ring and dew-cap is a matter of taste (I don’t).

Internally the tube has several knife-edge baffles and is painted matt black, but it isn’t as well flattened as a Takahashi tube, or as thoroughly baffled as an Astro Physics one.

With the focuser in place, the minimum tube length is 24”, but removing it shortens the tube to just 21” (or even 20” if you remove the dew shield) – that is just carry on portable, something rocking-horse-poo rare in a 5”-class APO. The FLT-123 is actually shorter than a Takahashi TSA-102 (and many other 4” refractors) and not much bulkier either (though a little heavier).

That weighty lens makes the OTA front-heavy, but it’s a yin and yang thing because it balances a heavy camera or binoviewer nicely.


The Starlight Instruments Feathertouch Focuser probably needs no introduction – it’s the best there is, apart perhaps from specialist imaging focusers from FLI or whoever. This one is their 3” diameter/3.5” travel drawtube model that is a rack-and-pinion type, not a Crayford. It has a dual-speed pinion with chunky metal knobs and its own lock, whilst the drawtube has a friction lock too. The drawtube has a scale on the outside for precise focus when imaging and is heavily baffled internally to kill stray light.

The Feathertouch is all machined from a hard, stainless steel and is designed to hold heavy eyepieces or larger cameras without image shift, sticking or sudden racking out. In practice, it has no trouble handling an EOS5D and the AFR-IV reducer, or a heavy Denkbinoviewer and a pair of Tele Vue Panoptics, remaining smooth and progressive and free of image shift. The focuser body has a large brass lock-knob on top which you only need with heavy gear; it is progressive and shift free.

Another nice feature of this larger Feathertouch is the rotator. The focuser body attaches to a simple rotating collar so you can easily change the angle of eyepiece or camera by adjusting the tension on three brass set screws. It’s not as sophisticated as the capstan-wheel rotator on the larger 3.5” focusers, but has the advantage that you can easily remove the whole focuser body, shortening the OTA for travel. Starlight sell a blanking plate to seal the tube if you do this.


This is not an unusually heavy 5” APO, but it’s heavier than its compact dimensions suggest. It will need a medium-sized mount.

My Vixen SX2 mount takes the FLT-123 just fine and is rock-steady with it, but it does need a 5Kg counterweight (less than most mounts because the motors are positioned as a balance), rather than the 2.5 Kg needed for, say, my AP Traveler.

Other medium sized mounts – HEQ5 and 6, smaller iOptron mounts etc – should take it fine, but it would overload an EQ5.

It comes with the usual CNC William Optics rings and a Vixen dovetail plate, but other plates should afix to the hole pattern, including Losmandy D-dovetails and Astro-Physics plates.

William Optics FLT-123 with dew-shield extended.


The WO FLT-123 comes with a nice, logoed semi-rigid case that is very well padded and protective, but is nowhere near carry-on portable. As explained elsewhere, you would need to remove the focuser (no threading required – just slacken off three set screws around the rotator) and get a different case.

Mine also came with a William Optics AFR-IV 0.8x reducer/flattener.

Attaching a finder would require an aftermarket shoe for the Feathertouch, but at this focal length finding things is easy with a low-power eyepiece.

In Use – Daytime

The FLT-123 gives outstandingly crisp and false-colour free daytime views, even at powers way beyond any spotting scope (120x plus). Chromatic aberration levels are very low, even on dark branches or feathers silhouetted against a bright sky. But it’s probably academic – this telescope is too heavy to be birding or spotting scope like an AP Traveler or TV-85 could be.

In Use – Astrophotography

As the full-frame unprocessed image of M42 below (in a slightly misty sky) shows, the field is flatter than you get with an all-spherical F6 triplet like the AP Traveler. The corners have slight darkening, but coverage is good. For APS-C chips you could probably get away without a flattener to start with.

The fast focal ratio mean you can take single frames for stacking (like this one) from just 30s exposure at ISO 1600, meaning your tracking doesn’t have to be perfect (I only eyeball-aligned the mount roughly for this shot).

The next shot was the same target and exposure, but using the William Optics AFR-IV field flattener. By reducer standards, this one is quite cheap, but it speeds the FLT-123 up from F6 to F4.8, flattens the field very nicely and only modestly increases violet blur. Note that for the FLT-123, the middle back-focus setting of 76mm on the reducer produced the flattest field (the back focus is adjustable from 66mm to 86mm). I have included 100% corner crops from both images to show the effect of the flattener.

My final image is the Moon, cropped but with no other post-processing. It’s sharp and very detailed, given the limitations of the small image scale.

Full-frame image of M42 with Canon EOS 5D and 100% corner crop

Full-frame image of M42 with Canon EOS 5D using WO AFR-IV 0.8x reducer, with 100% corner crop showing much tighter stars and good coverage.

Crop of gibbous Moon through FLT-123 with Fuji XM-1, otherwise unprocessed (no sharpening, contrast pushing etc).

In Use – The Night Sky

General Observing Notes

One big disadvantage of larger triplets is their weight - those kilos of glass cool slowly. The FLT-123 is still showing thermal effects in its star test an hour from a warm room. It may be grab-and-go portable, but cool-down is more like a big-scope.

The FLT-123 may look like a compact four-inch, but its almost-five-inch aperture reveals a lot more. In general, the Moon and planets show significantly more detail and deep sky objects are brighter and display more structure in their nebulosity than with a four-inch refractor of similar quality.

Due to its short focal ratio, the FLT-123 has more off-axis field curvature than an F8 triplet, more than an explicitly flat-field design like a TeleVue NP127 or Takahashi FSQ. However, compared with an F6 doublet or an all-spherical F6 triplet, field curvature is well-controlled, especially when using a really flat eyepiece like an Ethos.

Star Test

This is a 0.988 Strehl lens (see test report above), so you would expect (and do get) a near perfect star test. That’s most impressive for a triplet of this size and focal length.

The Moon

The view of the Moon is just incredible through this telescope. It is as good as the very best at this aperture (which is to say a Takahashi FS-128). What does that mean? In short, a much more beautiful, detailed and involving view than through smaller telescopes.

At 123x (surely an auspicious power for this scope) with a 6mm Ethos, almost whole Moon is visible and it’s just so sharp and full of fine detail, so full of false-colour-free contrast – a really breath-taking view that makes you want to linger. One advantage of the flatter-than-most (for F6) field is that the Moon is sharp right across, even when filling the field – one of the reasons it looks so good.

A seven-day-old Moon, still just on the crescent side of first quarter, is gorgeous. The grouping of Theophilus, Cyrillus and Catharina look wonderful, as does Rima Hyginus on the terminator; but it’s Mare Serenitatis that holds my attention. There is just so much you don’t normally see – wrinkle ridges and craterlets, fine gradations in the tone and hue of the lava plains. Bessel, lying alone in the Mare, is spectacular – picked out with stunning contrast, its wall shining in the rising sun, floor half filled with black shadow. Nearby Plinius with its two central craters, is a feast of detail too and in moments of quiet seeing, a craterlet (C) appears in Posidonius next to prominent A. The outstanding optics means no flare, no bleeding of light to spoil the absolute hard crispness of the terminator, Apennine peaks catching the sun.

I could go on … and I did, just enjoying and exploring when I was supposed to be testing.


For most of the 2016 Mars opposition period Mars was very low (17° altitude or less) here and so mostly sunk in poor seeing. However, just a week before opposition (with Mars at nearly 18 arcsecs) I did manage to catch come excellent seeing and quite a lot of albedo detail on Mars at 246x with a 2-4mm Nagler zoom on the 3mm setting. My sketch from that night shows Syrtis Major, darkening around the pole, Mare Tyrrhenum; bright Hellas and bright blueish limb cloud too.

The view of Mars with the FLT-123 on that occasion was just outstandingly good. I could see no difference between this scope and a typical LZOS F8 triplet on Mars, a most impressive result (Mars is often tough for fast APOs).

Much later – nine months after the opposition when Mars was a tiny 4” across – I got another really steady view. The FLT-123 easily took a magnification of 295X with a TV 2-4 zoom, showing a perfect orange disk with hints of surface markings and no flare or false colour. That said, at such a high mag the merest nudge on the microfocuser snaps it in.


As with Mars, the FLT-123 gave a perfect view of Jupiter, free of stray light, false colour or softness. Jupiter is a low contrast object, but the FLT-123 showed lots of belts and embedded dark storms – more detail than any typical portable scope.

Deep Sky

The fairly flat (despite its short focal ratio) and wide field of the FLT-123, along with its pin-point stars and large-ish (for a refractor) aperture, give wonderful deep sky views. With a 13mm Ethos, M42 shows colour and knots in the nebulosity, wide arms and central spike. Tested alongside a good four-inch the difference on M42 was very obvious.

Auriga’s Starfish and Whirlpool nebulae show their characteristic shapes and lots pin-sharp stars, some with colours. The Pleiades are fabulously brilliant jewels, embedded in wisps of nebulosity and velvety-black space. The crab nebula shows its shape and stands out brighter than through smaller APOs.

The FLT-123 FLT may have been designed with the deep sky imager in mind, but is one of the few faster APOs I have tested that is also really outstanding at high power on the Moon and planets. This is a genuinely do-it-all-superbly telescope.


The FLT-123 is both a fast astrograph and an uncompromised planetary scope in a (just about) carry-on-portable package. Considering its large aperture and short focal length, it controls aberrations remarkably well. It will take high powers as well as a premium F8 triplet like an LZOS 100/800 or a TSA-102. The F6 lens reveals just a trace more chromatic aberration on Venus than an LZOS 100/800, but normally it’s false-colour-free. Its field is not as flat as a TV NP 127’s, but it is sharper at high power and considerably more compact too. It is larger of aperture than the AP Traveler and a bit flatter of field, but it is a lot less portable and slower to cool. You can’t (quite) have it all.

Build quality of the tube and rings is very high and the Starlight Feathertouch focuser is just the best around. The William Optics tube is thoughtfully constructed to give plenty of in-focus for binoviewing or imaging. The LZOS 123/738 in this package is a strong contender for that ‘best all-round scope’ crown. The only real downside is the (justifiably) high price of LZOS lenses.

So, if I was forced to own just one single telescope, this would now be it – a small scope that pushes into the performance league of larger APOs and did a super job of every task I set it, from ultra-high power planetary viewing to effortless deep sky imaging.

The LZOS 123/738 is another very fine lens from the Russian company. In a premium package like this WO FLT-123 it makes an outstanding multi-purpose, portable APO and gets my highest recommendation.