It’s easy to tell when I’m reviewing a Borg. My desk is littered with small circular objects of mysterious function and I look perpetually puzzled. Borg is Meccano for scope nerds, but that means this review is all part numbers and metric thread sizes.

Borgs have always had the twin advantages of modularity and high quality, lightweight components. But now Borgs also use premium quality fluorite (yes, proper mineral fluorite, not high-fluoride glass) objectives from Canon/Optron.

That would be exciting enough, but this 90FL boldly re-creates the optical spec’ of one of the first small, fast multi-purpose apochromat astrographs – the now discontinued Takahashi Sky-90.

Compared with the Sky-90, the Borg 90FL has even more accessories and options that make it ideal for the imager, including a stunning new F4 reducer. But does it still have the Sky-90s Achilles’-heal - its massive, collimation-sensitive lens cell? Let’s find out.

At A Glance


Borg 90FL



Focal Length

500mm (360mm with reducer)

Focal Ratio

F 5.6 (F4 with 0.72x reducer)


~390mm (15.5”) imaging, 470mm (18.5”) visual


~1.5 Kg as Shown

 Data from Borg/My own.

What’s in the Box?

Answer: lots of little boxes! The 90FL was sold as a package, but like any Borg what you actually get is lots of tiny boxes for all those connectors and adapters.


Design and Build

There are so many tube options for the 90FL that it’s hard to write a definitive review: various focusers and tube components can be combined in so many different ways.

The version on test here is a compact, 80mm tube version with one of the smaller helical focusers and no drawtube. It is primarily an astrograph, but with an eyepiece holder slotted in, it works for visual too. It’s preposterously light for a 90mm refractor: just 1.5 Kg - half the weight of the Sky-90.

You want a complete component list? Oh alright then:

2591 – 90FL objective unit

7101 – 150mm long 80mm diameter tube

7801 – M77.6 to M68.8 adapter

7835 – M68.8 helical focuser

7352 – M57 rotating ring

7872 – 0.72x reducer

This 90FL set was sold as a complete set, but it isn’t ready for use: for astrophotography you would need to add a T-ring and M57 adapter; for visual an M57 eyepiece holder.


The 90FL objective is made by Canon/Optron, just like Takahashi’s objectives. Its specs are identical to the old Takahashi Sky-90, at 90mm aperture and 500mm focal length, giving F5.6. It is also, just like the Sky-90, a front-surface fluorite design; in other words, it’s a Fraunhofer doublet with the fluorite crown at the front. I checked this with a laser as usual – you can see the laser disappear in the fluorite because the mineral (fluorite is crystalline, not a glass) scatters less light than any glass.

However, the laser test reveals a key difference from the Sky-90. The Sky-90’s objective had a very large air space (13mm) between the elements to reduce aberrations, but it made for a huge, adjustable cell and collimation problems. The 90FL still has an air-gap to correct aberrations better than a foil-spaced doublet; but the air-gap is modest, the cell a conventional size and non-adjustable.

The objective has some of the best coatings I have seen and, like all front-surface fluorite lenses, appears particularly transparent (why do you think coatings contain fluorides?)

The glass and fluorite elements sit in an objective ‘unit’ that incorporates a sliding dew-shield. Again, it’s a light-weight but classy piece of Japanese engineering that slides with just the right weight and clicks into place. The objective unit is 15cm long and weighs a kilo, terminates in a male M75 P1.0 thread (I think).

At the front of the lens unit is a curious dew-cap. It threads-off like a Tele Vue cap (fiddly in the dark with numb fingers). But Borg have added an inset plastic plug. You could just take out the plug, but doing so would stop the lens down to 80mm. I am informed that this is to accommodate 82mm solar filters.



There are endless tube options and everything threads together. But this can get tricky. Why, for example, do the main tubes have to have different threads at either end (M75 at one end, M77.6 at the other) and different again from the focuser? Yes, Borgs are modular like no other telescope, but you need adapters (sometimes several stacked) to connect things up. Borg adapters are quite expensive (£25-£40 each) and costs mount quickly.

The OTA I bought ended unhelpfully in an M57 female thread: I was immediately buying more adapters in order to connect a camera or eyepiece. Usefully included, though, was a very compact camera rotator ring.

The 90FL as shown is an imaging setup with a 150mm main tube. The UK dealer recommends a shorter main tube for visual use, but I found that swapping in a long 2” eyepiece holder (part # 7509) in place of the reducer (along with a 1.25” diagonal) gave a perfect focus range for most 1.25” eyepieces. For 2” eyepieces you would really need to add in a drawtube for more focus travel.

The 90FL is carry-on-size portable in any configuration, imaging or visual. But of course you can just take it apart to go in an even more compact bag.

Borgs now come only in satin black, which is attractive and stealthy (possibly a serious advantage for urban astronomers) but shows every mark and print.

Borg components tend to have micro-ridge baffles machined in for stray light suppression and everything is painted flat black inside. But they also supply the main tube with a large piece of flocking material to line it with – nice touch.


If you Google images of a particular Borg model, you’ll find that every example looks different. This is because you can choose between numerous Borg focusers and then put them in different places in the OTA. Most Borg focusers are helical, but they make a Crayford and a rack-and-pinion too and you can get adapters (are you sick of that word yet?) to fit a Feathertouch.

The focuser in this setup is the ‘standard’ 68.8mm thread helical focuser for the 80mm tube set, but you could fit others – larger or smaller – if you prefer.

I really like helical focusers and this one works well – up to a point. But it gets a bit stiff and sticky with larger loads and travel isn’t enough at just 20mm. It does have a lock screw for imaging which produces little or no image shift.

However, a warning for imagers: I found that this focuser struggled with a heavier camera. I would recommend the larger (75mm thread) focuser for serious imaging.

90FL in imaging configuration

90FL in visual configuration


The 90FL is in the very smallest and lightest class of APOs: little bigger than an FS-60 and much smaller than most 90mm refractors. Consequently, it will go on any mount and hardly taxes the little TakTeegul, which keeps plenty of counterweight travel in reserve for accessories. The Teegul was really designed for 60mm-class scopes, making the point about how lightweight the 90FL is.

Being able to use the smallest mounts is a major advantage for imagers (and observers) who travel with their scope.


In a sense, any Borg is all accessory. Enough said. Go and take a look at the Borg website to understand the huge range of options.

The most significant accessory – included with this set as standard – is a 0.72x reducer (part 7872). It reduces the focal length to just 360mm (F4) and is a seriously premium piece of glass (well four pieces – it’s a quadruplet). The reducer is dedicated to the 90FL and 107FL and threads into the focuser.

My 90FL package came with another useful accessory, a rotator: just a thin friction ring with three set screws, I really like it for either visual or imaging.

Dedicated 0.72x quadruplet reducer for Borg 90FL

In Use – Daytime

I noted minor false colour when focusing through silhouetted branches at 100x with a 5mm Nagler, but little in-focus.

The view remained perfectly sharp at 143x with a 3.5mm Nagler.

With the 2mm setting on a Nagler 2-4mm zoom, giving a ridiculous 250x, the view was dim but still sharp, suggesting excellent optical quality.

The 90FL plus reducer makes an excellent telephoto lens, sharp, fast and flat.

Bay sands – Fuji APSC 1/2500 ISO 1000, Borg 90FL at F4 with reducer.

In Use – Astrophotography

As you would expect at F5.6, the field is quite curved at full frame without a reducer (zoom in on the sample star field below).

With the 0.72x reducer, I didn’t find the field quite as well corrected as I was expecting, with slight distortion of stars towards the edge. Only later (sadly, after I'd returned it) did I find out why: the reducer as delivered and shown is configured for the 71FL! For the 90FL, the body needs to be dissasembled and the smallest spacer removed.

Off-axis darkening is a problem with Petzval astrographs like Takahashi’s FSQs, but the Borg 90FL/0.72x reducer combo’ avoids this to give excellent coverage at full-frame.

At 360mm F.L. with the reducer, the field is super-wide. F4 means short exposures too: I was amazed to find the Flame nebula bright and clear in an exposure of just 30s at ISO 1600. See for yourself in the (totally unprocessed as usual) single frame below, taken in slight haze. Note that enormous field of view.

The 90FL’s natively short focal length works against it for prime focus snaps of the Moon, but cropped they are clear, sharp and contrasty nonetheless.

M36 – full frame Canon EOS 5D 30s ISO 1600, Borg 90FL F5.6 (no reducer).

Flame Nebula - full frame Canon EOS 5D 30s ISO 1600, Borg 90FL at F4 (0.72x reducer)

Crop of Moon at Dawn - Fuji APSC, Borg 90FL at F5.6

In Use – The Night Sky

General Observing Notes

It’s a minor annoyance, but the dew cap is one. It’s tedious to unscrew and dropping it would be all too easy. Then you get it off and the push-fit central section falls out ‘cos it contracts when it’s cold.

I was worried that a smaller objective air gap might mean more aberrations than the Sky-90, but that wasn’t apparent to me: clearly technology has moved on. In general, the Borg 90FL makes a much nicer general purpose visual scope than you might expect from an astrograph. But you will need eyepieces that work well with fast focal ratios (like most from Tele Vue).

Cool Down

Cool-down is super-fast. It’s usable almost at once. Another big advantage in a quick-look or travel scope.

Star Test

The star test is good, probably better than 1/4 PV. There was just a hint of colour on Rigel in the star test.

The Moon

The 90FL gave a really good view of the Moon with no false colour that I could find, even on the bright limb. Contrast was outstanding, as usual for fluorite doublets. The Borg delivered lots of lunar detail that most scopes of its diminutive stature wouldn’t.


A perfect dazzling crescent at 100x with a 5mm Nagler and virtually free of false colour in focus, but with a modest amount of false colour – proper green and purple – either side: a bit more than you get with, say, an FC-76 (a longer focus fluorite doublet, also made by Optron). By 142x you can’t focus the false colour away and the bright planet does shed a little stray light, but this is a seriously harsh test of an F5.6 doublet.


Mars was just 4.6” across, weeks before disappearing behind the Sun. Nonetheless, at 143x it showed up as a perfect tiny red disk through the 90FL, without the softness and stray light I recall the Sky-90 generating on the Red Planet.


Though low in the west, Jupiter showed a surprising amount of detail – significantly more than with the 3” refractor I had alongside. At 143x with a 3.5mm Nagler, I could make out various cloud belts and the polar hood and the GRS just coming around the limb. The view was perfectly sharp and free from stray light or false colour – comparison with a longer focus FC-76 at the same magnification were favourable (whereas the Sky-90 underperformed the FS-78 in a similar test).

Deep Sky

The Pleiades looked wonderful, with only a small amount of curvature showing on stars near the edge and no appreciable astigmatism or coma.

M42 looked brighter and with more direct vision structure than through smaller scopes (60-80mm).

Rigel was an easy split at 100x, with the companion well outside the glare from Rigel A: more evidence the 90FL isn’t just a low-power imaging scope.


The 90FL objective is outstandingOptron have achieved a fast f-ratio without sacrificing optical quality. All aberrations are well-controlled. Off-axis curvature and coma are at F6 triplet levels, even without the flattener. False colour is well suppressed for such a fast doublet. The 90FL may have the optical spec’ of the Sky-90, but it’s free of that scope’s high-power softness and collimation issues. Designed as an imaging scope, it still performed very well on the Moon and planets. Meanwhile, the 90FL gave lovely wide, flat deep sky views with eyepieces designed for fast f-ratios, like Tele Vue’s.

The 90FL is really remarkably light and compact for its aperture – it looks and feels like a 60mm scope. It significantly outperforms other (50-80mm) refractors of similar size and weight. This is a fantastic feature that makes the 90FL in a class of its own for portability and performance.

Mechanically, the 90FL is beautifully made in that Borg way, but it’s easy to go to adapter hell. In the end, I had to take photos to remind me which way everything went together for different modes (does part #7352 go before or after the reducer?) And I do think Borg should supply everything needed to get you imaging (or viewing), instead of leaving you scratching your head over an M57 thread and poring over the parts catalogue.

Whilst we’re talking negatives, the helical focuser was just about up to the job, but with such a steep light cone, a finer focuser is really needed and it did get a bit stiff and graunchy under heavy loads. You’d need the bigger M75 focuser for serious imaging.

Coverage with the 0.72x reducer (as with the bare objective) was exceptionally good on full-frame and a real win over most Petzvals. But I didn’t find the field quite as well corrected as I was expecting, almost certainly because the 90FL was delivered with the reducer configured for the 71FL. Nonetheless, some spectacular wide-field images are possible with short exposures at F4.

As a luxury grab-n-go or travel scope, the 90FL gets my highest recommendation. As an imaging machine, it makes a smaller, lighter, more flexible alternative to an FSQ. But be prepared to get to grips with all those adapters and thread-sizes.

Borg 90FL on Takahashi Teegul mount with dewshield extended.