Borg
90FL Review
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 an F4 reducer. But does it still have the
Sky-90s Achilles’-heal - its massive, collimation-sensitive lens cell?
Let’s find out.
Note: I eventually bought another 90FL
as a travel scope for my own use, so I’ve updated this review.
At
A Glance
|
Telescope |
Borg 90FL |
|
Aperture |
90mm |
|
Focal
Length |
500mm
(360mm with reducer) |
|
Focal
Ratio |
F 5.6 (F4
with 0.72x reducer) |
|
Length |
~390mm (15.5”)
imaging, 470mm (18.5”) visual |
|
Weight |
~1.5 Kg as
Shown |
Data from Borg/me.
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
Uniquely,
Borg don’t sell telescopes. Instead they sell objectives and a huge kit
of parts to build the specific OTA you want. At the time of writing, Borg offer
the following Canon/Optron fluorite doublet objective units:
· 55FL (55mm F4.5) reviewed here
· 72FL (72mm F5.6)
· 90FL (90mm F5.6) on review
· 107FL (107mm F5.6)
I’ve
also reviewed the now-discontinued but wonderful 67FL (here).
The smaller
Borg lenses can be configured in either 80mm or 60mm OD tubes. The larger objectives
are intended for 80mm tubes, but you could still build a 60mm setup if you
really want. However you build them up, Borgs are
almost always exceptionally light for their aperture – a huge advantage
for portability, but for ease of mounting too.
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. But there are other ways to do it: you could choose a different
focuser or a longer tube for visual only; you could spec carbon for the tube
instead of aluminium. You could even put the focuser behind the objective for a
dedicated camera lens.
In terms of competition,
Takahashi’s current FSQ-85 is an obvious one, Astro Physics’ 92mm
Stowaway another, but there are others from the likes of William Optics and
Askar.
Optics
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. Like Borg’s other FL objectives - and just like the Sky-90
– it’s 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.
F5.6 in a
more conventional fluorite doublet sounds like a recipe for semi-APO correction
levels. But as we will see, the 90FL objective is remarkably well corrected for
chromatic aberrations. Fabrication quality on the two examples I’ve seen
has been truly excellent. The coatings are some of the most transmissive I’ve
seen. Like all front-surface fluorite lenses, the 90FL 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. Removing the plug reveals an 82mm thread to accommodate 82mm
photographic filters for (most likely solar) imaging.
The 90FL has
a larger air-gap than a simple foil-spaced doublet, but much smaller than the
13mm one on the Sky-90.
Tube
Unlike
Borg’s smaller objectives, the 90FL really needs an 80mm setup. Borg make
a range of 80mm OD tubes, but in this case it will be either the 150mm (part
7151) or 205mm (part 7803) for the OTA’s main tube. Both tubes are very
lightweight but finely fabricated, containing a knife edge baffle and flocking
material in the case of the 205mm.
All Borg’s
80mm aluminium tubes come in satin black. It’s attractive and stealthy
(possibly a serious advantage for urban astronomers) but shows every mark and
print. For significantly higher cost you can also get 80mm tubes in carbon
fibre.
The 90FL as
shown below is an imaging setup with the 150mm main tube. Threading a long
2” eyepiece holder (part # 7509) onto the M57 thread at the back of the
focuser in place of the reducer allowed most 1.25” eyepieces to focus (in
a 1.25” diagonal with a standard 2” to 1.25” insert). However,
this does put more strain on the focuser. The longer 205mm main tube with a
standard 2” eyepiece holder might make more sense in a purely visual
setup. A Borg advantage is that you could buy both tubes and swap as needed
(they’re not expensive).
Whichever
tube you use, the objective unit just screws straight onto the M75 female
thread at one end. At the other end of the tube is a an M77.6 thread and you
will need an adapter for a focuser. In this case, the adapter reduces the
thread to M68.8 (part 7801) for fitment of a Borg helical focuser (see focuser
section below).
The 90FL is very
compact and lightweight in any configuration. With the helical focuser shown,
the OTA weighs about 1.5 Kg (2Kg including rings and plate) – about half
the weight of the optically identical Takahashi Sky-90. Any configuration,
imaging or visual, will be carry-on portable. But of course you can just take
it apart to go in an even more compact bag.
Complete
90FL imaging setup broke down (from L to R):
2591 – 90FL objective unit
7151 – 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 (shown
incorrectly configured for the 71FL)
For astrophotography you would need to
add a T-ring and M57 adapter; for visual an M57 eyepiece holder.
90FL with the
150mm tube but configured for visual with a long 2” eyepiece holder (part
7509).
Focuser
If you Google
images of a particular Borg model, you’ll find that every example looks
different. This is because you can choose between various Borg helical focusers
and then put them in different places in the OTA. Borg also make a Crayford and
a rack-and-pinion too and you can get adapters (are you sick of that word yet?)
to fit a Starlight Instruments Feather Touch.
The focuser
in this setup is the ‘standard’ 68.8mm thread helical focuser (part
7835) for the 80mm tube set. I really like helical focusers and this one works
well – up to a point. It features a lock screw for imaging which produces
little or no image shift.
The original
focuser I had got sticky with even moderate loads, but I read that Borg had
made improvements in later versions and the latest (early 2025) was much
better: it coped fine with a normal diagonal and an Ethos eyepiece. However, it
might still become sticky with a heavier camera or the largest 2”
eyepieces.
Mounting
The 90FL is
in the very smallest and lightest class of APOs. It’s the same weight as
a Takahashi FS-60 and almost half the weight of Askar’s tiny FRA400!
Consequently, it will go on any mount, even a tiny Takahashi Teegul, designed
for 60mm-class scopes. On Vixen’s lightest equatorial, the AP, it
balances with just a 1Kg counterweight. Almost any other imaging setup will
need one of the larger weights. Being able to use the smallest mounts is a major
advantage for travel, but it also means you can image stably with a much
smaller mount setup – an under-appreciated benefit.
The 80mm OD
tube is a standard size. You could get cheap 3rd party rings or
splash out on Borg’s own, which are very light and finely machined.
Alternatively, one of Takahashi’s 80mm clamshells work well too.
Accessories
In a sense,
any Borg is all accessory. Fewer than there once were, Borg still market a huge
range of parts and accessories. The most significant – 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.
Other
accessories you’re likely to need include (with part numbers):
·
7000 –
Oasis Camera Mount Adapter, to fit a Borg wide camera mount to the focuser or
reducer (a standard 42mm T-ring will vignette at full frame)
·
500X - Borg
wide camera mount for your camera (e.g. 5005 for Canon EOS)
·
7501, 7506,
7508 etc - Various 2” eyepiece holders with different optical path
lengths
·
7317 –
1.25” eyepiece holder (you could use a Takahashi or Baader instead)
·
7522 or 7523
– adapter to fit a standard camera T-ring or 1.25” eyepiece holder
to the M57 focuser back
My 90FL
package came with another useful accessory, a rotator (7352): 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 star field
around M36 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 smallest
spacer needs to be removed from the reducer assembly.
Off-axis darkening (vignetting) is a
problem with Petzval astrographs like Takahashi’s FSQ-85, but the Borg 90FL/0.72x
reducer combo’ avoids this to give excellent coverage at full-frame. At
360mm F.L. (F4) 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.
Despite its short focal length (just
20mm more than a TV-76) and so small image scale, the 90 FL takes a good snap
of the Moon, even in mediocre seeing as here.
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). Off-axis distortion is due to incorrect
spacing in the reducer.
Snap of the Moon with Borg 90FL/Canon
EOS6D MkII in average to poor seeing.
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. But the
extending dew-shield works well and is long enough to combat pesky urban
streetlamps.
I was concerned
that the more conventional objective design would mean worse chromatic
aberrations than the Sky-90, but false colour isn’t a problem, even at
high powers. The Borg 90FL is much more capable than a 60mm or 76mm refractor
and comes surprisingly close to the 92mm AP Stowaway whilst being much lighter
and quicker cooling.
With simple
eyepieces like Plössls, there is a lot of off-axis field curvature
from about 50% field width, though centre field is perfectly sharp. To kill
those off-axis aberrations for visual use, flat field eyepieces with built-in extenders
– Tele Vue Naglers, Ethos, Delos etc – will do the trick.
Cool
Down
Cool-down is
super-fast. It’s usable almost at once. Another big advantage in a
quick-look or travel scope when compared with a triplet or Petzval.
Star
Test
The star test
is excellent – sharply defined, identical rings either side of focus.
There was just a hint of false colour on Rigel in the star test.
The
Moon
A waxing 10.5 day Moon was a feast of sharp detail to explore with the
90FL. I watched the sun gradually rise over the Bay of Rainbows - wrinkle
ridges smoothing out, the triangular shadow of Promontorium Laplace and
isolated peaks nearby shortening, the palisade of Montes Jura dazzling in the
dawn sun.
Further south, I spotted a strange circle, like standing stones, that
I’d never noticed before near crater Euler, picked out on the terminator north
of Copernicus in Mare Imbrium. Continuing on down the terminator, I explored
the radial rilles south of Hippalus on the edge of Mare Humorum, trying to see
how far they extend.
I noted minor
false colour focusing through the bright limb at 142x, but little in focus.
Venus
A perfect
dazzling crescent at 100x with a 5mm Nagler - virtually free of false colour in
focus, but with a tint of green and purple either side, a bit more than you get
with, say, a Takahashi FC-76 (a longer focus fluorite doublet, also made by
Optron). By 142x there was some false colour and stray light in focus, but this
is a seriously harsh test of an F5.6 doublet.
Mars
I was lucky
to catch Mars and Jupiter on an evening of almost perfect (really) seeing just
after New Year 2025. It was chastening to realise just how much difference
seeing makes, even to the capabilities of small scopes like the 90FL.
Mars was
14.4” across, just a couple of weeks before its 2025 opposition (but
still relatively small compared to its angular size near more favourable oppositions).
I was genuinely surprised by the
view. Many faster doublets give a slightly soft image with a blur of
unfocused red light around the planet due poor correction in the orange and
red. Not the 90FL, which gave a sharp view in focus and just a trace of colour
either side.
At 135x with
a Tele Vue Ethos 3.7mm (200x with a 2.5mm Nagler showed no more), the 90FL
delivered a perfectly sharp orange disk with significant albedo detail. I could
make out a bright northern polar cap, the large dark area of Mare Acidalium below it. In the centre west was the bright,
featureless Tharsis desert. But I was particularly surprised to make out fine fingers
of darker albedo reaching up from the south – detail I usually expect to
need a much bigger scope to see.
Jupiter
If the view of Mars in perfect seeing
had been impressive, Jupiter was astonishing. At 135x with the Ethos 3.7mm I
could see space-probe levels of detail. What I thought was the GRS turned out
to be a different storm, or perhaps just an oversized festoon or barge. I could
make out other white ovals and dark spots infesting the equatorial belts, lots
of them, small and large. The equatorial belts no longer looked solid, but
highly variable in thickness and tone. Between them was what looked like another
cloud belt, where the festoons drag and coalesce. The north and south polar
hoods revealed loads of fine bands too. I could go on and I did – viewing
for ages, just drinking in the astonishing view (honestly one of the best of
Jupiter I’ve ever had).
To my eye there was minimal false
colour on Jupiter, in focus or out. But I did notice that my iPhone’s
camera – more sensitive than my eye at the shortest wavelengths - picked
up a diffuse violet band around the planet.
Deep
Sky
The Pleiades
looked wonderful through a 19mm Panoptic giving 26x, 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).
With smaller
scopes, Rigel usually takes a bit of looking and waiting to split - for the
faint companion to pop out from the main star’s glare. But on that night
of fine seeing, at 135x with the Ethos 3.7mm, Rigel was so immediately and obviously
double I had to check to make sure I’d got the right star. Partly this
was due to the seeing, but the 90FL’s excellent optics too.
Summary
The
90FL objective is outstanding – Optron have achieved a fast f-ratio
without sacrificing optical quality. For visual use, false colour is surprisingly
well suppressed for such a fast doublet. It gave truly outstanding views of the
Moon and planets, subjectively closer to a fine 4” APO than a 3”. It
gave lovely deep sky views too, but you’ll need fancy eyepieces like
Delos, Ethos etc for a flat field.
For imaging, coverage at F4 with the
0.72x reducer (and natively at F5.6) was exceptionally good on full-frame. With
the correct reducer spacing (see above), off-axis aberrations would likely have
been well controlled too. F4 is unusually fast and captures faint nebulosity
even with short exposures. The only issue is a little more violet bloat on O-A
stars than slower fluorite doublets and ED triplets.
The 90FL is in a class of its own for
portability. It looks like a 76mm and weighs about
the same as a TV-60, yet significantly outperforms other (50-80mm) refractors of
similar size and weight.
Mechanically, the 90FL is beautifully
made in that ultra-light Borg way. But to view or image, you’ll need to
get to grips with the wonderful world of Borg adapters, even if you buy a
complete setup.
The only real downside for me is the
7835 helical focuser. It works fine for light loads, but might not cope with
heavy cameras. It has the advantage of being much lighter than a conventional
focuser (which is why I chose it again for my second 90FL), but a Borg rack and
pinion or a Feather Touch would be better for imaging.
As an ultra-portable visual scope, the
90FL gets my highest recommendation. As an imaging machine, it makes a smaller,
lighter, more flexible alternative to an FSQ-85. But be prepared to get to
grips with all those adapters and thread-sizes.
Borg 90FL on Takahashi Teegul mount
with dewshield extended.