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
|
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/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
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
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.
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. 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.
Tube
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.
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 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
Mounting
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 Tak’ Teegul,
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.
Accessories
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.
Venus
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
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.
Jupiter
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.
Summary
The
90FL objective is outstanding – Optron
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.