Borg
67FL Review
The Borg 67FL was a limited production
run and is no longer available new. But when one came up for sale I had to have
it. The reason is simple – it’s insane optical spec’ makes for a pocket-size
(really) telescope that performs much like a good 3” apochromat. You want
extreme portability and performance? Here you go.
Once I’d bought it, though, I held it
in my hand and had doubts. Would its radical numbers make for a poor optic?
Let’s find out.
At
A Glance
|
Telescope |
Borg 67FL |
|
Aperture |
67mm |
|
Focal
Length |
300mm |
|
Focal
Ratio |
F4.5 (F3.8
with 0.85x reducer) |
|
Length |
270mm
visual |
|
Weight |
880g
visual |
Data from: Me
What’s
in the Box?
It’s a Borg, so just lots of component
parts in little boxes.
Borg 67FL configured in a Series 80 OTA
with helical focuser.
Design
and Build
The 67FL objective threads straight
onto Series 80 tubes, or onto Mini Borg tubes using the flared adapter you see
above. Apart from that, it’s impossible to be specific, because there are
numerous different combinations of focusers and drawtubes you could combine
depending on your needs.
Optics
Borg 67FL and
Takahashi FS-60 objectives for comparison.
The Borg 67FL has a fluorite doublet made by Canon/Optron
in Japan, like others in the Borg range and like many Takahashis.
A laser test shows the fluorite element is at the front in a conventional
Fraunhofer configuration, like older ‘FS’ Takahashi’s, rather than recent (and
also much older) ‘FC’ Tak’s that have the fluorite at
the back as a Steinheil.
In my opinion, the
front-element makes for a slightly better optic (though many will disagree) if
only because having the more transmissive fluorite up front makes for a
slightly more transparent lens (fluorite transmits more light
than glass, hence its lack of scatter in the laser test).
So far, so
typical. However, in terms of its focal ratio the Borg 67FL is a radical beast.
Why? Consider the Takahashi FS-60 (see above), for many years the definitive
small, fast apochromat. The FS-60 has an aperture of 60mm and a focal length of
355mm giving F-5.9. In comparison, the
Borg 67FL has a focal length of just 300mm, giving an F-ratio of just F-4.5,
even though its aperture is larger.
Now F-4.5 is
incredibly fast for any refractor, but for a doublet it’s pretty amazing. Fast
is good for imagers, decreasing exposures and widening the field. But all
aberrations tend to increase with aperture and shorter f-ratios. We might
expect the 67FL to show lots of false colour and off-axis field curvature and
to be limited to low powers. However, the 67FL has an air gap between the
elements in its objective, compared to the FS-60 that just has little foil
spacers. That air gap allows better correction of aberrations and helps explain
how the 67FL can get away with being 1.5 F-stops faster than the FS-60.
Tube
In this case, the 67FL is in a Series
80 (i.e. 80mm o.d.) tube that allows a 2” visual back, but it still weighs just
880g in visual trim – even an FS-60C weighs 25% more. Meanwhile, the objective
comes with a flared adapter to fit 60mm Mini Borg components which would make
an even lighter OTA.
That size you see in the photos isn’t
due to a drawtube or the need for extensions – it really is that short in use,
in fact for some eyepieces you need to shorten the tube! That length also
includes the fixed dew-shield.
Borg 67FL and
Takahashi FS-60C compared.
Focuser
The focuser is Borg’s standard helical,
part 7835, to fit Series 80 tubes. It is decent, but lacks travel and can get
stiff under heavy loads.
Mounting
The 67FL is
small enough to mount on the lightest photo tripod.
Accessories
Borg’s own 80mm tube ring is very
light, but a bit thin and short on padding. A nicer option is the 80mm
Takahashi tube ring for the FS-60C which has an offset plate which helps with
balance. Borg make lots of other useful accessories, such as an extra-wide
T-mount (see OTA in reducer configuration below).
The only reducer sold with the Borg
67FL was the doublet 0.85x MiniBorg reducer (part
7885) which only works with APS-C. I’ve discussed this in detail below.
It’s possible other Borg reducers will
work. I got some incredibly good terrestrial images with the new 0.72x 90FL
reducer, but wasn’t able to test it on the night sky.
In
Use – Daytime Viewing
The 67FL makes an excellent spotting
scope when used with eyepieces designed to cope with F4.5, like Tele Vue’s.
An old 15mm Panoptic gave wonderful
wide-field views at 20x.
At 60x (about max for most spotting
scopes) with a 5mm Nagler, the view is still absolutely
sharp, quite flat, detailed and with minimal false colour. Things stay this way
to at least 100x – impressive.
At 120x with a 2.5mm Nagler the view is still quite sharp, but shows quite a bit
of false colour, perhaps a little more than Takahashi’s FS-60 (an Optron fluorite doublet of similar spec’), but smaller than
the extra aperture and shorter focal ratio of the Borg might suggest.
In
Use – Daytime Photography
For terrestrial photography, test
images of silhouetted branches show very similar false colour levels to
Takahashi’s FS-60; however, without a flattener, the Borg has much more severe
off-axis curvature as you would expect due to its very challenging f-ratio.
0.85x MiniBorg doublet reducer (part 7885)
This
small and quite cheap APS-C only reducer came with the 67FL, the only one sold
with it as a package and apparently the reducer best suited to it.
Terrestrial images using the 7885 reducer (see below for connecting it) are good with an APS-C
chip (for which it is designed). Used like this, the 67FL converts into an F3.8
255mm telephoto that works well and produces sharp images to the corners of an
APS-C frame.
For the
recent Jupiter/Venus conjunction I was up at four and got my big scope out. But
in the event, the planets only appeared for a minute or two over my neighbours’
roof before the clouds closed in. A hand-held snap from the very corner of my
balcony with the Borg 67FL was all I got – at F3.8 it is fast enough to use
hand-held in low light.
0.72x Quadruplet 90FL Reducer (part
7872)
I also tried the 0.72x quadruplet
reducer for the 90FL and 71FL which threads directly into the M57 thread on the
focuser. In this configuration, the reducer is designed for a focal length of
400mm (i.e. the 71FL), so it’s not quite right for the 300mm focal length of
the 67FL.
Nonetheless, I got some really fabulous
terrestrial images (see below) – razor sharp to the edge, with loads of fine
detail zoomed in and with minimal false colour at 216mm F3.2 and proving what a
fine objective the 67FL is.
Connecting
a reducer - example
This section is intended as a resource for anyone
wanting to figure out how to connect their camera to a Borg 67FL; if that’s not
you, skip it!
Borg’s
great strength and pain-in-the-*** is all the adapters they sell to allow
almost anything to fit anything else. To fit a reducer to your camera will take
a few of these adapters because you need to get the spacing right or you’ll
have nasty off-axis aberrations.
As an
example, connecting the basic 0.85x APS-C reducer (part 7885) to my Fuji
X-mount camera needed the following parts (there are other solutions):
7885
- Reducer
7920
– M57 to M49.8 female
7602
– M57 extension 20mm
7000
– M57 to M49.8 male
5016
– Borg X-mount adapter (ends in a M49.8 thread)
Now,
I’ll go through a sample calculation with my Fuji X-mount camera to show you
why you need the 20mm extension.
For the
0.85x doublet APS-C reducer (Part 7885), the distance from reducer’s interface
flange to the camera’s image plane needs to be 55mm.
The
Fuji X-mount has a focal flange distance of 17.7mm (this is a key fact about
your camera which you can easily check online, it is the distance between the
mount surface and the sensor). To the flange distance, you need to add 7mm for
the Borg X-mount adapter (the standard T-mount adapter is too long), another
8mm for the M57 adapter on the back of the reducer (part 7920) and a further
5mm for the X-mount M49.8 to M57 adapter (part 7000). That lot sums to 37.7mm,
leaving a further 17.3mm to space out. The closest Borg come to that is the M57
20mm extension tube S, (part 7602), or just use the 20mm eyepiece holder which
is also a 20mm M57 tube! Job done … eventually.
Borg
67FL configured with MiniBorg 0.85x reducer.
Terrestrial
image with Borg 67FL + 0.85x reducer: Fuji X-Trans APS-C camera.
Terrestrial
image with Borg 67FL + 0.72x quadruplet super-reducer: Fuji X-Trans APS-C
camera.
In
Use – Astrophotography
The
reducer gives very wide and flat APS-C frames – so wide you can start to image
whole regions of sky. Another advantage of such a short focal length is that
you only need rough polar alignment for exposures up to a minute or two, which
will be enough for most purposes if stacking. The images of M31 and M42 below were
taken by just rough-aligning the Teegul mount and one
minute plus showed no tracking errors. Coverage is excellent on APS-C.
With
such a wide, fast field, things appear in single raw frames that I’ve never seen before – such as the North American Nebula.
The field is commendably flat too, with stars developing no more than a trace
of coma/astigmatism in the corners. There is some violet bloating on bright O-A
stars, but it’s no too bad at all. Sadly, I never got a chance to try the 0.72
reducer on the night sky.
At its
native F4.5, or with a reducer, the 67FL has just too little image scale for
the Moon, but Moon-rise shots will work out fine.
Straight
from the camera as always – no stacking or processing, just reduced in size.
Pleaides
with Borg 67FL, no reducer: 30s at ISO 3200, Fuji X-Trans APS-C.
M31
with Borg67FL + 0.85x reducer: 40s at ISO 3200, Fuji X-Trans APS-C.
M42
with Borg67FL + 0.85x reducer: 30s at ISO 3200, Fuji X-Trans APS-C.
In
Use – The Night Sky
General
Observing Notes
I have tested
a lot of small refractors now and I was pretty sure what the Borg 67FL would be
– an astrograph compromised for visual use. I was quite wrong.
At higher
powers, Naglers delivered a reasonably flat field. At
lower powers, there is a lot of curvature, along with some coma and astigmatism
from about 60% field width. Even so, the central section is large and flat
enough to give great views of almost anything, without the off-axis aberrations
intruding. Other, lesser eyepieces, work less well with the F4.5 focal ratio (Naglers are tested to F4, but few others can say the same).
The
configuration shown worked well for a range of 1.25” eyepieces from a 32mm TV Plossl to a 2.4mm Nagler zoom.
But for big 2” eyepieces you would need to replace one of the white tube
segments with a drawtube (which Borg sell) for more focus travel, but why would
you bother?
You see, the
67FL requires some conceptual recalibration. A 32mm Plossl
(maximum FOV for 1.25”) gives a magnification of 9x and a field width of 5.2
degrees, well into binocular territory. However, at this magnification exit
pupil is 7mm and to older eyes DSOs will look no brighter than through a 50mm
scope. Now plug in a 55mm Plossl and you get a
magnification of 5.5x and a field of 8.8 degrees – wider than almost any
binoculars, great. But at that magnification, the exit pupil is a stupid 12mm.
No one has pupils that big and so the 67FL just became a 40mm scope, even for
young eyes (which can dilate much more than older ones).
Cool
Down
Instant,
except on the frostiest night, when it’s very fast.
Star
Test
Good. Better
than you have a right to expect at F4.5.
The
Moon
A night with
a first quarter Moon, scudding clouds and periodic showers is where a tiny
scope like the Borg comes into its own: put it out, get observing almost
straight away and then whip it back in if the weather turns ugly.
You would
expect a 67mm F4.5 doublet to show semi-APO levels of false colour on the Moon
and at 60x with a 5mm Nagler I’m prepared for lilac
wash; but it’s not there. At this magnification, the Moon looks fantastic –
cold fluorite whites and hard greys, with no false colour to note, even
focusing through the limb.
At this point
many owners who have bought this little scope as a wide-field astrograph are
going to run out of eyepieces. Fortunately I like tiny
APOs, so I can push on to a 3.5mm Nagler. This
eyepiece normally gives the highest power I will ever use in a small scope, but
in the 67FL it magnifies the image by just 86x - still well within the Borg’s
capabilities. That expected softness and chromatic aberration still haven’t
materialised.
What now? Who
owns eyepieces shorter than 3.5mm? I bought a 2.5mm type 6 Nagler
for rare occasions like this: the tiny Borg is one of the few scopes where it’s
of any use. Now I have a magnification of 120x. It’s a good thing that these Naglers are parfocal and I can keep the focus point from
the Nagler 3.5mm, because that helical focuser is
struggling: the steep light cone means you really need a micro-focuser at this
power.
120x frames
the Moon to perfection – it almost fills the field. Surprisingly, it’s still a
good view – reasonably sharp and bright and detailed. The central peak in
Alphonsus is just catching the morning sun. Strange Cassini crater is still
surrounded by dark shadow. Hyginus rille is easy to spot with its craters.
Leaning back from the eyepiece, it’s hard to believe the view is from this
finder-sized telescope, a scope too light for the counterweight on the Teegul, that makes a TV-60 or FS-60 look big.
I tried a
back-to-back comparison with the FS-60C on a waning crescent one frosty
pre-Christmas dawn and found that the 67FL showed no more chromatic aberration,
was just as sharp and significantly brighter – no apparent downside to that
short (F4.5 vs F5.9) focal ratio.
I am surprised what a good view this tiny astrograph
gives of the Moon – if you have the right eyepieces.
Mars
Low in the
morning sky and at just four arcsecs in size, in the run-up to next year’s
(2018) opposition, Mars was a tough target for a small scope. The Borg held up
at 150x with a 2mm Nagler zoom and delivered a
perfect tiny gibbous disk with just a touch of flare and false colour.
Jupiter
Jupiter focused crisp and without false
colour. I saw the equatorial belts and polar darkening, a hint of GRS, at
100x-150x with a Nagler zoom.
Deep
Sky
The Pleaides
were lovely and sparkly with hints of nebulosity through a 13mm Ethos, or 15mm
Panoptic. The field curvature is obvious at the edge, but doesn’t spoil the
view. The clusters running up through Auriga (M35-37) looked lovely, with
pin-sharp stars and strong colours, likewise the double cluster. I had great
views of the double cluster too – masses of sparkly stars.
Orion’s sword
region was less positive. The Great Nebula (M42) looked good with pin-sharp
stars and the Trapezium easily resolved at 20x with a 15mm Panoptic; some
structure in the nebula too. But stars in other parts of the sword were then
severely distorted by field curvature and astigmatism, spoiling the overall
view.
You wouldn’t
expect an F4.5 scope to resolve much in the way of doubles, but with the 2mm
setting on a Nagler zoom giving 150x, epsilon-Lyrae resolved nicely in good seeing, with epsilon-1 easy
at 2.6” and epsilon-2 a bit harder at 2.3”.
The 67FL makes a surprisingly good all-round visual
scope.
Summary
The Borg 67FL is a good example of why
I enjoy reviewing small telescopes, because it really surprised me. I expected
a dedicated astrograph, but what I discovered was the smallest general purpose
astronomical telescope I have tested. Most of the views through it were typical
of a good 3”-class APO, but this is a scope that makes a TV-76 seem huge and
unwieldy.
The 67FL is not a super-APO, but false
colour levels are typical of a 60mm F6 doublet – an FS-60 or TV-60. It can
easily take powers that would trouble an expensive spotting scope by day. By
night, only Venus really generates much nasty purple fringing. The Borg 67FL
gives excellent views of the Moon and planets that would trouble a Questar;
much better than an old TV Ranger, too.
Field curvature is strong at low powers
on deep sky, but the sweet spot is large enough for most things. With a reducer
giving under F4, some incredible wide-field images are possible, with lots of
nebulosity in single frames straight from a DSLR. With the helical focuser and
fast f-ratio, it works well as a handheld telephoto, too.
There are a few downsides. The 67FL is
too light to balance, even on the Teegul micro-mount.
It is capable of magnifications well over a 100x, but needs eyepieces shorter
than 3mm to do so, eyepieces most people don’t own. Then again, many eyepieces
won’t cope with F4.5 (e.g. Pentax XWs).
The basic reducer is fine for APS-C,
but not ideal for full frame. Finally, that standard Borg helical focuser is
stretched with this lens. It would be an even better scope with a Feathertouch, or a better helical.
All
that is just nit-picking, though, because if you need a proper telescope the
size of a finder, this is it. If the TV-60 is a ‘briefcase scope’, the 67FL is
a ‘handbag scope’. The Borg 67FL is probably my favourite ultra-portable scope
to date: too bad they only made a few.
The Borg 67FL just shouldn’t be any
good – F4.5 seems silly in a doublet. But in fact, it is excellent for visual
on anything you point it at. No other ‘proper’ telescope comes close to being
this portable (or arguably this flexible either). For imaging, there is some
violet bloat; but stunningly wide, fast frames are possible with a reducer.