Borg 50FL Review
I am a huge fan of Ed’ Ting, but I do
blame him for one thing – my near-obsession with small refractors. Twenty years
ago, I read and re-read his evocative reviews of tiny marvels like Takahashi’s
FC-50. So when Borg re-booted the FC-50 with a
fluorite objective by Canon/Optron like the original,
I simply had to have one. Yes, it was preposterously expensive for a scope the
size of your finder. Like I said, blame Ed’.
But is the Borg 50FL actually useful
for anything? Evidence from other tiny scopes isn’t encouraging, but this isn’t
any small scope. I knew from experiments stopping down high-end refractors that
an effectively perfect 50mm aperture can do a lot, especially on the Moon (and
so for eclipses too). Sadly, one of Borg’s older mini-scopes, the 45ED, fell a
long way short of that ideal. But when I reviewed the 45ED I did note that I’d
like to try the 50FL. Now let’s find out if this tiny and expensive sliver of
fluorite does any better.
Note: the 50FL has been discontinued,
but is still available from some Borg dealers at the time of writing. The 55FL
sounds similar, but is much faster at just F4.5 – better for very wide-field
imaging, not so much for eclipses.
At
A Glance
|
Telescope |
Borg 50FL |
|
Aperture |
50mm (2”) |
|
Focal
Length |
400mm |
|
Focal
Ratio |
F8 |
|
Length |
Depends on
tube configuration |
|
Weight |
~900g,
depending on config. |
Data from Borg
What’s
in the Box?
As with all Borgs, the answer is lots
of little colourful smaller boxes, each containing some component or adapter.
Design
and Build
Borgs are basically Mechano
for astronomers, so it’s impossible to be specific here. The 50FL can be built
into a huge range of different tube and focuser configurations, depending on
your needs: visual, astrograph, telephoto, spotter or whatever.
A commonly sold visual configuration,
certainly in Japan, was based on a Mini-Borg drawtube and a tiny (but
expensive) Borg Crayford focuser connecting into the Mini-Borg components on an
M57 thread.
In comparison, the OTAs on test here
are based on the larger series 80 components that makes it possible to
configure it as a telephoto lens with a helical focuser.
Optics
A quick note
about fluorite and objective types are in order. Read, tune out or skip to
taste.
The 50FL is a
conventional Fraunhofer type doublet with the front (crown) element made not
from glass but from a piece of synthetic fluorite (note no ‘flour’ in fluorite,
soft though it is), a crystalline mineral. Fluorite scatters less light than
glass, which is why it is revealed by its absence in a laser test (the unscattered beam disappears in the fluorite, but not in
glass – see above). In this case, having fluorite as the first thing the light
hits (after the coatings) makes the lens that bit more transparent too.
An
alternative arrangement is to have the fluorite at the back in a more unusual
configuration called a Steinheil. This protects the
fragile mineral (which is good), but is probably not quite such an ideal
optical design (it needs steeped curves grinding, for one thing). Many older
fluorite objectives, including the original Takahashi FC-50, have this
arrangement.
So, in reality, the Borg 50FL uses a different
optical design to Takahashi’s collectable FC-50, even though it’s also an F8
fluorite doublet.
A laser test
further reveals that the 50FL lens has a significant air gap – it’s not just a
simple foil-spaced job. Air gaps can work as a kind of ‘virtual lens element’,
giving the designer more freedom to correct aberrations. In many of those older
Steinheil designs, like the FC-50, the fluorite was
uncoated too, but not the Borg 50FL, which has top class multi coatings on all
surfaces.
Like the
FC-50, the 50FL is made by Canon/Optron in Japan.
Unlike the FC-50, its cell, though finely engineered, is not colimatable.
Three Canon/Optron fluorite objectives: Takahashi FS-60, Borg 67FL, Borg
50FL.
Borg 50FL is
a front-surface fluorite (Fraunhofer) doublet with a substantial air gap.
Tube
The tube you
see here is based on Borg Series 80 components, not the usual (for this lens)
Mini Borg. It looks a bit strange and un-wieldy, but means it can use a bigger
helical focuser and fits straight into the Takahashi FS-60
clamshell on my little Teegul mount.
Another option I use is a 150mm long
80mm tube that allows the 50FL to be used as a telephoto lens with a screw-fit
(rather than push-fit) attachment.
Focuser
In this case, the focuser is the
standard Borg helical focuser for the Series 80 tube set (part 7835). I like it
for use as a telephoto, but it lacks travel for visual use and can get stiff
under heavy loads. In any configuration, the 50FL will be quite a bit longer
than many other small Borgs because of its longer focal length. For example,
the 55FL has a focal length (and so OTA too) 15cm shorter.
Mounting
Any
configuration of the 50FL is light enough to go on any small mount or photo
tripod. On my Teegul, the 0.6Kg counterweight is
really too heavy for it.
Borg make a
tiny single-arm fork mount which should be perfect.
Accessories
In a sense
Borgs are all accessory, but an obvious one for the 50FL is a reducer to turn it
into an excellent small astrograph. It’s a sharp lens with very low false
colour and reduces very nicely to create a sharp, lightweight imaging scope.
There are three reducer options for the 50FL:
·
0.85x MiniBorg doublet reducer (part 7885): 340mm F.L. F6.8. The
cheapest option, this reducer gives good results with a minimal increase in
false colour and a nicely corrected field across an APS-C frame.
·
0.7x triplet
reducer (part 7870): 280mm F.L. at F5.6. Should work with full-frame, but has a
design that easily introduces tilt. Discontinued, but still available at the
time of writing.
·
0.72x quadruplet
reducer (part 7872): 288mm F.L. at F5.8. Intended for the 71FL/90FL, this works
with the 50FL because it has the same focal length (400mm) as the 71FL.
Japanese photographers have used it with the 50FL to great effect, producing
full-frame images with excellent coverage that are sharp to the very edge at
F5.8. It is expensive, however.
Borg 50FL
configured as a telephoto lens with the 0.85x Mini Borg doublet reducer.
In
Use – Daytime
Daytime views are very sharp up to 100x
(about the limit for any 50mm scope). I could only coax the faintest trace of
false colour out of it by focusing through a crow in silhouette. The 50FL would
make a good lightweight spotter or birding scope; the only problem is the small
aperture makes the view a bit dim at high powers in dull conditions.
Daytime photos are tack sharp
centre-field and free of chromatic aberration, with lots of detail. The 50FL
was noticeably better than the 67FL in this respect. It would be just the job
for arty Moonrise shots without a flattener, just crop-out the slightly soft
edges. However, if you want to use full frame then you will need a flattener
and even for APS-C a flattener gives better coverage and saturation.
Photographers in Japan have obtained
outstanding terrestrial images using the quadruplet reducer (part 7872)
intended for the 71FL/90FL (see above), but I found results with the cheaper MiniBorg reducer (part 7885) were good for APS-C: very
sharp centre-field, with minor softening in the corners.
Borg 50FL with 0.85x Mini Borg reducer:
Fuji X-trans APS-C with no processing.
In
Use – Astrophotography
Images of brighter DSOs are quite
doable natively with the 50FL. Exposure times obviously reflect F8 and there is
some off-axis curvature (see first image below). Otherwise the results are
surprisingly good for such a small aperture, with notably low levels of
blue-violet bloat.
With the 0.85x MiniBorg
reducer, an almost perfectly flat APS-C field results, with good coverage and very
little coma in the corners, yet still nicely sharp on axis. Violet/blue
bloating is increased only very slightly. This configuration is capable of excellent
wide-field deep sky results at a focal length of just 340mm giving a field of some
3.8°
across the long axis of a typical APS-C sensor.
Native images of the Moon are sharp and
detailed for the aperture, suggesting the 50FL would work well for eclipses,
better than one of the very fast Borg scopes like the 67FL.
M37: 50FL at prime focus with no
reducer, single APS-C frame.
M36: 50FL with 0.85x Mini Borg reducer,
single APS-C frame.
Moon through 50FL: cropped and with slightly
pushed contrast.
In
Use – The Night Sky
General
Observing Notes
The 50FL’s
longer light cone makes it usefully easier to focus than, say, the 67FL. Focus
is still very snappy, though.
Cool
Down
Pretty much instant.
Star
Test
The 50FL
shows some under-correction, possibly by design for imaging. But in-focus stars
revealed perfect diffraction rings around a larger than usual Airy disk (small
aperture means large Airy disk). There was no false colour in the star test,
even on Sirius.
The
Moon
A
magnification of 80x with a 5mm (in this case Nagler
Type 6) eyepiece gives a great view of the Moon: super-sharp, high-contrast,
with no false colour and a lot more detail than you might expect from 50mm.
Things were just as good at 100x with a 4mm eyepiece. Upping the magnification
to 114x with a 3.5mm eyepiece still gave an excellent view, with none of the
softness or wash-out I got with the 45ED at higher powers.
I was
surprised by how much I could see on a Moon just past 1st quarter,
with all the major highlights – Pitatus, the Great
Wall, Clavius on the terminator, Tycho
and the Appennines, Rima Hyginus too. Two days later
I enjoyed Sinus Iridium on the terminator, with its wrinkle ridges and the
pointed shadow of Promontarium Laplace.
To put this in
context, views of the Moon resolved a little less than my old Questar, but were
very comparable with the much larger Borg 67FL. All the features in my ‘Photogaphic Lunar Atlas’ by Chong, Lim and Ang were
discoverable. For a 50mm scope, that’s quite something.
Jupiter
In poor seeing I was easily able to
make out Jupiter’s equatorial cloud belts and darkened polar regions.
Mars
Early in an opposition year, with Mars
just 5” across, the 50FL revealed a minute orange disk, crisply defined and
with no false colour at 114x.
Deep
Sky
What can you
expect from 50mm? Actually, I was surprised by the deep sky performance of the
50FL. It is almost flat to the edge with a good eyepiece and delivers very contrasty views of bright DSOs – thank the front surface
fluorite and excellent coatings.
The Orion
Nebula showed lots of extended nebulosity and a hint of colour at 27x with a
15mm Panoptic. Likewise, The Pleiades were sparkly diamonds in mist – perfect.
A 50mm
aperture was never going to be great for doubles, but the 50FL easily split
Castor. I might have glimpsed Rigel B.
The 50FL gave
the best views I’ve seen of deep sky through any 50mm optic (premium binos included).
Summary
The 50FL is another small refractor
that isn’t quite what I expected. It is extremely sharp and contrasty
at high power and gave lovely views for such a small aperture. Views of the
Moon were very good up to 100x and more, similar to something like an 80mm Maksutov - pin-sharp and full of detail, suggesting the
lens is very well figured and polished. But the 50FL isn’t just a visual scope for
high-powers on the Moon and planets.
The 50FL is a front-surface fluorite
doublet with the best modern coatings, so it gets out of the way of the light
better than just about any other design. Consequently, visual deep sky is
better than you would expect and so is deep sky imaging.
APS-C imaging of brighter DSOs is
perfectly possible using the 50FL without a flattener - giving sharp stars across
a good-sized field and minimal violet-bloat. The 50FL would be ideal for
prime-focus imaging of eclipses, too. However, with a reducer/flattener, really
wide field deep sky images are possible.
The 50FL is also ideal as a cheap,
super-sharp, low false colour, lightweight terrestrial telephoto lens.
Very highly recommended for wide-field
imaging with a reducer or as a telephoto lens. Also recommended if you must
have the smallest, lightest high-power scope for quick looks at the Moon and
planets or eclipses.
Borg 50FL with its favourite visual
target: The Moon.