Borg
55FL Review
Borg has produced a lot of different
objective options over the years, some great, some not-so-great. I loved the 67
FL, the 45ED not so much.
But if you want one tiny do-anything
scope – from birding to imaging – Borg’s approach has a lot
to recommend it. Buy the objective and then add parts to build the different
OTA configurations you need.
One of the most tempting options for
this is the 55 FL on review here. It’s a proper fluorite doublet made by
Optron like a Takahashi, but with a very short focal ratio of F4.5. The
dedicated flattener drops that to just F3.6 – ideal for very wide field imaging
or as a high-fidelity telephoto.
Note: this is a long and complex review
with lots of Borg part numbers.
Contents
Option 1 – F3.6 Imaging/Telephoto
Option 2 – Terrestrial/Astro visual with drawtube
(optional focuser)
Option 3 – F4.5 Astro visual without drawtube
In Use – Terrestrial Photography
In Use – Observing the Night Sky
At A Glance
|
Telescope |
Borg 55FL F3.6 Astrograph/telephoto (tube Option
1 below) |
|
Aperture |
55mm |
|
Focal Length |
200mm |
|
Focal Ratio |
F3.6 |
|
Length |
215mm w/o caps |
|
Weight |
750g incl. camera adapter |
|
Telescope |
Borg 55FL F4.5 terrestrial/astro-visual (tube Option
2 below) |
|
Aperture |
55mm |
|
Focal Length |
250mm |
|
Focal Ratio |
F 4.5 |
|
Length |
245mm |
|
Weight |
575g |
|
Telescope |
Borg 55FL F4.5 Astro-Visual (tube Option 3 below) |
|
Aperture |
55mm |
|
Focal Length |
250mm |
|
Focal Ratio |
F 4.5 |
|
Length |
235mm |
|
Weight |
480g min |
Data from me.
What’s in the Box?
Lots of smaller
boxes. Little green ones! Here for your pleasure I’ve included just the
main ones for the objective and reducer.
Design and Build
All Borgs follow the same basic design
philosophy: choose an objective, then build up the OTA(s) you need from a huge
set of thread-together high quality but ultra-light components. Then later you can use the parts kit to build new
configurations, either with the same objective or a new one. But note that here
I’ll only cover some of the possible configurations.
(NB: Save
money with this one weird trick! Don’t overtighten your Borg components,
the threads can seize so they can’t be freed.)
Borg 55FL configured for astro/visual
next to a TV Ethos eyepiece for scale.
Optics
The Borg
55FL is an air-spaced 55mm/250mm (F4.5) Fraunhofer (positive fluorite crown at
the front) doublet made by Canon/Optron.
For a
doublet, F4.5 is very fast. For comparison, Takahashi’s FS-60, also an
Optron fluorite doublet, is F5.9 as is WO’s ED-doublet Zenithstar 61. Tele Vue’s venerable TV-60 is F6. Due
to this, the 55FL’s front fluorite element has strong curves and is
remarkably thick in the centre for such a small lens. Unlike many cheaper
objectives, reflections suggest all surfaces have top-line multi-coatings. These
factors (and perhaps a fancy flint element) help explain why this is a
typically expensive Borg objective.
Unlike most
of Optron’s Takahashi lenses, but like most
Borgs, the lens isn’t just a simple foil-spaced design – it has a
substantial air gap between the crown and flint. The large air gap means a more
complex cell and may also add to cost. It allows better correction because the
lens designer can use the gap as a virtual lens element and so false colour is
low for the spec’, though not completely absent. However, a large air
space can cause problems beyond cost. Per Roland Christen’s blurb for
Astro Physics’ 110 GTX:
“Wide airspaces cause two problems that we
want to avoid - one is sensitivity to decentering and
the second is long cooldown time.”
Cooldown time isn’t an issue with such a small lens,
but I have seen a few objectives with wide air spaces that suffer decentring,
including a Takahashi Sky90, a Takahashi FOA-60 (returned by me to the dealer)
and a Tele Vue NP-127. Sadly, as we will see, this particular 55FL does too,
but I assume this is a fault and isn’t typical.
Tube
As usual
with Borg there is no fixed OTA setup (see above), but let’s look at some
possibilities. Options 1-3 are the ones you’d likely pick, but there are others
you could try.
In all
cases, the tube is M57 and the focuser (if used) is a helical behind the
objective - see the focuser section for details. Adapters exist for
Borg’s larger Series 80 tube, but excessive light path may be a problem
for this objective.
Note also
that for visual use I’m assuming a 1.25” eyepiece holder and a
diagonal. For straight through, you’d need an extra extension tube in the
mix.
Option 1 – F3.6 Imaging/Telephoto
R to L:
·
55FL
objective unit
·
M57 Helical
focuser (part 7761 or 7758)
·
60mm
extension tube (part 7604)
·
Reducer DGQ55
·
Oasis M57
Camera Mount Adapter (part 7000
– easy to remember!)
·
Borg wide
camera mount for your choice of camera (part 500X,
e.g. 5005 for EOS)
This
is the set you would most likely choose for astro-imaging or terrestrial
telephoto and ends in the camera adapter of your choice. It focuses from ~5m to
well past infinity and operates like any manual prime.
The DGQ55
reducer slots into the 7604 extension as the main ‘tube’ and
projects into the focuser, but this isn’t a problem. The DGQ55 ends in an
M57 thread so you then just need the adapter and camera mount, no other spacers
required. It’s a very neat and compact solution.
This
OTA/Lens is 215mm long, weighs 750g including the camera adapter.
Option 2 – Terrestrial/Astro visual with drawtube
(optional focuser)
R to L:
·
55FL
objective unit
·
OPTIONAL 7761
or 7758 focuser OR 20mm extension tube (part 7602)
·
Mini Borg
Barrel DX-S’ drawtube assembly (part 6010)
·
Borg M57 to
T-2 adapter part 7522
·
Borg short
eyepiece holder (here replaced by Baader’s)
This
the set you’ll likely choose for visual use only. It focuses from
infinity down to just ~1.5m with the focuser or ~ 4.5m without, so you could
use it for birding, general nature viewing, or even insects, as well as
astronomy.
The DX-S
drawtube assembly is the main ‘tube’. You could get the version
with an attached ARCA dovetail (part 6011). The DX-S drawtube is a nice piece of kit
– light and beautifully engineered, internally lined with flocking
material. Note: I’ve used Baader’s ‘Ultra-short’ T-2 eyepiece
holder rather than Borg’s – it’s cheaper and has a clamp ring
instead of a set-screw. This OTA weighs 575g without the ring and is 245mm
long, but Option 3 is even lighter for astronomy only.
Note: don’t
want the weight/expense of a focuser but do want the ultra-close focus? Replace
the focuser with a Borg 7602 20mm M57 extension. Want to try imaging at F4.5?
Just replace the T-2 adapter and eyepiece holder with the Oasis Camera Mount
Adapter (part 7000) and a Borg wide camera adapter (part 500X, e.g. 5005 for
EOS) for your camera.
Option 3 – F4.5 Astro visual without drawtube
R to L:
·
55FL
objective unit
·
7761 or 7758
focuser
·
60mm M57 extension
tube (part 7604)
·
20mm M57 extension
tube (part 7602)
·
Borg M57 to
T-2 adapter part 7522
·
Borg short
eyepiece holder (here replaced by Baader’s short T-2)
This
is the lightest set and the easiest way to convert an F3.6 astrograph to visual:
you need only buy the 7522 adapter, an eyepiece holder and the 20mm extension, all cheapish items, if you own the astrograph reducer set (Option 1
above).
The 55FLs
light cone is so steep that for
astronomy the helical
focuser’s travel is sufficient for every 1.25” eyepiece I tried. So
here the drawtube is omitted and a Borg 60mm M57 extension (part 7604) is the
main ‘tube’ with a further 20mm (part 7602) extension tube. Alternatively
just use the 7604 extn with a longer eyepiece holder. This OTA is incredibly
light and short at 480g/235mm - even with a ring, about a third the weight of most
60mm F6 doublet OTAs (e.g. a Takahashi FS-60). But note this OTA focuses down
to only ~~50m, so isn’t ideal for terrestrial use.
The widest
1.25” eyepieces (a 32mm Plössl etc) will give you a 6.2° FOV –
about the same as traditional 10x50 bino’s. But if you want even more for
an ultra-rich-field scope, you could build an OTA with a 2” eyepiece
holder (e.g. part 7509), but again too much light path may be an issue.
Focuser
The steep
F4.5 light cone necessitates a fine focuser, both for imaging and high power
visual (for lower powers you can get away with just a drawtube). The only focusers that are both fine
enough and work in terms of light-path length are Borg’s M57 helicals, positioned behind the objective.
Borg
recommend the 7761, but in the photos you see its forerunner, the 7758, which
has a coarser action but is otherwise similar. The 7758 focuser works well and I
found it precise enough, both for visual up to 100x and imaging/telephoto. Because
it doesn’t have to support a heavy diagonal/eyepiece or camera, it is
always smooth and free of image-shift. Compare the rear-mounted 7835 M68.8
focuser in my 90FL review which got stiff and graunchy under load.
Mounting
The 55FL is
ridiculously light and so for visual use it is stable on literally any photo
tripod and head, even ones intended for compact cameras. For imaging with a
DSLR, you could probably get away with a mini tracking mount like a Vixen Polarie. I intend to try this and report back. Larger
travel mounts will take it with ease.
The head in
the photos is a Manfrotto MVH400AH ‘BeFree’ (yeah, I know) on a cheap carbon tripod. The
whole setup weighs a couple of kilos and can be picked up in one hand. It vibes
more than I’d like but is usable up to 100x once it settles.
Accessories
Borgs are
all accessory, and you could play around with focusers, eyepiece holders,
various ring and plate options etc.
For imaging
you’ll need a reducer or flattener. The 55FL gets its own dedicated 0.8x
reducer, the DGQ55 (part 7880), that takes the focal length down to 200mm
(F3.6): see above.
To attach a
camera to the reducer, you could use the M57 T-ring adapter (part 7522), but
the camera adapter 7000 and a Borg wide-T will give much better coverage. Borg
make wide-T camera adapters for most DSLR and compact lens mounts.
The cheap
0.85x reducer (part 7885) might work, but I haven’t tried it on the 55FL.
I had good results for APS-C with the 67FL, but spacing is awkward. Borg sell a
generic 1.08x flattener (part 7108) and a 1.4x extender (part 7215) if you want
more image scale.
If you want
a rotator, Borg do a miniature one with an M57 thread (part 7352), but check Borg’s
assembly diagram carefully to get the spacings right.
In Use – Astrophotography
The 67FL was
a mess of adapters for imaging; not the 55FL/DGQ55 which makes for a neat and
simple OTA (see tube Option 1 above).
Optically it is impressive – at
200mm/F3.6 it’s fast and has a huge and well-corrected field with excellent
coverage at full-frame. I’ll add some deep sky images when I have some
good ‘uns to share, but for information, below
is a 1000x1000 pixel bottom-left corner crop of a random star field.
The
corner crop shows just very minor distortion of stars at the field corner and
minimal vignetting with Borg’s own wide-T adapter. You can see the minor
objective decentring here too.
At 250mm/F4.5
(I.e. without the DGQ55 reducer), the 55 FL has a very curved field, but
you might be able to correct (some of) that in software.
The FL55
takes decent snaps of the Moon, but that’s mainly useful for moonrises
and conjunctions. The shot below of the 2023 Jupiter-Moon-Venus conjunction
wouldn’t have been possible with a narrower FOV; zoom in to see
Jupiter’s moons even at the field edge – most telephotos
wouldn’t do that.
100% bottom-left
corner crop with Borg 55FL F3.6/Canon EOS 6D MkII 30s
at ISO 3200.
Feb 2023
conjunction of Jupiter, Moon and Venus: Borg 55FL F3.6/Canon EOS 6D MkII.
100% crop of
original frame alongside pasted zoom-in of Moon with Borg 55FL F4.5/Canon EOS
6D MkII.
In Use – Terrestrial Photography
For
terrestrial photography the 55FL + DGQ55 reducer (tube Option 1 above) makes an
excellent manual F3.6 200mm prime unless you need macro. It focuses
conventionally and is as light and short as an OEM 200mm. It has a good range
of focus (down to about 4-5m). It is fast enough to hand hold, with no
vignetting at full frame, low fringing and excellent sharpness edge to edge.
One of the
reasons I bought the 55FL was to image atmospheric Moon rises against
interesting backdrops. It’s great for this with a much wider field than
most astrographs and better optical quality than most telephotos.
Borg 55FL as
a 200mm telephoto.
Full-frame
snaps with Borg 55FL/Canon EOS 6D MkII. Straight from
the camera.
Crops of a
full moonrise with Borg 55FL/Canon EOS 6D MkII.
In Use – Daytime
The 55FL
makes a fabulous daytime spotter, smaller than most prismatic scopes and
sharper. But it’s obviously not waterproof!
My usual test
of viewing silhouetted branches generates a trace of false colour above 50x out
of focus and some in-focus by 100x, but in normal use at spotting scope
magnifications it’s false-colour-free.
The view is
sharp edge-to-edge with a Nagler or Panoptic and not far off with a basic TeleVue Plössl. Most (all?) Tele Vue eyepieces work down to
~F4. Others may not.
With the
drawtube extended it focuses very close – down to 1.5m.
In Use – Observing the Night Sky
Most will
buy the 55FL as a wide-field astrograph or super-sharp 200mm telephoto. But the
benefit of Borg is that you can easily convert it for visual use.
General Observing Notes
I mainly
used T6 Naglers and found that they seemed to suite the 55FL well, giving a
perfectly corrected field with pinpoint stars to the stop. The F4.5 light cone
will challenge some eyepiece types.
The maximum
magnification is ~100x with a 2.5mm eyepiece (a rather specialist item). At
that power, perfect focus is such a fine point that you could almost blow on
the ring of the helical to get it. Still, that helical is precise enough for
the job; many rack and pinion focusers just wouldn’t be.
You
don’t need the drawtube for astronomy, but it does make a nice rotator
– keep it fully retracted then just slacken to adjust the angle of
eyepiece or camera with no image or focus shift.
Cool Down
Instant.
Really.
Star Test
At first
glance, the star test seemed excellent with sharp rings both sides of focus.
Only when I returned to it under better seeing did I notice that the defocused
rings, though properly concentric, were illuminated slightly more to one side.
More tellingly, an in-focus star showed Fresnel rings distorted to one side at
100x – the lens is suffering from some minor misalignment, probably
decentring. For
imaging this is probably a non-issue, but it does prevent the lens from giving
its best at high power for visual use.
The Moon
At 100x with
a 2.5mm Nagler T6 the Moon almost fills the field to give that
‘porthole’ view.
A six-day
crescent is a feature-packed phase with the grouping of the Theophillus,
Cyrillus and Catherina just coming out of the terminator’s black shadow,
Theophillus’ central peaks just snagging the dawn.
To the
north, the wrinkle ridges in Mare Tranquillitatis look exaggeratedly rough and
dramatic. Posidonius shows its central crater and in the south Maurolycus
stands out from the rest.
As far as
false colour is concerned, there is none, not even focusing through the limb;
but at 100x the Moon isn’t especially bright in such a small aperture.
Overall, the 55FL surprises as a really
satisfying quick-look scope for the Moon.
Venus
A small but
brilliant Venusian disk showed minimal false colour, even out of focus, an
excellent result for such a fast doublet.
Jupiter
Jupiter at 100x was free of false colour, but looked slightly unsharp
and didn’t have the contrast it should, likely a result of the decentring
noted above.
Deep Sky
The Pleiades
looked great at 50x with a 5mm Nagler – the whole group fitted nicely in
the field, all pinpoint and sparkly. Other larger DSOs looked as good as they
can at this aperture.
Castor was
an easy split at 100x, but I noticed larger airy disks than I’m used to
from the small aperture.
For
visual use the 55FL comes quite close to a TV60 or Takahashi FS-60C, but
it’s much smaller and lighter than either.
Summary
For what the
55FL brings to imaging – a super wide, flat field – many are buying
dedicated astrographs or telephoto lenses. The 55Fl makes an interesting
alternative.
Yes, the 55FL
is more expensive once you add in the reducer but consider:
1)
With the
reducer it’s an F3.6 astrograph with excellent coverage, low bloat and a
super-wide field
2)
Short and
light and with no attached mounting hardware, it’s a superb terrestrial
200mm telephoto lens, fast enough to hand-hold
3)
Add a short
extension and you’ve got the smallest mini visual astro’ scope,
capable of 100x or more for quick looks or travel, weighs just 600g
4)
Swap the
extension for a drawtube and the 55FL is the sharpest birding/nature scope
you’ll ever use that goes on the smallest head and focuses down to ~2.5m
Imagers. Photographers.
Visual astronomers. Birders – we tend to live in gear-silos, but the 55FL
bridges them all. And unlike
other Borgs I’ve reviewed, it all works with a minimum of adapters and no
gotchas.
The 55FL is the first Borg that fulfils
the Swiss-army-knife promise for me – it literally does everything. Highly
recommended, despite a high price.
