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

At A Glance. 1

What’s in the Box?. 1

Design and Build. 1

Tube. 1

Focuser. 1

Mounting. 1

Accessories. 1

In Use – Astrophotography. 1

In Use – Terrestrial Photography. 1

In Use – Daytime. 1

In Use – Observing the Night Sky. 1

General Observing Notes. 1

Cool Down. 1

Star Test. 1

The Moon. 1

Venus. 1

Jupiter. 1

Deep Sky. 1

Summary. 1

At A Glance

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.

Later, you can use the parts kit to build new configurations, either with the same objective or a new one.

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

This objective is an air-spaced 55mm/250mm (F4.5) Fraunhofer (positive crown at the front) fluorite doublet made by Canon/Optron.

Unlike most of Optron’s Takahashi lenses, but like most Borgs, the lens isn’t just a simple foil-spaced design – it has a more substantial, though not huge, air gap. This allows better correction because the lens designer can use the gap as a virtual lens element, but it adds cost (perhaps explaining why, like all Borg FL objectives, this is a costly item).

Due to the short focal ratio, the front fluorite element has strong curves and is remarkably thick in the centre for such a small lens.

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.

But as we will see, some magic with the air space and perhaps a fancy flint glass means false colour is low, though not completely absent. Unlike many cheaper objectives, reflections suggest all surfaces have top-line multi-coatings.

But a large air space can cause problems. 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 I’m assuming a 1.25” diagonal for visual use. 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 imaging or terrestrial telephoto.

The DGQ55 reducer slots into the 7604 extension as the main ‘tube’. The DGQ55 ends in an M57 thread so you then just need the adapter and camera mount, no other spacers required.

This set focuses from ~5m to well past infinity, for terrestrial or astronomy.

This OTA/Lens is 215mm long, weighs 750g including the camera adapter.

Option 2 – F4.5 Drawtube (optional focuser) Terrestrial/Astro visual

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.

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 set 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.

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.

This OTA weighs 575g without the ring and is 245mm long, but Option 3 is even lighter for astronomy only.

Option 3 – F4.5 Astro-Visual with no drawtube for 1.25” Eyepieces

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 to convert an F3.6 astrograph to visual: you need only buy the 7522 adapter plus an eyepiece holder 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. But note this OTA focuses down to only ~~50m.

So here the 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).

Note: the widest 1.25” eyepieces will give you a 6.2° FOV – about the same as traditional 10x50 bino’s. 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

Perfect focus is an absolute point, so you need 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 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, just the objective, it is always smooth 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.

The 55FL’s light weight and a short FL mean you could probably get away with a mini tracking mount like a Vixen Polarie for imaging with a DSLR. 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 old 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 reducer which makes for a 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 the 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.

Full-frame snap 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

Despite that steep light cone, Naglers give a perfectly corrected field with pinpoint stars to the stop.

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.

I mainly used T6 Naglers and found that they seemed to suite the 55FL well. The F4.5 light cone will challenge some eyepiece types.

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.