Borg MiniBorg
50 and MiniBorg 45ED Review
MiniBorg
parts: Drawtube, EP holder, 50mm and 45mm ED Objectives
If my TMB175 is at one end of a spectrum of refractors that I
can’t go beyond, then the MiniBorg is the other. If a smaller, lighter, more
compact astronomical telescope exists, then I don’t know about it. This is a
telescope that you could put in a pocket; then again it’s the same aperture as
a regular binocular barrel or most finders. Everyone wants a truly portable
telescope, but does this one actually do anything useful beyond making a very
configurable (and expensive) finderscope?
The Way of
the Borg
Borgs are modular. Read that again, because it’s by far the most
significant thing about this unusual range of... well, of telescope parts
really. With Borg you don’t buy a
telescope, you buy a kit of parts to
construct the telescope you need. If I was seriously rich, I would buy the
entire Borg parts list just so I could play constructor with it: you can
imagine me sitting cross-legged on the floor of my study making weird looking
scopes surrounded by tubes, lenses and adapters. What fun!
There is a serious side to this approach, though (and a downside
too, you can probably guess what it is). Borgs are very flexible because all
the components use common metric thread sizes. If you want to build a finder
from the MiniBorg parts list, you can - or a terrestrial spotter, a camera
lens, a little astro’ scope, a guider, a monocular.
That flexibility is great, but it’s also very expensive. By the
time you’ve bought the 50mm lens, the drawtube body, a helical focuser (which
you could fit in various places), some extensions and adapter rings to get it
to focus and finally an eyepiece holder, you will have spent the equivalent of
a very decent small fast-food APO.
If you want to equip the MinBorg with a 60mm lens, or an APO lens,
the cost goes up even more and can easily get into premium 60mm APO (Tak’
FS60/Tele Vue TV60) territory.
Design and
Build
The basic MiniBorg 50 has an F5 achromatic doublet similar to that
in many finderscopes, but of good optical quality, attached to a simple draw
tube with a ¼-20 mounting for a photo tripod. The OTA construction is light
weight, but high quality – all metal with a nicely blacked inside and a single
baffle.
The threads on the lens cell and the back of the drawtube are
standard M57 and so all sorts of accessories – camera adapters, push fit
eyepiece holders, helical focusers etc – can be attached in a huge range of
configurations. For use as a tiny astro’ scope, though, you can get away with
Borg’s simple set-screw 1.25 adapter and the diagonal and eyepieces of your
choice. The sliding drawtube works fine as a focuser in most cases.
As I explained above, the Borg modular concept means you can substitute
the basic 50mm achromatic objective with either a 45mm ED APO (see comments
later) or a larger 60mm achromat or ED APO.
In Use
I was surprised at how much I like the Mini 50. It may just be a
finder with interchangeable eyepieces (and an expensive one at that), but it
works well. At lower powers, bright DSOs like the Orion Nebula and star fields
are a delight with a huge wide-field view on offer, even from basic EPs like
Plossls. Critically for a quick-view scope, cool-down is virtually instant, so
you can use it straight from a warm house.
The moon is nice, crisp and contrasty up to about 50x
magnification, after which the chromatic aberration spoils the view a bit. But
at low-medium powers CA is not the problem you would expect if you’ve tried
larger F5 achromats. At, say, 20x there is very little even during daylight
use. Why is this? Is the Mini50 some kind of semi-APO or something? The reason
is that for achromats, you will recall, there is a criteria for acceptable CA
levels of F=1.22D, where F is the focal ratio and D is the aperture in cm. Now
for a six inch refractor this is going to be about F18 (think Cooke refractor),
but for a 2” APO it’s down at about F6, hence the MiniBorg’s reasonable
performance in this respect.
On planets ... well what do you expect from 50mm!? You can clearly
see Jupiter’s moons, the main equatorial cloudbelts and Saturn’s rings, but not
the Cassini division. I tested the MiniBorg side-by-side with a 70mm Russian
Maksutov: the Borg was much easier to use and unexpectedly provided nicer views
on everything I pointed it at, Moon included. For medium power daylight
spotting it works well too.
The MiniBorg
50 proves that for a super-portable small telescope a simple, quality achromat
is all you need.
The Borg MiniBorg 45 ED
The MiniBorg
configured with the 45mm ED objective. Its longer focal length means you need
to thread an extension tube into the mix.
I like the Mini50 at lot, but chromatic aberration meant it
couldn’t go to high magnifications, so I decided to upgrade to the expensive
45mm ED objective with the idea of having a miniature lunar and planetary scope
to take away on trips. This is presumably what Borg intended – larger achromat
for low power views, smaller apochromat for high powers. Trouble is, I didn’t
like the result nearly as well as the original 50mm achro’.
The 45mm ED objective looks identical to the 50, but has a longer
focal length. Given that 1.22D rule I mentioned earlier, it’s impossible to
understand why this might be. Making an F5 APO 2” doublet should be easy, or
just buy one from a binocular manufacturer. Anyway, that longer focal length
requires an extension tube to be screwed onto the OTA. What’s more, the
in-focus range is very tight and the draw-tube won’t do: you need to buy a
helical focuser to accurately bring this objective to focus, adding weight and
cost. All this makes it very expensive: the objective alone is over £150.
For the Moon and planets, the 45 ED is little better than the 50,
captures a bit less light and is has a longer focal length and therefore
narrower field (so not as good for star fields). The 45ED is, for some reason, unpleasantly
hard to focus. In practice, the 45mm certainly doesn’t produce any false
colour, but it suffers image breakdown above about 60x (everything goes
washed-out and grainy). This was a big surprise and a disappointment and may be
due to poor polish on the lens: a good 45mm APO should in theory handle at
least 90x.
If you think this is all you can expect from such a tiny aperture,
you’d be wrong. My 1964 Swift Model 838 (a 50mm F14 achromat) has a perfect
star-test and can do remarkable things for a 2” scope, showing considerable
Lunar and planetary detail: four belts, dark polar hood and some dark storms on
Jupiter, for example. It can also split doubles down to the Dawes Limit of
about 2.3” and easily takes 100x magnification and more.
Needless to say the Mini 45 falls well short of this standard,
confirming my suspicions of poor optical quality, which is unacceptable
considering the premium price. If you must have a 50mm APO, then consider
hunting down a Takahashi FC50 on the used market instead.
Summary
The Mini 50 has a definite charm and utility.
No other scope comes close to being this portable. It’s not an APO, but is
really about quick low power looks anyway. What’s more, that modular approach
means if you get bored with using it as a quick-look scope, you can turn it
into a finder (which I did), or a straight-through terrestrial spotter (which I
also did). Although it’s an F5 (with a vast field as a result), chromatic aberration
is not a problem. At moderate powers the views are sharp and bright.
The 45ED is a lot less charming, though maybe
mine was a bad one. Even if so, I can’t really see the point in paying for a
45mm F7 APO; I should have known better.
The MiniBorg
50 is the most portable astro-scope available and is recommended; the 45ED
isn’t.
Addendum: recently Borg have introduced a version of the Canon/Optron
50mm F8 fluorite lens (originally found in the Tak’ FC50) to the MiniBorg
range. Despite my negative experience with the 45ED, I’d love to try it out …
watch this space!