Borg Mini Borg 45ED Review
The Mini Borg
configured with the 45mm ED objective. Its longer focal length means you need
to thread an extension tube into the mix.
I like the Mini Borg 50 achro 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. I bought it then wondered if I was crazy.
The Mini Borg 45ED may just be the smallest apochromat ever (or at
least it was, until Borg made the 36ED). It boasts less aperture than a regular
binocular barrel and many finders. Everyone likes tiny apochromats, but is
‘apo’ even a meaningful concept at just 45mm? And the 45ED is an expensive
sliver of glass; even used I recently saw one go for almost the price of a used
Tele Vue TV-60, ouch!
A few other 50mm apochromats do exist and Borg make several,
including the 50FL (tested on this site) and the 55FL. But unlike the 45ED,
those two are fluorite doublets made by Canon/Optron,
the very same that made the objective for the original (and now highly
collectable) mini-apo, the Tak’ FC-50. The 45ED
however is not an Optron lens and isn’t fluorite
either. Does that matter? Here I aim to find out.
At A Glance
|
Telescope |
Borg Mini Borg
45ED |
|
Aperture |
45mm |
|
Focal
Length |
325mm |
|
Focal
Ratio |
F7.2 |
|
Length |
168mm |
|
Weight |
380g |
Data from Me/Hutech.
Design and Build
Borg are less a range of telescopes than a catalogue of telescope
parts. This has some big advantages: Borgs are very flexible because all the
components use common metric thread sizes and it’s easy to upgrade the
objective or focuser down the line.
That flexibility is great, but it’s also very expensive. By the
time you’ve bought the lens, the drawtube, 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 upgrade the Mini 45ED to a 60mm ED lens the cost
goes up even more and can easily get into premium 60mm APO (Tak’
FS60/Tele Vue TV60) territory and beyond.
Optics
The Mini Borg 45ED is an F7.2 ED doublet. It’s an apochromat – it
says so on the dew shield. Indeed, whilst we don’t know what type of ‘ED’ glass
they used, you might expect that a 45mm F7 ED doublet
would be effectively false-colour-free. After all, that’s the same recipe as
the much larger TV-85, which has an excellent reputation.
Interestingly, you’ll note that the 45ED OTA is 40g heavier than
the 50 Achro, despite being physically identical. Is
this the result of all those heavy metal atoms in the ED glass (joking, I
hope).
Tube
This is the standard basic MiniBorg set,
except that it needs an extension tube due to the 75mm extra focal length of
the 45ED objective. You could build a Series 80 tube set using a bit of Borg
magic in the form of adapter #7459 (see below).
In the case of this Mini Borg OTA, the objective is attached to a
simple draw tube. The OTA construction is light weight, but high quality, with
a nicely blacked inside, a single knife-edge baffle and no plastic.
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.
As I explained above, the Borg modular concept means you can
substitute the 45mm ED APO lens for a larger 60mm achromat or 60mm ED apochromat,
extending the OTA with additional tube sections if required.
Focuser
The sliding drawtube shown above works fine as a focuser with the
50mm achromatic objective, but the 45ED needs a finer focuser. There are
numerous helical focusers to choose from and Crayford and rack-and-pinion
options too. The most commonly encountered in a Mini Borg OTA is the tiny
1.25”-only helical shown below that threads into the Mini Borg draw tube. I
eventually resorted to buying one for the 45ED.
A Mini Borg
50mm objective in a Series 80 tube with larger helical focuser. This is the
50FL.
Mini helical
focuser, threads into the Mini Borg tube.
In Use – The Night Sky
General Observing Notes
The 45ED is, for some reason, unpleasantly hard to focus. In
practice, the Mini 45ED certainly doesn’t produce any false colour though, just
as theory suggests.
The Moon and Planets
Disappointingly, given my reasons for buying it, the 45ED is
little better than the 50 Achro for the Moon and
planets - not due to false colour like the 50, but because it suffers image
breakdown above about 60x (everything goes washed-out and grainy). This was a
big surprise and may be due to a poorly polished 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. The 1964 Swift Model 838 (a 50mm F14 achromat) I reviewed 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. A Zeiss 50/540 – another ancient 50mm achromat - is much the same.
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.
Deep Sky
The 45ED captures 23% less light than the 50 Achro.
It has a longer focal length and therefore a narrower field as well. The upshot
is that it’s not as good for star fields or for the bright DSOs that the 50
displays quite nicely.
Summary
The Mini 50 Achro has a definite charm and utility. It’s cheap (for a
Borg), effective and ridiculously portable. It’s not an APO, but at moderate
powers the views are sharp and bright, day or night.
The 45ED spoils
those good things and is a lot less charming, though maybe mine was a bad one. Yes,
it’s CA free, but extremely dim and unable to take significant magnification.
If you think that’s just down to its small aperture, you should try a Zeiss
50/540 achromat.
If you must
have a miniature apochromat (and I have
always wanted one for reasons I can’t explain, so I’m not judging), then Borg’s
own 50FL or Takahashi’s original FC-50 are a better bet.
The MiniBorg 45ED isn’t recommended, if
this sample (bought new from Hutech) is anything to
go by.
If you want a
miniature apo, this one might make you happier than the Borg 45ED: Takahashi
FC-50.