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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 couldnít 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 Ė 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 Ďapoí 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 Takí FC-50. The 45ED however is not an Optron lens and isnít fluorite either. Does that matter? Here I aim to find out.

At A Glance


Borg Mini Borg 45ED



Focal Length


Focal Ratio






 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 itís easy to upgrade the objective or focuser down the line.

That flexibility is great, but itís also very expensive. By the time youíve 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 (Takí FS60/Tele Vue TV60) territory and beyond.


The Mini Borg 45ED is an F7.2 ED doublet. Itís an apochromat Ė it says so on the dew shield. Indeed, whilst we donít know what type of ĎEDí glass they used, you might expect that a 45mm F7 ED doublet would be effectively false-colour-free. After all, thatís the same recipe as the much larger TV-85, which has an excellent reputation.

Interestingly, youíll 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).


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 Ė camera adapters, push fit eyepiece holders, helical focusers etc Ė 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.


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 1.25Ē-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 Ė The Night Sky

General Observing Notes

The 45ED is, for some reason, unpleasantly hard to focus. In practice, the Mini 45ED certainly doesnít 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, youíd 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 2Ē 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 2.3Ē and easily takes 100x magnification and more. A Zeiss 50/540 Ė 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 itís not as good for star fields or for the bright DSOs that the 50 displays quite nicely.


The Mini 50 Achro has a definite charm and utility. Itís cheap (for a Borg), effective and ridiculously portable. Itís 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, itís CA free, but extremely dim and unable to take significant magnification. If you think thatís 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 canít explain, so Iím not judging), then Borgís own 50FL or Takahashiís original FC-50 are a better bet.

The MiniBorg 45ED isnít 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.



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