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 canít go beyond, then the MiniBorg is the other. If a smaller, lighter, more compact astronomical telescope exists, then I donít know about it. This is a telescope that you could put in a pocket; then again itís 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 itís by far the most significant thing about this unusual range of... well, of telescope parts really. With Borg you donít 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 astroí scope, a guider, a monocular.
That flexibility is great, but itís also very expensive. By the time youíve 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 (Takí 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 ľ-20 mounting for a photo tripod. The OTA construction is light weight, but high quality Ė 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 Ė camera adapters, push fit eyepiece holders, helical focusers etc Ė can be attached in a huge range of configurations. For use as a tiny astroí scope, though, you can get away with Borgís 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.
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 youíve 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 2Ē APO itís down at about F6, hence the MiniBorgís reasonable performance in this respect.
On planets ... well what do you expect from 50mm!? You can clearly see Jupiterís moons, the main equatorial cloudbelts and Saturnís 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 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. Trouble is, I didnít like the result nearly as well as the original 50mm achroí.
The 45mm ED objective looks identical to the 50, but has a longer focal length. Given that 1.22D rule I mentioned earlier, itís impossible to understand why this might be. Making an F5 APO 2Ē 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. Whatís more, the in-focus range is very tight and the draw-tube wonít 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 doesnít 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, youíd be wrong. My 1964 Swift Model 838 (a 50mm F14 achromat) 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.
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.
The Mini 50 has a definite charm and utility. No other scope comes close to being this portable. Itís not an APO, but is really about quick low power looks anyway. Whatís 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 itís 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 canít 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 isnít.
Addendum: recently Borg have introduced a version of the Canon/Optron 50mm F8 fluorite lens (originally found in the Takí FC50) to the MiniBorg range. Despite my negative experience with the 45ED, Iíd love to try it out Ö watch this space!