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Borg 76ED Refractor
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Borg 76 ED
Fig.1 - The little APO stands on a balanced fluid video head for easy star gazing.
I bought this little APO in Spring 2002 to replace a Konus Vista 80 mm F/5 (one of the many short-tube variations). The short-tube turned out to be very good "rich field" telescope, but sharp views were limited to low magnifications (less than about 40-50x). I previously owned a ETX 90, which conversely was good for relatively high magnifications, but not for rich fields (no greater than about 1° true field of view).
My expectations, for this third little scope, were to get both rich fields and sharp views at (moderately) higher magnifications. It had to be a travel scope, to carry in vacations, and meeting airline cabin luggage restrictions. Outstanding planetary performance and very high magnifications was not necessary (for planets I had got bigger guns). Cost effectiveness was more important than being able to reach the very edge of high magnifications of its size. I did not plan to do any photography; but indeed the scope has photographic attitudes, which I never exploited.
A lot of information can be found about this scope, beginning from the manufacturer site. I will not go into details that can be easily found elsewhere. I only mention that the scope is designed around a "modular" paradigm, which combines different parts to obtain different optical sets. Mine is one of the pre-defined "standard" sets, based on a 76 mm ED doublet 500 mm focal length, an extendable tube, a helical focuser with micro metric scale (revealing the photographic aims of the designers), a 7x50 finder.
A travel scope
Fig.2 shows a travel set, composed of the collapsed scope, finder, 1.25" diagonal, two Pentax eyepieces, one filter, and a barlow. As shown the overall size is similar to a typical 15" notebook (except thickness) and it is evident that it may fit into a shoulder bag that can be carried everywhere, including airline cabins, mountain trails, camel trips into the desert, scuba diving :-) etc.
Fig.2 - A shoulder-bag travel set, compared to the size of a compact 15" notebook.
When the scope is in use the tube is extended thanks to a "draw tube", which is locked by means of two knobs, as shown in Fig.3. A third knob locks the focus for photography. Additional extension tubes may be fitted behind the helical focuser, should they be needed - but they are not needed when a diagonal is in use.
I place the assembled scope onto a photo tripod with fluid head, as shown in Fig.1 (Manfrotto 190 tripod and 128RC head). A counterweight balances the scope so that the clutches are never locked, but used only to provide the right "fluid" resistance. The result is a scope that can be used in a dobson-like fashion, panning the night sky and star hopping. At low and medium magnifications (up to 100x) the mount is petty stable. Above 100x it is still acceptable. The only drawback is that the balance mass weights almost as much as the scope (2kg). In case of airline travel I think it is better to put the tripod and counterweight into the registered luggage (rather then trying to carry it into the cabin). Alternatively one might buy those heads that use springs to produce virtual counter weights. Other solutions are also possible.
One defect of the draw tube is the existence of a little play, which causes small misalignments between scope and finder when the two knobs are tightened. With practice one learns how to tighten the knobs in order to best repeat the tube radial position. Another source of misalignment between finder and scope is due to the finder socket which, unlike other designs, has play. Again, with practice, one learns how to best repeat finder-scope alignments.
The above are the only two mechanical imperfections that I found. The remaining of the scope is well built and meets my expectations. Honestly I would have preferred a Crayford focuser, but I have to admit that the helical focuser performs close to a single speed crayford focuser and surely much better than a rack and pinion one. I think that only dual speed crayford design can be noticeably better.
Fig.3 - Detail of the extension tube.
OpticsAnd here is the main part of the review: how good are the optics and how does the scope perform?
Let us first look at coatings. Fig.4 shows the reflected images of a bright halogen lamp. From left to right they are produced respectively by: a Pentax XL14, the Borg 76 ED, a Chinese 10x50 binocular (top), another Chinese 15x70 binocular (bottom). The photo is a little dark because exposition is set on the bright reflections. The inserts show magnified views of the reflections, for better evaluations.
Fig.4 - Coating performance (left to right: Pentax XL14, Borg 76 ED, 10x50 and 15x70 Chinese binoculars).
The Pentax eyepiece (left) makes dim reflections (notice the brightness of the parabola compared to the bulb); of course there are many reflections because of the many lenses. In comparison the 15x70 binocular (bottom right), appears to be surprisingly good, close -if not better- to the eyepiece. Conversely the 10x50 (top) is poorer (actually it has coatings, but they are not very effective). The reflections of the Borg lens are somewhat brighter than the eyepiece or the good binocular; but better than the poor binocular. Bright spots are seen in the enlarged image of the 76ED lens. They are due to small stains: in fact, the lens has never been cleaned. If they are not considered, elsewhere the brightness of the 76ED reflection is closer to that of the (pristine) good binocular.
In daylight use a difference in light throughput between the two binoculars is obvious (that difference probably includes the effects of the coatings of all the remaining surfaces -prisms and eyepieces-, not only the objectives). At night reflections and ghost images, especially near the moon and bright lamps, are seen in the 10x50 but not in the 15x70. In other words I consider the 15x70 as a reference point for "good coatings".
The Borg, which I use with Pentax eyepieces, does not show ghost images. I think that a doublet, like the Borg, is not as critical as an optical system with many surfaces, like the eyepiece or the binocular. That may justify, in my opinion, the slightly reduced specifications for its coatings.
In fact, besides having no noticeable internal ghosting, the telescope seems to have very good light throughput, especially if compared to typical SCT. In several occasions, with naked eye sky limiting magnitude of 6 or slightly above (Bortle class 3), I have been able to record stars of magnitude about 13. In the same conditions I recorded limits of about 14.2 for my 8" SCT , 14.7 for a friend's 10" SCT, and 16 for my 16" dobson. Contrast is also very good: the dark side of the moon is seen past the first quarter, dim stars near the moon limb are easy, and the dim parts of nebulae (e.g. the Big Lagoon) are bright and not confused with stars halos (there are not halos except at high resolution, see below).
High resolution. Fig.5 shows an artificial star test done by imaging the Sun as reflected on a small pin cap (if someone plans to do similar tests I recommend to be very careful: to choose dim and small reflections, not stare at them in the eyepiece -better to risk the replaceable camera sensor- and not to point the Sun for any reason). The pin was set ten meters apart, which is not infinity but only 20 times the focal length and which may have introduced some additional spherical aberration. In addition there are little spurious reflections.
The astigmatism shown in the picture is not always present. I have found two causes for it: one is excessive tightening of the barlow lens, the other is incomplete cooling (astigmatism is the last sign to disappear after cool down). Otherwise, especially in mild summer nights, the scope shows perfect round rings (and definitely better resolution) and perfect collimation.
Intra focal (left) and extra focal (right) ring patterns show obvious differences in my opinion. Above all, the intra focal image is softer. Blue comes to focus first than red (the residual chromatic aberration). In monochromatic light the star test would look much better. The three central shots are relative to near focus. They are overexposed: the bright central spot is the airy disk and first ring fused together. Over exposed pictures show the diffused light and the second and further rings. At best focus (between second and third shot) there remains both a slight amount of diffused light and the rings are slightly brighter than I would expect for an unobstructed scope. I would rate the first ring (as I saw under the sky) as bright as a typical 35-40% obstructed SCT (not as dim as it should be in an unobstructed telescope).
In evaluating Fig.5 note that magnification, thanks to the zoom of the digital camera, is very high. The bright spot (first ring+Airy disk) looks almost a "point" in direct view at 100x. To have an idea of what the rings (bright stars) look like in a star test under the real sky one should shrink the pictures like, for example, in Fig.6.
In fact, under the sky at best focus there is no noticeable sign of color (centre below). The moon limb has no color and planet colors are "natural".
Fig.6 - Appearance of star test under the sky at medium high mags.
The scope suffers for some residual SA, in my opinion, and for the "softening" and diffused light. When I first saw the above star test I called the shop, fearing it was defective or a poor sample. They were kind and allowed me to star test another. The comparison showed a very similar behavior. Thus I believe it is a design trade-off. In fact the scope is only a doublet with ED glass and has to fulfill many contrasting requirements: cost, short focal length for portability, wide flat field for photography, resolution etc. I personally would have liked a little more of resolution at the expense of photographic performance (not at the expense of portability and focal length), but I understand that perhaps many other people would prefer the opposite.
Thus, how does the scope really behave under the sky in terms of high resolution?
As for what concerns roll-off magnification I would say the images start to soften at about 100-120x (35-40x per inch), but in good seeing and thermal conditions I have been able to use magnifications of 150x and above (I mean useful magnification). On Jupiter, to make one example, I have been able to see the GRS occasionally. But there is not that much to see on one planet either at 120x or 150x (In fact I prefer the views with 10" and 16" dobsonians at 400-600x).
A double star that I have found to be a test bench for this scope is epsilon Bootis. Its separation and brightness difference is such that the secondary star happens to be on the first ring and is not much brighter than the ring itself (ring is brighter than it should be in a perfect unobstructed scope). The double is "split" in the sense that a brightening is seen on the ring.
Below 100x views are definitely crisp (I have acute sight and still see the airy disk). 100x is also the highest usable magnification for some deep sky viewing (the scope definitely has deep sky ability, especially in dark sky). Star clusters look fine and faint stars appears up to that magnification. Galaxies like M81 and M82 look fine.
I like this scope and it responds to my portability, cost, rich field and medium-high magnifications expectations. I would have preferred a little more of crispness at very high magnifications at the expense of photographic performance (which I do not exploit) but not at the expense of compactness and cost. Although I own a couple of medium and big dobsonians I keep this scope in case of a trip in the African desert....