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BST Starguider Binoviewer review

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A review of a budget Binoviewer.

As a preamble, binoviewers split the image so that it can be viewed by both eyes. These were originally invented for microscopes, where they are usually described as 'binocular viewers'  or the like, and seem to be in common use. If you are good in the workshop it is possible to convert a microscope binocular viewer for astronomical use - the used microscope viewers tend to be cheaper.  Why use a binoviewer? Essentially because two eyes are better than one. If you want to see what a binoviewer might do for you without investing a three-figure sum, try looking at this screen (or your HDTV) using just one eye, then the other one, then both together. Which is better?   So why are they not universally used? (1) cost and complication. (2) Light loss. (3) Adds about 10cm to optical path length.

BST Starguider appears to be a trading mark of "Sky's The Limit" who supplied this item at a cost of £120.  They say that it is identical to the Arcturus binoviewer sold in the USA.  The device and packaging however are not marked with any name or identifying marks. In general appearance it looks like other budget binoviewers (a lot of the budget binoviewer brands come from the same factory.) It comes with two 32mm Plossl eyepieces, a x1.8 Barlow and a x3 Barlow, which is more accessories than most of these outfits offer.

The unit appears fairly well made and is packaged with sealing bags, and various plastic caps. The eye-spacing is controlled by a hingeing arrangement (like conventional binoculars) The eyepieces are gripped by compression rings and at least one can be individually focused. The eyepieces are claimed to have a FOV of 48 degrees with 22mm eye relief. They are unmarked and look budget, and I suspect that given the clear aperture of the binoviewer itself is more like 25mm, some vignetting must occur.

I tried the binoviewer first in daylight, using my 127mm Mak on a AZ-4 mount. In summary, it does work, and gives a distinctly 3-D view of satellite dishes and the like. The aluminium nosepiece can be unscrewed and replaced by one of the Barlows.  Getting focus in a Mak cassegrain telescope was no trouble at all, but the magnification seems higher than expected. Hopefully the Barlow lenses will allow focus to be attained in other designs of telescope.

At night, on Mars and Venus, I could not get a clean image with my right eye without the Barlow lenses, but with x1.8 I could get a clean image with both eyes. The problem seemed to be with my eye, as I re-tried after rotating the binoviewer through 180 degrees. I compared the results with a regular 15mm Plossl and confirmed that the binoviewer results were acceptable considering the observing condtions.

On the Moon there was a striking 3-D effect, and picking out fine detail seemed less of a strain when using both eyes.  Castor (double) was well split with the x1.8

Last night I tried it on Jupiter with the x1.8. (magnification x100 - x120?) The seeing was awful, but I could make out two cloudbelts which seemed to have uneven edges. I have seen (glimpsed) more detail with regular eyepieces in previous seasons, but using both eyes seems less of a strain.

I am still waiting for a 20mm Plossl eyepiece  to make up a 20mm pair.

I bought a second 20mm Revelation Astro Plossl and was able to try them on 14th March. I found that with these I had difficulty in merging the images. This is not a fault with the binoviewer; the 32mm eyepieces still work fine. One of the Revelations has a safety groove on the barrel while the other has not. It dawned on me that the 32mm Plossls supplied are specially designed to minimise the path length, as the entire barrel apart from the eyecup goes inside the eyepiece holder. Not so with the typical astro eyepiece! With some fiddling around I was able to get the images to merge, and found that the 20mm eyepieces work quite well on Jupiter. Some extra winding of the Mak focus knob is needed to get them in focus (it is easier to do this on a bright star or planet).  On the Moon, the magnification seemed higher than nominal. With the x1.8 Barlow there was marked off-axis chromatic aberration, worse than with the 32mm eyepieces.

In summary, binoviewing does work, and if you want to try it out with an entry-level device, this model, at half the price of some of the other popular binoviewers, seems worth buying.


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