After a break of nearly 20 years, I got bitten by the astronomy bug again. Although I work in the field of intellectual property protection (patents), I studied astronomy at the university. During that time I was able to peek through professional telescopes at prime sites, which killed my enthusiasm for backyard observing. Later, from a suburban location in the Netherlands (mag 4-4.5) I did some casual planet viewing with a vintage Dynamax 8-inch SCT. This scope, with no corrector coating and weird star images, fully showed its age of 25 years. Nevertheless Jupiter and Saturn looked nice, with sometimes a hint of a Jovian satellite transit. Every summer I spend a month in rural France with rather dark sky conditions (Milky Way visible to the horizon, mag 6-6.5), so slowly I got more and more interested in my old passion of deep sky observing. On warm summer nights I would linger over the Sagittarius star clouds with a pair of twenty-year-old 10x70 binoculars which still gave awesome views in a 4?field. Eventually, I concluded that I wanted more aperture and more magnification. For comfort and ease of viewing, I decided to go with big binoculars. There were some peripheral conditions:
As they would only be used for a few weeks, during vacation, they couldn抰 be too expensive. In case I lost interest again, I didn抰 want to be stuck with heavy, costly binoculars.
I wanted at least 80mm objectives
I was fed up with straight-through viewing
I wanted variable magnification, not fixed
I first considered Miyauchis, but in Europe they are very expensive. Moreover, I heard good and bad stories about them, ranging from 損inpoint star images? to 搑ainbow-like, comet-like images.?When I discussed this with the local Vixen importer, he suggested the BT-80M. For those who do not know it, this is an 80mm binoscope with a focal length of 900 mm in a 500 mm tube (they have a built in tele-extender). The BT-80 has 1.25?eyepiece turrrets angled at 45 degrees, using amici prisms. At 5 kilos this scope needs a sturdy mount, so I bought the Vixen Custom-D altaz with counterweight and slow motion controls. It works very well, as you can use it both for sweeping or for detailed study at high magnification using the slowmos. In fact, this mount is so good that on holiday I even used it with a TAL 200K Klevtsov朚aksutov optical tube assembly weighing about 12 kilos. I had to add about 3 kilos to the counterweight but the mount held, although clearly it was pushed to its limits. (There are some more excellent reviews of the BT-80 on Cloudy Nights. I plan to write a review of the Klevtsov in due time.)
The BT-80 in a (selfmade) case, with additional 6x23.5 finder glued to one tube.
My first pair of BT-80抯 was a demo model that I got from the importer. It had some scratches on the tube but looked OK. The importer exchanged the orthos that come with it (which had a ridiculously narrow field of 1.1? with a pair of fine Celestron 25mm plossls, which yielded about 1.5?actual field of view at 36x. I collected the scope on a Friday in December 2001 and, in a rare feat for a newly bought scope, could immediately use it the same night. First object: Jupiter. WOW, the giant planet was floating in a 3D sky surrounded by moons and many stars. I noticed three things immediately:
The 3D experience, which you have to see to believe. Jupiter looked so BIG at 36x. I checked this, and when I closed one eye the image indeed appeared smaller. Somehow the brain (mine at least) gives you the illusion that the image is bigger when viewed with both eyes. With my old SCT, Jupiter at 40x was a tiny disk with two main equatorial belts, but when using the BT-80 I easily saw four bands. The planet was surrounded by stars, which contributed largely to the 3D effect. Using the SCT, I was not used to such dark, glareless backgrounds, which allow you to see dim stars close to bright objects (like Jupiter or the Moon).
I cranked up the magnification to 75x, using a pair of 12 mm Vixen lanthanums ?and yes, a big disadvantage of a binoscope is that changing magnification is very expensive. You need two premium eyepieces, and the BT-80 has a liking for lanthanums. My enthusiasm faded when the big Jovian ball (at 75x) showed fuzzy-streaked features.
Although the view was very impressive, with lots of detail, something was wrong. An examination revealed that the right tube had focusing problems. All I know from star testing is that whatever you see should at least be concentric and circular. The right tube at focus showed Capella with streaks like a small comet, and out of focus it showed a bunch of oval rings having their origin far off-center. As the BT-80 has a factory installed collimation, there was nothing to do but to return it to the importer, who was surprised because this problem did not show up during terrestrial observing. As there were no more BT-80s in stock, he told me he would order two more and I could pick out the best one. They arrived within a month and I took one of them, hoping for the best.
Then it happened. Misery. Storms and rain prevented any test for the next three weeks, so I had to just sit and look at the BT-80, not knowing how it would perform. The sky finally cleared, and again first light was on Jupiter and Capella. The BT-80 performed beautifully, showing only a violet haze around brightest objects (after all it is an achromat) 卋ut only when using the lanthanums at 75x because the 25mm Celestrons did not come to focus. I discovered that the turn and clamp barrels of the BT-80 are 31.75mm inside diameter exactly, which means zero tolerance for oversized eyepieces. The Celestrons went in just to where the clamping point starts. Combined with the (nice) helical focusers that have a (very) limited travel range, this meant that focusing was a no-go. (Strangely, however, my first pair of BT-80抯 apparently had a slightly wider barrel.) As has already been noted in one of the Cloudy Nights reviews of the BT-80, the combination of tight barrels and a short focus range really limits the choice of eyepieces. I went eyepiece shopping, taking the BT-80 with me (once burned, twice shy). The only low-power eyepiece that I could get into the barrel and into focus was the expensive 40mm lanthanum. At about US$200 apiece, this worked out to US$400 for 22x. Ouch! But they worked like a charm, delivering an almost 1.9?AFOV with virtually no chromatic aberration and revealing stars to be tiny white points to the very edge of the field.
I was afraid of vignetting, because the amici prisms seemed a little smaller than the free opening of the 40 mm eyepieces. It is not noticeable, however, and I could detect virtually no magnitude loss when moving a dim star from the center to the edge of the field. Although the 40mm Vixens are very expensive, I am convinced they should be standard equipment for the BT-80. Jupiter shows its bands clearly, and Saturn is definitely a ball with rings at 22.5x, even when moved to the edge of the field. I know many amateurs would say that other 80mm binoculars at 20x have FOVs from 2.5-3.5 degrees, but often only the middle 60% is flat and pinpoint. According to other reviews, some expensive 80mm binoculars do not show the rings of Saturn at all. Therefore, the concession made to the field of view is largely compensated by the quality of the image, when using high quality eyepieces. Those Vixen guys apparently thought it all out. And yes, I now have 4 pairs of lanthanums, which have cost me a fortune (40mm for 22.5x, 20mm for 45x, 12mm for 75x, and 9mm for 100x).
I must say that 100x is about the limit with regard to the optical quality and light input. Luckily, the BT-80 is capable of collimating the two 100x images, which allows for stunning views of the moon and doubles. The double-double in Lyra is a definite 3D split at 100x. I am not sure whether the image breakdown above 100x is due only to the achromat objectives or also due partially to the amici prisms. A fluorite version might settle the issue.
Now for some deep sky experiences.
M42 is stunning, with green nebulosity everywhere. On this object the binocular effect is marvelous. When viewed with one eye, M42 is just a weak nebula, but opening the other eye is like switching on a light amplifier and adding 3D. Definitely the most impressive view of M42 I抳e ever seen, with the four stars of the trapezium floating around (and this from a suburban location). At 75x the trapezium shows a hint of the 5th star. The bright, high contrast views, coupled with the 3D effect, are fabulous. At 45x the nebula is also very impressive.
At the dark site in France, M57 at 75x looked like a 3D eye staring at you. M81 and M82, in the same field of view at 45x, were like looking through a porthole in space. The Double Cluster in Perseus was amazing, with stars everywhere. The two clusters really showed their depth. I tried looking through only one eye and then suddenly opening the other. At that instant the image was boosted, depth appeared, and stars that were at the edge of detection easily stood out against the background. The binocular effect works like magic. M13 at 100x was almost completely resolved, but the biggest surprise was M31 with its satellites M32 and M110. The view at 22.5x knocked my socks off. M31, surrounded by a myriad of stars, was all over the 3D field, and the dark lane was visible. It looked like a picture. When I looked with only one eye, the splendor was completely gone. I would like to see this through the BT-80抯 big brother, the BT-125, which I surely will buy in the distant future.
Pros and Cons of the BT-80
Pros:
Not very expensive (around $1000)
Field much flatter, sharper and more contrasty than many other (sometimes more expensive) 80 mm binoculars.45-degree viewing
Interchangable eyepieces and a magnification range. This enormously increases the versatility and the range of viewable objects. Together with the comfortable 45-degree viewing, I believe the BT-80 can keep up with straight-through 20x100 binoculars when used near the zenith.
Cons:
Rather heavy and bulky
Needs pairs of expensive eyepieces
Actual field of view limited to about 1.9?/li>
I love them for the deep sky, but the planets and the moon keep impressing me, too. They are definitely a keeper. Although I purchased a 200mm Klevtsov Cassegrain for the real planetary work, I would very much like to have a binocular with 100 or 125 mm fluorite objectives, but that is getting expensive. Maybe for pure deep sky the BT-125 would do fine. Certainly it would be better for my bank account! |
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