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Introduction:
Binoculars are a great way of touring the night sky, the image is the 'right way up' so it's easy to find your way about by reference to a simple star chart. Many objects are better viewed through binoculars, whether it's sweeping through rich star fields in the Milky Way or a wide-angle view of a comet's tail - in fact anything that requires more than a couple of degrees of real Field of View (FOV). Larger 'astronomical' or 'night observation' binoculars offer a different view to that of standard binoculars, they provide a brighter image revealing fainter stars, nebulae and galaxies, and usually have a slightly higher magnification which shows star clusters to better advantage. They do not replace standard binoculars which are still the best instrument for hand-held scanning at low power.
General-purpose binoculars, such as 7x50 or 10x50 models, are ideal for hand-holding but have limited light-grasp and magnification. Larger binoculars are perhaps less convenient to use, if the magnification is stepped up above about 12x they are definitely going to require a mount of come kind to prevent tremors ruining the image. Another factor is weight - if the object glass diameter is increased above 70mm to provide the necessary extra light-grasp they become heavier and bulkier, and so no matter what the magnification binoculars of this size demand a sturdy tripod mounting. All big binoculars will have the facility for tripod-mounting and the Miyauchi 20x77s are no exception.
As both amateur astronomer and engineer I tend to be very critical of the optical performance and also the mechanical construction of binoculars, I have a second pair of Minolta 10x50 Classics (the better of the two models Minolta sold under this name) and before purchasing these I spent a long time browsing shops trying out various models before finally buying the Minolta's. To be truthful I never found a single pair at any price that I couldn't find some fault with. A disappointing number were very poor indeed, so it was with trepidation that I took the Miyauchi's out for first-light under the stars to see how they would perform.
The retail price for Miyauchi BS 20x77 binoculars in the UK varies, most outlets list them at just under 1000UKP but I found a retailer selling them for nearly 300UKP less. The internet is a great way of shopping! However, buying binoculars by mail-order is a Bad Idea under normal circumstances because you really need to examine them first-hand and (of course) optically test them to see if they are suitable for the application. Astronomy is particularly demanding of optical performance, normal terrestrial use during daylight may reduce the impact of optical defects, but turn them on the stars and anything less than optical perfection becomes glaringly obvious. There are many other aspects of binoculars which are user dependent - such as the amount of eye-relief required, the feel of the focusing mechanism, or the amount of diopter correction needed. Being unable to travel to examine them in person meant I had to take many listed specifications on trust (another Bad Idea!).
Mechanical Construction:
The Miyauchi 20x77's are available either with straight-through viewing and central focusing, or with eyepieces set at 45 degrees and focusing achieved using individual eyepiece adjustments. For astro use the 45 degree offset is a real advantage and the loss of a central focusing mechanism is no real hardship (and possibly even a benefit). The binoculars weigh in at about 2.6 Kg, and a convenient handle is fitted on the top for carrying, it's also a useful grip point when mounting them onto a tripod.
The Miyauchi's offer a convenient 45-degree viewing angle, ideal for astro use where most of the time is spent looking up.
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Given that these binoculars were relatively expensive I was not exactly over-awed by the build quality. Don't misunderstand me - I'm not saying they are poorly made, but those little details of fit and finish which are only present in the best quality instruments were missing. Overall I would describe the Miyauchi's as 'functional' or perhaps 'utilitarian' but not exceptionally well finished. The aluminium frame for the object lenses and 45-degree prism assemblies is a machined one-piece fabricated (welded) housing which is at once very strong and light weight. There is no central hinge - thankfully because it's a weak point and the bane of anyone trying to maintain accurate collimation - and interpupilliary distance is therefore set by rotating the two erector prism housings holding the eyepieces. The mechanism which keeps the two prism assemblies equidistant and symmetrical as interpupilliary distance is adjusted consists of a small hinged block sliding in a channel, the fit of the block in the channel is too loose and it's possible to move one of the assemblies a little without the other one moving. One would normally expect this adjustment to be governed by a gear segment which offers more precise control. Whilst I don't think this has any practical impact on performance it's not the sort of thing I like to see.
Interpupilliary distance is set by pushing the two eyepieces together or away from each other, the mechanism that controls this is mechanically a bit sloppy.
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The objectives have convenient built-in lens hoods which can slide forward as needed. Again, the fit here is a bit more sloppy than I like to see and while fine for terrestrial observations where the binoculars are horizontal, with the binoculars pointed upwards there's not a lot stopping them from sliding back down again. I would like to have seen some sort of positive detent mechanism holding them in place (either retracted or fully extended). They also seem to be a little on the short-side for effective dew protection but I have yet to test them under severe conditions.
 The Miyauchi's are supplied with built-in lens hoods which slide out as required.
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 The interior of the tube assemblies seem to be well baffled.
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The interior of the binoculars seem to be well baffled and blackened to prevent stray reflections. The objectives themselves are of a 4-element semi-apochromatic design incorporating an ED (extra-dispersive) element for improved colour correction. The lenses are multi-coated and this is clearly revealed by the multiple red and green reflections when held up to the light. The eyepieces are easily removed with a firm pull, being held in place by nothing more than a neoprene O-ring around the upper part of the barrel. Although I have no technical specs I understand them to be of reverse-Kelner design offering a 50 degree AFOV. The eyepieces are interchangable and 37x eyepieces are available as an option (but at 140UKP a pair these seem a bit expensive for what you get). The eyepiece barrels are a little less than 1-1/4" diameter, and I did try to see if other eyepieces from my collection would work but without success. I've still got some more work to do here because it would be nice if wide-angle astro oculars could be substituted.
The eyepieces offer a full 20mm of eye-relief, which is excellent for high magnification (20x) binoculars because many other models provide perhaps half this making them unsuitable for eyeglass wearers. However, I found a problem with the rubber eyeshields which, try as I might, did not appear to fold down. With the rubber eyeshields in place there was no way I could see the full FOV. As it happens it's a fairly easy job to remove them, a curious metal ring which sits over the outside of them (and doesn't seem to do anything useful, it certainly doesn't hold them in place) is first removed and then the rubber eyeshield itself can be pulled off. The ring, whatever it's purpose, cannot now be replaced because it fits over the outer diameter of the rubber eyeshield and is held in place by friction. It's no loss, but unfortunately neither would the eyepiece covers fit because, again, these slip over the rubber eyeshields. While neither of these is a desperate worry another problem then became apparent - with the eyeshields removed there was actually a little too *much* eye-relief making it difficult to keep your head in the right place to get the best view, and worse, there was a great tendency to accidentally knock the lenses of my eyeglasses against both metalwork and lenses of the eyepieces. I put up with this situation for a short while before deciding to 'trim' my eyeshields. This is not really a DIY job but I happen to have the necessary tools, I made a mandrel in my lathe over which the eyeshields were stretched, and then cut them to about half length using a special knife tool. Now they are just right for me when wearing glasses - but anyone else without glasses might have a problem!
 The rubber eyeshields needed to be 'trimmed' to half length before they could be used.
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Individual eyepiece focusing is a knack which is quickly learned, which is just as well because a potential Miyauchi owner will be doing a lot of it. The eyepieces are far too loose to hold their setting, the slightest knock will cause them to turn and lose focus, and from what I can see there doesn't seem to be any simple way tightening up the action. At the moment I'm toying with the idea of fitting small locking screws to each eyepiece although drilling and tapping through the skirts looks tricky. I think it will require disassembling the eyepiece focusers, which may reveal a better solution (I'll let you know). Whatever, something will have to be done because making constant adjustments is driving me nuts and it's even more annoying because stable focus is the one major benefit you are supposed to get in return for giving up central focusing.
The front lens covers are also desperately loose. With both covers on pointing the Miyauchi's downwards and giving a slight shake causes both to fall off. Sometimes they don't even need a shake. Seeing as these are new binoculars the fit is not going to get any better.
The binoculars are well endowed with mounting points. Underneath there is a circular plate with a standard 1/4" x 20 TPI thread which will fit onto any standard camera tripod, although at 2.6Kg these binoculars will need a particularly heavy one for stability. In addition there are lateral mounting points on the sides of the tube assemblies which accept Miyauchi's fork mount. I opted to make my own fork mount which makes use of these, and for a tripod I used "Stubby" (a home-made tripod described on my website).
 A plate and 1/4" x 20 threaded hole can be seen on the underside, this can be used to mount the binoculars on a heavyweight camera (or video) tripod.
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 With an 18" extension "Stubby" does a fine job of carrying the Miyauchis fitted to my home-made fork mount.
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Optical Performance:
Shiny paintwork aside, the critical question has to concern optical performance. Overall, I think this can be described as 'Excellent' and it's clear this is where most of the manufacturing costs have gone. I do have a few quibbles though. During daylight testing the colour correction is mostly very good - but only if your eyes are held precisely on-axis. Move your head even slightly whilst viewing and it's possible to evoke some remarkably colourful artifacts, especially on high-contrast objects such as a pylon viewed against the bright sky. Interpupilliary adjustments for your own eyesight thus needs be set exactly - if it isn't then one or other eye will be off-axis with the same result. The sweet-spot for best performance is really quite small so keeping your head in the correct position can be a little tiring.
Brightness and contrast are also excellent, thanks to the BAK4 prisms, good interior baffling, and high quality lens coatings. The FOV is very sharp in the center 50%, and remains very nearly as sharp until close the edge (about 95%) where it then drops off perceptibly. There is no evidence of the usual pincushion/barrel distortions or spherical aberrations. Mind you, this has been achieved with a fairly modest 50 deg apparent field of view (AFOV) - a bit less than offered by most good quality 10x50 binoculars.
My Miyauchi's had first-light on 6th April 200, the evening of a notable conjunction of Moon, Jupiter, Saturn and Mars, so this provided an excellent opportunity. Both the atmospheric transparency and 'seeing' were reasonably good (amazingly), and the crescent moon was tack-sharp and a really beautiful sight at 20x. I could clearly distinguish Saturn's rings from the planetary disk and also faintly detect the cloud belts on Jupiter (by now well past opposition and fairly low in the west). Turning to M42 in Orion I could just split 4 stars in the Trapezium (an amazing performance in itself) and this could also be achieved across about 90% of the FOV (tested by moving the subject from side to side in the field of view). There was just an impression that the image got softer at the edges and this reduced sharpness and contrast enough to prevent all the stars being resolved. A few bright star clusters were examined, M37, M38 and M35 all looked fantastic - pin-point star images. No problems of false colour were encountered with normal viewing although this could be provoked on Jupiter and the moon's limb if I looked through the binoculars with my eyes a little off-axis (not a sensible thing to do really). Star colours (real!) were beautifully defined, I don't think I have ever noticed such clear colour contrasts before. Locating objects in the 2.5 degree FOV can be a bit tricky at high elevations, the 45-degree eyepieces are more comfortable but they mean you don't simply 'point and look'. I have tried using the handle as crude pointer but something a little more sophisticated is needed I think. The handle is quickly detachable so in theory could be replaced by a Telrad or other finder. There is so much else to look at on other nights that it will take a long time to fully evaluate the Miyauchi's performance on different objects, but based on this performance it should be an enjoyable experience.
In conclusion, the Miyauchi 20x77 binoculars appear to be very strongly made and are optically excellent. There are a small number of design flaws (the rubber eyeshields and lens caps for example) and also the mechanical construction leaves something to be desired (the eyepiece focusing mechanism being the most irritating). At the asking price of 1000UKP these defects really should not be there. Nevertheless, I know I can correct the mechanical problems and the superb optical performance makes up for them. At the end of the day, it's what you can see with them that counts and it's here the Miyauchi's deliver the goods.
Well, like I said earlier, something had to be done about stiffening the action of the eyepiece focusers! In the end I bit the bullet and drilled and tapped the eyepiece skirts to accept a pair of small 4mm locking screws. This works fine for me but would accept it's hardly a good solution for most people. I had considered trying to fit a thin sheet of teflon into the gap between skirt and focusser body, maybe this would still be a solution for most people should the problem bother you also.
A definitive answer to the excessively free moving eyepiece focussers - a pair of locking screws.
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...and this is my modified binocular mount, designed for comfortable viewing and (most importantly) a high degree of stability. I found that when seated the tripod legs tended to get in the way, the 'T' arm you can see in the photos moves the binoculars away from the tripod. Eyepiece height is fully adjustable for viewing anything from horizon to zenith. The main failing of commercial parallelogram mounts is a lack of stability, this one is made from 1-1/4" alloy barstock, 3/4" stainless steel pivots and has large clamp knobs to lock it up tightly. The fork ALT-AZ movements have friction bearings which hold the binoculars wherever they are pointed, and the mount doesn't need a counterbalance. High stability is necessary because I want to increase the magnification using additional eyepieces, what may be passable stability at 20x turns into a disaster at 50x.
 
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| The solid offset fork mount allows comfortable viewing and good stability. |
A problem with higher power binoculars is the actual FOV is fairly small and the Miyauchi 20x77s offer just 2.5 degrees, which can make it a little tricky to get the desired object into view. To compound this difficulty the 45-degree eyepieces make pointing the binoculars less intuitive - especially at higher elevations where you can't 'sight' along the top. With high power eyepieces (30x) the problem becomes acute. Until now I had been using the carry handle as a very rough pointer but this was becoming tedious, I was getting frustrated searching around Ursa Major (near the zenith) for some of the brighter galaxies because I could never be sure the binos were pointing in the right direction. An optical finderscope is listed as an optional extra but I decided to fit one of the unit-power reticle finders that are more usually associated with telescopes. A Telrad is just a bit too big to mount conveniently on top so instead I used the Rigel Quickfinder, much more compact at just 1-1/4" square and standing vertically instead of horizontally like the Telrad. The Rigel also offers an adjustable reticle with variable pulse rate and brightness - useful for lining up on dim objects where a normal illuminated reticle tends to wash out the view. This finder really works, it's so much easier place objects in the FOV even if they can't be observed directly naked-eye, all that's needed is to estimate the position with reference to nearby stars.
   
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| The Miyauchis fitted with a Rigel Systems Quickfinder, and at right - an actual view through the finder while centered on the crescent moon. |
Mounting the finder required a bracket to hold one of the two plastic bases which come with the Rigel, the handle had to be reversed to make room but it still works fine like this. The finder body simply clips onto the base with a couple of catches and is easily removed for transport. The outer of the two circles provided by the Rigel's reticle represents a 2-degree FOV, the inner circle 1/2 degree.
  
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| A bracket provides a platform for the Rigel's base to sit on. The finder body can be quickly removed for transport. |
A pair of 30x eyepieces were purchased after some 2 months of use because I thought the additional magnification would help extract the maximum performance from the Miyauchis. These optional eyepieces are labelled 'Wide Angle' although there is no indication of their angular FOV (possibly because they were designed for use in more than one model of binocular). However, using a distant pylon as a rough measure it seems (supprisingly) that the real FOV is very similar to that provided by the 20x eyepieces, which means the 30x eyepieces offer around 75 degrees of apparent FOV. A wide angle such as this is usually accompanied by significant deterioration in edge of field performance but no problem was noted on terrestrial subjects. Using a field of stars it's clear there is some softening starting about 75% towards the edge of field, but it is not serious and with the huge 75 deg apparent field it's almost within peripheral vision for a centrally placed object. I would therefore still describe edge sharpness as 'very good' with the 30x eyepieces (compared to excellent at 20x), and colour correction remains excellent though with the same caveats as described above. There was more field curvature than with the 20x eyepieces but it's not noticeable under normal conditions, it's necessary to look for it by panning across a field with straight lines in it (such as power or telephone cables). There is one drawback of possible significance, the eye-relief is substantially reduced to about 13mm compared to 20mm for the 20x eyepieces. This would be of no consequence at all unless you wear eyeglasses (as I do), but luckily the short rubber eyecups fold down rather more easily than the 20x eyepieces and it's barely possible to view the entire FOV with them folded. However, if you have thick prescription lenses in your eyeglasses you might find it impossible to see the full FOV even then. Personally, I dont like having to press my face against the eyepieces whilst viewing so reluctantly I've removed the eyecups completely. Unike the 20mm eyepieces at least the lenses of my glasses don't strike the rear lens of the eyepieces with the eyecups removed, and viewing the full FOV is certainly more comfortable.
At 175 pounds (UK price) the 30x eyepieces cost just a few pounds less than I paid for a decent pair of Minolta 10x50 binoculars a few years ago, although I suppose another way of looking at it is the price is also similar to that of a single TV Radian. The 30x eyepieces give little away to the 20x eyepieces in terms of performance so if you can live with the reduced eye-relief I can recommend buying them. Certainly, if the option were there I would suggest buying the binoculars fitted the 30x eyepieces in the first place.
 
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Left: one of the 30x wide-angle eyepieces with it's rubber eyecup turned down. Right: Showing the difference between 20x (left) and 30x (right) eyepieces, the latter has a smaller diameter field lens and significantly less eye-relief. |
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| How to give an idea of what the image scale looks like? Well, this digital camera shot of the moon was taken through the right-hand side with the 30x eyepieces fited. The image is cropped and doesn't show all the detail which is available visually but it's not bad. I'm not recommending the Miyauchis as ideal for astrophotography :) |
©Chris Heapy 2000.
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