Given the satisfaction I had in drawing nebulae with the 11X80 Celestron binoculars, I decided to increase the diameter and, above all, to increase the famous visibility factor (magnification x diameter).
CHOOSING GIANT BINOCULARS
I'm not a fan of dual instruments (something will definitely stay in the closet!), but I needed a portable instrument to use on winter nights, above the inversion layer.
I initially leaned toward the 30x125 Perl Vixen, but I was worried about the inevitable chromatic aberration. In any case, beyond 100mm in diameter, the choice is very limited, and in the end, the Miyauchi 141 was the one that best met my aspirations: good quality/price ratio (try adding the cost of two 130 mm Takahashi or Astrophisics APO refractors!), very compact (for the diameter, the weight of 12 kg is a record), usable for both terrestrial and astronomical use (thanks to the 45° prisms).
What's not to be overlooked is the instrument of a well-known brand: you were on the safe side...
From this point of view, my confidence was as great as the fame of the Japanese company.
I deliberately overlooked the ultra-short focal ratio of these binoculars (f/4.4) – I didn't expect to find the high image quality of the Canon 15X50 Canon binoculars, which are a benchmark for me in this field..
However, I had also purchased the additional 45x eyepieces, which a diameter of 141 mm should support very well!
After a delay of about a year in construction and delivery, I finally managed to carry out the first optical tests.
According to the Japanese, the delays in the construction of this model were caused by the change in the type of glass used: natural fluorite is now banned in Japan for health safety reasons and has been replaced by another type of synthetic glass, even better than the much-vaunted fluorite.
DEFECTIVE BINOCULARS?
But, alas, the giant Miyauchi fails even the Snap Test: the focus isn't clear, and you search in vain for the ideal focus position at 25x. At 45x, the binoculars are unusable! However, there is a central area of the image with better quality, which suggests that the alignment of the components could be improved.
Among the various possible defects, one might assume that astigmatism and chromatic aberration are very limited in an instrument with this level of finish. At 25x magnification, the stars (or any bright point) appear as vertical green whiskers associated with nearly horizontal red whiskers. Traces of magenta/green coloration were visible on the Moon, indicative of the typical correction of apochromatic triplets. No residual blue-violet secondary spectrum, proving I hadn't been sold two achromatic lenses! At 45x magnification, the colored tufts became even more evident, giving the impression of a halo—in other words, an undeniable loss of contrast. This caused the instrument to be returned to Japan for a few months.
AFTER THE TECHNICIANS' INTERVENTION
At Miyauchi, there's only one optical technician authorized to "touch" the 25x141 binoculars, and he found a problem with the mounting of some metal parts near the objective lens. However, he overhauled the entire binocular to ensure it was as perfect as possible. You may be wondering how the second test went: in fact, the binoculars performed much better, offering a nearly perfect image at 25x.
|
Optical defects emerged during the second test |
||
| Defect | at 25x | at 45x |
| Curvature of the field |
Modest, variable depending on the area of the field (more marked in the upper right and left quadrants) | Modest |
| Astigmatism | Due to the astigmatism in my left eye | Instrumental (right tube) joined to the one of my left eye (central area and two lower quadrants) |
| Aberration chromatic |
Residual color on a faint outer crown (better correction in the lower quadrants) | Appreciable chromatic residue (discrete correction in the center, more evident in a large peripheral area) |
| Distortion | Weak in the center, extremely weak everywhere else on the field | |
FIELD TEST
At 25x the solar granulation is almost always visible (due to atmospheric turbulence) and the sunspots are well defined: in the best conditions I find the image quality of my Celestron C14 telescope.
Between you and me, I never thought a Celestron optic would ever serve as my benchmark for contrast. Jupiter, at 25x, is frankly unattractive: the satellites are well-defined, although their wisps are oriented in the direction of my left eye's astigmatism. The planet's globe is too dazzling, and the chromatic aberrations are noticeable.
What happened to the 45x in the field? Focusing becomes difficult – a beginner will search in vain for the ideal position. It, of course, exists, but frankly it is not found: there is therefore a defect in the 'Snap-Test' which reveals a narrowness of the ideal "infocus" zone. More detail appears on the Moon, but the image is poor in contrast. Furthermore, the instrument is somewhat more sensitive to atmospheric turbulence. Solar granulation is also more difficult to see, and it's not always easy to see additional detail in sunspots.
Still regarding the Moon, the image observed with my right (normal) eye is more precise in the left eyepiece than in the right one of the binoculars and therefore, with both eyes, there are no obvious differences between the two images. On Jupiter, the two northern equatorial bands are a little more colorful. The colored 'whiskers' (of instrumental origin) are evident on the bright stars: the show of the Pleiades is therefore messy. M13, at 45X, with the Moon in its first quarter, is at the limit of visibility: a swarm of thin stars can be seen in averted vision. M81 and M82, closer to the Moon, were a magnificent show at 45X. In terms of resolution, you presumably get 4.3" at 25X and 3.2" at 45X. It would be dishonest of me to claim separation of Epsilon 1-2 Lyr: although a rod-like shape is suspected, the resulting 'whiskers' suggest imaginary position angles.
In summary, the instrument is satisfactory at 25x while at 45x the image contrast is lower than expected.and – expires due to the astigmatism and chromatism observed in the right tube. This is a shame because the eye works in scotopic* vision under the best conditions with an exit pupil of 3.1 mm.
REFLECTIONS
- It was naive to believe that an apochromatic optical scheme is the absolute remedy against chromatism. : diameter is equally important and special glasses do what they can to reduce this problem.
- At 25x the image can be considered neutral. Using a friend's 20x100 Miyauchi binoculars (non-apochromatic), I was able to make terrestrial observations in parallel: the secondary spectrum was visible across the entire image (blue rather than yellow traces dominated). The chromatism problem on these 45x141 binoculars is due solely to the instrument's ultra-short focal ratio. The most renowned manufacturers (Takahashi, Astrophysics, Televue, TMB) have never gone this far with their apochromatic refractors.
- After careful consideration, I consider this large Miyauchi to be a somewhat "risque" object in terms of sophisticated optics! Perhaps it's more of a museum piece, but the problem is that I didn't buy it with that intent. I think the residual color (magenta/green) typical of "short" fluorite optics will remain a "natural" flaw in these binoculars. An interesting comparison of chromatic aberration can be attempted, on paper, between the Miyauchi at f/4.4 and the excellent Kowa 50x82 fluorite binoculars set more generously to f/5.5.
My calculations show that for a 141mm diameter, Kowa would have opted for an f/9.4 ratio for perfect color correction. Conversely, Miyauchi, for an 82mm diameter binocular, would have chosen an f/2.6!
At the last star party I introduced the Miyauchi to one of the attendees, who told me that in his opinion Miyauchi wanted to dare the impossible: a perfect optics at f/4.4 – he shares my opinion that these 25×141 binoculars remain a collector's item!
- Too short a focal ratio is certainly the cause of the reduced focus tolerance.
- The prisms contained in the rotating parts of the eyepieces do not appear to be the cause of the problem.
- The eyepieces (with a 'non-astronomical' diameter of 42 mm) are free of defects and interchangeable without any loss of quality: the image is neutral and without a trace of chromatic aberration, as with the old Zeiss ones. The eyepieces that offer 45x have a 14 mm focal length, and I took the opportunity to compare them with my Meade 14 UWA at the focus of the Celestron 14. The Miyauchi eyepiece has a narrower field, but the contrast is superior and it has better chromatic correction and light transmission. This is mainly due to the fact that the number of lenses in the Meade eyepiece is almost excessive.
- At this point, the question remains whether I should settle for the low-contrast image at 45x. Should I send the binoculars back to the manufacturer? Could the astigmatism be the lenses themselves, or rather a lack of rigorous collimation in the right tube?
- Of course, the overall quality is certainly sufficient for serious deep-sky work.
- Full of doubts and, at this point in the story, somewhat annoyed, I decide not to give up. It's necessary to address the problem from a technical standpoint and return to further in-depth tests to confirm or deny André Van Der Elst's comment: "The 25x41 binoculars? A model with a deterrent price!".
SECOND SHIPMENT TO THE COMPANY FOR AN INSPECTION
The first test of this instrument left its owner feeling discouraged, with the instrument far from perfect, especially at 45x. Solutions?…
First, I decided to send the binoculars back to their original location after a fruitful meeting with my colleague and friend André Van De Elst in Brussels—whom I thank here. At the same time, I wrote to the relevant engineer at MIYAUCHI to describe the problems I encountered at 45x, but the letter went unanswered. It would have been equally interesting to contact owners of the older fluorite model—but there are only a few of them in the world, while I'm likely the first to test the newer generation made of special glass.
NEW IMPRESSIONS AT 45x
(as we left it in the previous episode)
Those unfamiliar with the concept of "point-like focal pattern" would have understood it well by studying the image of a star through Miyauchi 141. They would have seen a non-point-like image, much wider than the diffraction pattern. With a little perseverance, they would have realized that the PPF became elliptical off-axis, a sign of astigmatism.
Furthermore, looking at the edge of the field, other geometric aberrations appear. A certain degradation of the PPF is normal at the edge of the field, which in this case manifests itself with a characteristic comet-like shape: the coma is almost natural since the incidence of the rays is very pronounced. Equally important is the field curvature, which cannot go unnoticed. Colors can be seen around the image of a star even on the optical axis. I have never detected significant spherical aberrations on either lens, and this is very positive, because this type of defect is generally noticeable in wide-aperture systems: evidently the corrective lenses inside are fully playing their role.
As for the focus search, one oscillates between the tangential focus and the sagittal focus, looking for the point of least aberration.
NEW FIELD TESTS
After its last summer stay in Japan, the Miyauchi 141 is finally ready to face the harsh reality of the stars. Its delivery is accompanied by a rather laconic message from MIYAUCHI: “The instrument is set to its maximum potential, we can’t do better!”.
For terrestrial observation, the first visual impressions are good. At 25x, the image quality is always satisfactory. But it's at 45x that the difference becomes noticeable compared to previous tests. Focusing is now easier, and within the reach of a careful observer. This is encouraging. But only the star test will tell us whether the instrument's response has improved.
NEW IMPRESSIONS ON THE FIELD
For reasons beyond my control, I was forced to correct the astigmatism by adjusting the rotation of the lenses using adjustment rings. I don't recommend this operation because it is very time-consuming, delicate, and requires multiple adjustments to collimation, parallelism, and inclination.
This was facilitated by the construction of a basic artificial target. This reticle of fine rods was placed approximately 600 meters from my site.
This target is very useful for adjusting the astigmatism of the two objectives and even for evaluating instrument resolution in the absence of atmospheric turbulence. At 25x, the image quality is very satisfactory. It can be described as having contrast across four-fifths of the field.
The "filtered" Rosette Nebula is magnificent at this magnification: it is no longer a vague circular corona as in the J11x80 binoculars, but a tangle of details, especially in its southeast portion—a major task for an astro-artist. The spectacle of the eastern region of Orion's belt deserves to be drawn in color: IC434 and NGC2024 in the same field, plus the small notch B33, the famous Horsehead. But in Taurus, I saw nothing of the filamentary network of the remnants of Supernova Simeis 141 observed under a perfect Alpine sky.
At 45x, the stars are much more pinpoint-like, and the image of Saturn and its "single" ring is no longer blurry and much more distinct. Some very small details appear on the moon (see the next section). I reach the limiting magnitude on a star of 13.75.
Drawing the field of NGC 2467 in the Puppis, I arrived at 13.44 m, at 18° above the horizon, which should correspond to a zenith magnitude of 14.40 under the same transparency conditions. In the region of NGC 7331, the galaxies NGC 7315 (12.50 m) and NGC 7342 (13.50 m) are equally visible in averted vision under an optimal sky. And to confirm the good contrast of the instrument, the famous Stefan Quintet stands out from the background of the sky as a vague, blurry patch that combines with the brightness of each galaxy in the group. The last winter observations with the 45x141 have comforted me in terms of contrast and resolution, of the nebulae in Auriga (IC410) and Cassiopeia (IC1848 and NGC281).
| Defects | 25x | 45x |
| Field curvature | Moderate (at the edges of the actual field provided, about 1/5 of the diameter) variable according to the lens and the dial | Ditto 25x, a little better in the lower quadrants |
| Astigmatism (at night) | Too weak, because of my left eye (1) | Weak (right instrument lens) (1) |
| Chromatic aberration (daytime) | Remarkable (it is noticeable from the peripheral edge for about ¼ of the real field shown). Improved if you tilt the direction of the gaze, variable according to the dial + chromatic aberration of my left eye (at night) (2) | Moderate and constant over a large part of the field (it is noticeable from the peripheral edge for about 1/3 of the real framed field) |
| Distortion | Very weak | Weak |
| Parallelism | – | – |
| Inclination | – | – |
| Aberration of sphericity | Weak | Undiscovered because it was masked by the curvature of the field |
Optical defects after the second return from Japan
- I remind you that these defects are of minor importance in binocular vision, since thanks to the double information perceived by the eyes, the brain is able to exclude the most serious defects. (for this reason, less chromatic abbrevation is noticeable in an achromatic refractor equipped with a binocular NDR viewer).
BINOCULAR RESOLUTION
What follows might make the illustrious Rayleigh, Dawes, and Sparrow smile, given that binoculars, due to their weak magnification, are incapable of achieving their theoretical separating power. Trying to see the Airy disk through binoculars is, in fact, impossible.
We must therefore be satisfied with modest values in the optical resolution analysis.
With the KOWA HIGHLANDER 50×82 you should be able to achieve a separation of 2.4”.
With the 141, you can hope to achieve an angular separation of 4.8" at 25x and 2.7" at 45x, under the right lighting conditions. In terms of limiting resolution, the 25x141 is the equivalent of a small 25mm diameter refractor, and the 45x141 is the equivalent of a 45mm telescope.
In medium light conditions I ran a series of tests on optical aiming.
At 25x, I obtained the extreme limit of 3.6", which confirms the excellent performance of the Miyauchi at this magnification. At 45x, the separation fluctuates between 2.5" and 2.8", depending on whether you observe the vertical or horizontal bars.
Let's see how it went with double stars, especially at 45x:
. if the separation is > 4.5”, the two components are well separated by a dark line.
. If the separation is between 2" and 4", observation is difficult; the two stars take on a rod-like shape, and the position angle must be used to verify the accuracy of the visual judgment. With this trick and choosing a pair within reach, Struve 559 in the Hyades, for example, with its 3.1" becomes easy.
So far my record at 45x is the double Struve 3050 (And), with 1.7” of separation.
The components have practically equal brightness and appear as an oriented rod – but the observation was performed under rare conditions of maximum contrast.
To validate these data I had fun solving the smallest lunar crater possible: you have to be careful because when observing near the terminator the brightness is very variable and sometimes the observations are random.
Searching at 45x magnification, Messier E and Gassendi Y, with their 5 km diameter, are clearly visible. At a good brightness, Gassendi R (4 km) and Puiseux C (3.5 km) are clearly distinguishable. Maestling G, with its 2.8 km diameter, still looks like a crater, in the sense that its two opposite edges are barely distinguishable.
Of course, numerous albedo spots are visible, replacing the much smaller craters, but we can no longer speak of resolution. Depending on the current Earth-Moon distance, a good resolution of around 2.8" (craters 5 km in diameter) is achieved. And with good brightness, we reach 2" with a minimum of 1.6" for Maestling G – a value similar to the double star in Andromeda!
It is very interesting to see 3 different methods of evaluating resolution (double stars, craters and optical sights) converge and square with the laws of optics.
EPILOGUE
My goal was to gain resolution and contrast at 45x near the optical axis. With the manufacturer's help, I was able to significantly reduce the astigmatism in the right lens and significantly reduce residual chromatism.
But let's not fool ourselves, since we move away from Gauss's conditions (that is, when in a compact optical system or in the presence of even a slight inclination of the rays, various geometric aberrations appear (Editor's note)
This is the price you pay for a very compact instrument.
But I want to point out the importance of the observer's efforts in the final adjustment of his instrument: in certain cases, superficial behavior or incorrect judgment can be the major source of image degradation.
In this case, from a “passable” instrument at 25x and mediocre at 45x, we have gone, at least on the optical axis, to an excellent instrument at 25x and very good at 45x.
Not bad for an instrument that has traveled more than twice around the Earth.
On this optimistic note, I leave you to reflect on my misadventures and urge you to always remain attentive and vigilant.
Translated by Giuliano Deserti and adapted by Paolo Morini
Fabrice Morat: A French amateur astronomer, he is a great enthusiast of visual observation. He uses a Celestron C14 and Miyauchi 141 binoculars effectively. He is an editor for Astrosur Magazine.