General Binocular Recommendations for Astronomy

General Binocular Recommendations for Astronomy
by Ed Zarenski
3-3-07

What do you recommend for a first binocular? What can I expect to see?
These are not uncommon questions at all, but I would say that they should at least stir up another 10 questions that need answers before you can make a decision. Before anyone can make any recommendations to you, YOU need to answer a lot more questions about yourself and your needs. For instance, What do you intend to See with this binocular? When you have developed some thoughts on each of these questions, and you begin to understand some of these basics about binoculars, then you can better help others understand your needs and they can more reasonably recommend a binocular that will work for you.

Here's some of what we don't know about you that would be helpful for making a recommendation:

Do you wear eyeglasses?
If so, will you always wear them when using the binocular? For eyeglasses, you need about 13-15mm of usable eye relief. Nearly all binoculars have recessed eye lenses and that uses up a portion of the stated eye relief. Consider a binocular with at least a minimum about 2-4mm longer than what you need. Usable eye relief of only 10-12mm is tight and you will lose a small portion of the field of view thru eye glasses. 8-9mm of eye relief is tight even without glasses. If you have eyeglasses on, your eyes will be so far behind the proper eye relief point that you will not be able to see a large portion of the field of view. Binoculars with only 8-9mm of usable eye relief are unsuitable for use with eyeglasses.

Do you know your Inter-Pupilary Distance (IPD)?
Some binoculars are not made to fit people with narrow set eyes. If you have eye pupils that are set only 56mm apart, a binocular that has a minimum of 58mm is no good for you. The Swift Ultralite 8x42 gets down to 51mm, but the almost identical Orion Vista 8x42 only closes to 59mm. All the Nikon Action Extremes get down to 55mm. Some of the larger 20x80 and 25x100 binoculars have a smallest (closest) setting of only 59-60mm. The Oberwerk 25x100 IF has an IPD range setting from 61mm to 71mm. The Celestron Skymaster 25x100 has an IPD range from 56mm to 74mm. Many children cannot use binoculars unless they close down to 56-58mm. Most binoculars open up as wide as 70mm.

Do you know how large your eye pupils are?
Under dark conditions, if your eye pupils open to 7mm, you would be able to use the full exit pupil from most all binoculars. BUT, if your eye pupils don't get any larger than 4mm or 5mm, it won't do you any good to carry around the extra weight of the glass needed to provide a very large exit pupil. 12x60, 10x50 and 8x40 are all examples of 5mm exit pupil binoculars. 20x80 has a 4mm exit pupil. 10x60 has a 6mm exit pupil. 10x70 has a 7mm exit pupil. 7x50 has a 7.1mm exit pupil.

If you have eye pupils that dilate only to 5mm and you use a 10x70 binocular that has a 7mm exit pupil, your smaller eye pupil cuts the effective aperture to 50mm and eliminates all the light outside that. You can pretty easily have someone help you measure your eye pupils in a fairly dark room. An easy method is to use a strip of paper with a series of dot sizes, sliced thru the middle of all the dots.

Will you observe in very low light conditions?
A 10x50 has only a 5mm exit pupil, but a 7x50 or a 10x70 has a 7mm exit pupil providing a far brighter image, and that would be useful in low light conditions, but ONLY IF your eye pupils open wide enough to take in that full size exit pupil. Generally, binoculars with such large exit pupils are used by people who can view from very dark skies. Or for example, for terrestrial observing, the choice might be between an 8x42 or an 8x56. The 8x56 would give brighter images and be more useful under low-light conditions. However, I consider binoculars with 7mm exit pupils as specialty binoculars and I would rarely recommend them to a first time buyer. Again, pay attention to eye pupils. As far as light goes, you can only use an exit pupil as large as your eye pupil.

Do you demand a waterproof binocular, or simply a weatherproof binocular?
There are waterproof binoculars, water resistant binoculars and binoculars that have no resistance to water infiltration at all. If you plan often to use your binoculars down by the seaside, you will most certainly want the ability to wash off the salt spray. For that you need waterproof. For astronomy it may not necessary, but it will keep moisture from ever fogging up the insides, and that's a good thing. I like knowing that my binoculars can get completely covered in dew and no moisture will get in.

Have you any experience hand-holding binoculars?
You need to give some consideration to how much weight you can hold for long periods or how much magnification you can hold for a steady view? My 7x35 Minolta Activa weighs only 24oz. The Swift Ultralite 8x42 weighs about 22 ounces. The Nikon Action Extreme 10x50 weighs in at 36 ounces. The Oberwerk Mariner 10x60 and the William Optic ED 7x50 each weigh over 48oz.

Generally holding anything over 10x is not so much an issue with the weight as much as it is an issue holding 10x power steady enough to stop the image from swimming around in your view. Some 15x70 binoculars weigh only a little more than 3.5#, but at 15x, every little movement of your hands or arms or even your heartbeat, will show up in the magnified image. Handheld tests show that both a 10x and 12x binocular can resolve equally as much as a 15x binocular when tested for limits on a resolution chart. I've handheld my 4#12oz Fujinon 16x70 for brief periods, but I wouldn't want to use them handheld for a full night. For this reason, some recommend the consideration of IS binoculars.

Have you any experience with wide-field versus narrow-field?
Things in motion put more demands on how much field of view you use. 15x70 binoculars usually have a little more than 4.0-deg field of view. The Nikon SE 12x50 has 5-deg. Many 10x50 binoculars have about a 6.0-deg field of view. The Nikon Action Extreme 8x40 has an 8.0-deg field of view. A 25x100 or 20x80 binocular will generally provide only 2.5-deg to 3.0-deg.

If you don't know the differences between what the fields of view (fov) look like, then you need to try several different sized binoculars with different fov. Put one edge of your view on the exact same spot for each binocular and make a note of how far across a wall it can see. Compare with the others. At a distance of 100 feet, every 1-deg field of view is 1.75 feet across the view. You need a 5-6-deg field of view to fit the Hyades. You need only 3-deg to fit the Pleiades or Orion's Belt with room to spare.

What difference do multi-coatings make?
Coatings are a very important product in the transmission of light. In this age of technology, no one should purchase new binoculars that have uncoated surfaces. An uncoated lens surface can reflect 4% of the light hitting it. Uncoated internal glass surfaces contribute to internal reflections and lowered contrast.

Magnesium Fluoride (MgF) single coating can reduce reflected light to 1.5%. Properly applied multi-coatings can reduce that reflectance to 0.5% or 0.25% per lens surface. A binocular may have as many as 14 lens surfaces. With the exception for extremely high level of performance of some superior coatings, even a properly multi-coated binocular can transmit at best only about 93% to 96% of incoming light. That same binocular, with all 14 surfaces only single coated with MgF, may transmit only 79% of the light. Subtract a few tenths of a magnitude from the Òhow faintÓ numbers for any binoculars that are not fully multi-coated.

What is Chromatic Aberration?
Binoculars can suffer from a number of aberrations but this one is often discussed. Chromatic Aberration (CA) is the result of the various wavelengths of light not reaching focus at the same precise point in the image. CA is a function of refraction. Light from different wavelengths is refracted differently. Blue, yellow and red wavelengths of light reach focus at slightly different points along the focal length axis of the lens. The most important thing the user should understand is that it is impossible to completely eliminate CA from a lens system. It can be reduced, but cannot be eliminated altogether. There is longitudinal CA and Lateral CA.

In an astro binocular, CA appears prominently in less than 1% of the objects viewed. So it might be considered one of the least critical aberrations on which to base a decision, unless of course you are buying your binoculars just to look at the moon. My advice is judge your choice of binocular on all the features that make a difference in the view 99% of the time. Honestly, the planets are not binocular targets, so the moon and a few very bright stars are the only astro targets that would be a consideration for judging whether CA is objectionable in the choices of astro binocular. For terrestrial viewing, CA becomes a more important factor. CA becomes a problem in extremely bright conditions, not in low light conditions.

What's all this talk about sharp field of view?
Sharpness of field image is deteriorated in the outer field of view by a combination of aberrations, the scope of which is beyond this discussion, but they are CA, coma, spherical aberration, curvature and astigmatism. Generally (but not always), a wider Afov binocular will suffer from more aberrations in the outer portions of the field of view. Much of those aberrations are contributed by the eyepieces, and it takes a very high quality eyepiece to successfully eliminate the aberrations in a wide field.

Think about the cost of the binoculars you have in mind, and then think about the cost of a pair of good 18mm to 22mm wide-field eyepieces. Usually, binoculars don't have the highest quality eyepieces. Therefore, somewhat narrower Afov binoculars, usually in the range of afov 60-deg or lower, have less aberrations in the outer field of view. Many Pentax binoculars use very narrow 50-deg to 44-deg Afov eyepieces, resulting in a narrow but extremely well corrected and sharp field of view. It doesn't do you much good to purchase a wide-field 20x80 with an Afov of 70-deg if the outer 30% is degraded. You may as well get a 20x80 with a 60-deg Afov and less aberrations.



Why do these binoculars look the same, but one is so much cheaper?
Probably the most important differences in any two binoculars that look the same but sell for much different prices is (A)the mechanical construction, workings and range of adjustment, and ((and very likely more important) the degree to which all the optical surfaces have been figured, polished and coated. There are other things that make a difference in the quality and most of them are not visible on the outside. Usually, cheaper is not just cheaper for no reason at all.

What should I consider for a mount?
Many times people ask, Why do you always say you can't mount an 8# binocular on a tripod that is rated for 8#. That is because the load rating for tripods is determined with the load weight directly over the tripod head and directly over the tripod column. Furthermore, load ratings are based on cameras with little to no magnification.

With binocular viewing, we often have a heavy load hanging off of one side. That is like a cantilever or an eccentric load. In all things mechanical, it takes a beefier support to hold an eccentric load than it does to take a direct load. Add to that 10x, 12x or 25x magnification and you can see the need for stability goes up exponentially. If the tripod is tall enough, you can mount a 2# to 4# binocular on most tripods ranging from $90 to about $150. People are often surprised to find out once they have spent $300 to $400 on an 8# 20x80 or 10# 25x100 binocular that they now must spend $300 to $500 on a substantial tripod and fluid head to hold such a large instrument.

Here is a summary of what you can expect to see:
Please keep in mind, you will not see as much handheld as you will with mounted binoculars. Assuming at least mag 5.5 skies:


How faint are the stars you can see with binoculars?
First judge how dark are your skies naked eye. If you can only see 6 of the stars in the Little Dipper, then you have no better than mag 4.5 skies. If you have skies that allow you to see all seven main stars in the Little Dipper naked Eye, then you have mag 5.0 skies. If you are able to see 9 or 10 stars in the Pleiades naked eye, then you have mag 5.5 skies. That's pretty decent.

Under mag 5.5 skies you can see stars just fainter than mag 10 with a 10x50, stars to mag 10.5 with a 12x50, stars to mag 11 with a 15x70, stars to mag 11.5 with a 20x80 and stars to mag 12 with 25x100. It is very difficult even with the best 70mm binoculars to see stars beyond mag 10 when handheld. Seeing stars at mag 10 requires a completely quieted binocular on a stable mount, viewing without touching the eyepieces. For mag 4.5 skies subtract a full magnitude. For handheld binoculars subtract another full magnitude or even more.

What type objects can I expect to see in various sizes binoculars?
In an 8x40, many of the smaller open clusters or fainter globular clusters and nebula such as M76, M26, M9, M10, M14, M71, M18, M1 and M78 are barely seen as a smudge or cannot be seen at all. 8x is a bit low for astronomy and it could be seen readily when compared to 12x or 15x on clusters. However 8x did perform very well on some objects. I easily found M13, M27, M29, M11, M35, M36, M37 and many other open clusters, but M11 was barely a smudge. On M36 you will see the glow of the cluster, but no stars will be resolved. Doubles 20Ó can be split, but none smaller. Even the smallest binoculars will see the 4 moons of Jupiter and can see bright Titan, although you would have difficulty even telling that Saturn is elongated.

10x50 binoculars can see many open clusters and in a good dark sky will find a large number of the brighter galaxies and nebula. Both M81 and M82 can be seen handheld in a 10x50, but M51 needs a mount. A few bright globular clusters can be seen, but they will be small, however they will appear distinctly non-stellar. M5 can be seen handheld, M10 is seen, but very small and M12 was just barely seen. In the Nikon AE 10x50 held braced, I saw M15 as a small but obvious globular. Doubles can be resolved down to about 16 arcseconds. M11 is still just a faint patch. You will be able to see the Ring nebula, but only just barely be able to see that it is a non-stellar spot. The broad galaxy M33 can be seen, but a similarly broad galaxy M101 will not be seen as it is too faint for 10x50 in mag 5.5 skies. Using a 10x60, M78 was just barely detected. The beauty of binoculars like 10x50 with a 6-deg fov is that objects like the Hyades can be seen all in one view.

12x50 binoculars will capture nice views of M42, M45, M11, M27, M71, Alberio, Sagitta, Delphinus, CR399, and the Cygnus Milky Way. Bright globular clusters can be seen, but they will still be small, however more will begin to appear as obvious globular clusters. Doubles can be resolved down to about 13 arcseconds. You will be able to see the Ring nebula distinctly as a non-stellar spot. 12x50 is still not enough to see M101 in mag 5.5 skies, but M1 can be detected easily and M78 just barely using a handheld Nikon SE 12x50.

In 15x70s and 16x70s, objects like the galaxies M65 /M66 can be seen. However M65 is difficult in a 15x70 and just a bit easier in the Fujinon 16x70. The companion NGC 3628 is not be seen in either. Portions of the North America nebula are visible. You can split doubles down to 12", and sometimes 10" with difficulty. M11 takes on the appearance of a glow with a faint star. M71 is still faint even in a 15x70. On the steadiest nights, although you might suspect resolution of all four components in the Trapezium, most people can only positively see three components. On M36, a 15x70 can separate only 4 to 6 stars, while Fuji 16x70 saw about 8 stars resolved. When small and faint M78 was seen with difficulty in a fine handheld 12x50, it was found quickly and easily in the 16x70. M101 can be seen under the best conditions, mag 5.5+ and excellent transparency. Saturn can be seen with a very little black space between the rings and the disk.

20x80s are solidly into the range of giant binoculars. Image scale grows. The field of view narrows to about 3-deg. You see more stars in clusters just due to the increase in magnification. In clusters like M44 the Beehive, an 8x40 will see only about 40 stars, in a 10x50 you might see 50 stars, in a 15x70 maybe 80 stars, but in a 20x80 over 100 stars. On M36, a 20x80 can resolve about 6-8 stars. You can sometimes pick out all four stars in the Trapezium, the closest being 8.7". You can see magnitude as deep as 11.2 to 11.5 and occasionally elongate some not-so-bright equal doubles in the 7" range. Some of the fainter galaxies such as M95 and M101 can be seen on good nights. On a night of mag 5.0 skies, M76, the Little Dumbell nebula in Perseus was not seen in a 16x70 but in a 20x80 it was very faint and obviously not star-like.

In a 25x100 binocular the field narrows to 2.4-deg. These binocular see stars at mag 12.0. When you view faint clusters in these binoculars you see stars that just were not there in any smaller binocular. On M36, a 25x100 can see about 20 stars. In M44 I've counted over 150 stars in a 25x100. Doubles as close as 6-7 arcseconds are clearly separated. M105 and its companion NGC 3384 make a nice pair. M57 is an obvious torus. M81 and M82 were both very easy and bright and both show extension beyond a much brighter core. Fainter galaxies seem to be easy pickings for these 100mm binoculars. While M66 is seen bright, and M65 is difficult in 16x70s, both along with the companion NGC 3628 are readily seen in the 25x100s. the pair M97 and M108 were detected in a 25x100. M101 was found quickly with a 25x100, but on the same night it was only suspected in the 16x70s. 25x100s make globular cluster M12 look like it is on the verge of resolution in the outer edges. 16x70s could see but could not resolve M12 at all.

So what is your goal?
Do you want to scan around and see the sky in general? Do you still need to learn your way around the constellations? Are you looking for a binocular that will help you find your way around when you are using your scopes? If so, get a low power wide field binocular. You might be content with an 8x40, but I'd go for a 10x50. If this binocular will do double duty as a daytime hiking binocular, maybe the 8x40 would suit you better.

Do you want to observe mostly open clusters and some of the brighter deep sky objects? Maybe you have no desire to lug around a 8#-10# binocular mounted on a 10# tripod, and sometimes you might prefer to just handhold your binocular for a few minutes? A low to moderate power and light weight 12x50 up to a 15x70 is a great choice for this. Consider the 15x70 if you will often mount it. Consider the 12x50 if you more often intend to use handheld.

Do you want to see both large and small clusters, hunt down faint objects and see extension in galaxies? A 15x70 might be right for you. Even a modest tripod can hold a 15x70. But if you want the most out of deep sky viewing and you will consider investing a good sum in a good mount, then you should be looking for a 20x80 or 25x100 size. Both of these expand your target list to include many and varied faint objects. For these, a reasonable knowledge of charts and DSOs must be already acquired or at least a strong desire.

If you're looking for a handheld binocular, then consider this; if you don't like the way a binocular feels in your hands, it probably isn't a good choice for you no matter what the specs indicate. One of the primary advantages of binoculars over telescopes is comfort and ease of use. No matter how outstanding a binocular is optically, if it isn't comfortable for YOU, it's probably not a good choice. If your looking for a large mounted binocular, you must consider whether you currently have enough of a mount or will you invest in a proper mount.

Don't rush into accepting recommendations because someone thinks this or that is great. Consider all the information provided to you, but get a binocular that fits you like a good pair of shoes. When you wear them, they get the job done right, and you can't even tell they are there.

As you can see, this is not a treatise comparing quality of binoculars. These models mentioned here are not necessarily recommended choices, but in the context of getting to the questions that need answers, they help make the point. I hope this article has helped you understand what you need to know about yourself and how you intend to use binoculars so that you might make a more informed choice.

Clear skies, and if not, Cloudy Nights
Edz
CN Binocular Forum Moderator