The approaching comet C/2025 A6 (Lemmon) is stirring up excitement as it may become a beautiful sight in the evening sky in October. Bright comets, like comet Lemmon may become, are especially well seen in binoculars. They may display beautiful long straight gas tails, curved dust tails, both sometimes with intricate detail. The coma may glow in distinct green or blue-green, sometimes yellowish colour. Occasionally a comet will develop a sunward tail, opposite to typical cometary tails. Overall bright comets look great through binoculars!
There is a myriad of binoculars on the market. It may be overwhelming trying to choose the right binoculars for you. Here are my thoughts on different sizes of binoculars for observing bright binocular and naked eye comets (6th magnitude and brighter), after observing over 40 such comets. My experience with observing comets with binoculars spans three decades with extended use of various 7×50, 10×50, 10.5×70, 11×70, 11×80, 15×80, 20×80, 25×100 and 20x110s. This is not a buyer’s guide, but I hope it perhaps helps you find the best binoculars for you.
Quick basics
There are a few things to know about binoculars. Most binoculars are characterized by two numbers, for example 10×50 or 20×80. The first number is the magnification (power) of the binoculars, that is how much in magnifies the view. The second number the diameter of objective (front) lenses on the binoculars. So a binocular with 10×50 written on it produces 10x magnification and has 50 mm objective lenses. A 20×80 has 20x magnification and 80 mm objective lenses. Some binoculars have zoom: these are marked with variable magnification 10-30×60 for example. I have not used zooms and in the past they had a bad reputation, but that may have changed. The field of view of binoculars is sometimes given in degrees, sometimes in units of distance. In the latter case it is usually specified as X meters/feet at Y meters/feet. Use atan(X/Y) to get the actual field of view.
Back side of the Telescope Express 15×70 binoculars with specs.
Some binoculars have “multi-coated” or “fully multi-coated” written on them. This refers to anti-reflection coatings on the optics in the binocular. Anti-reflection coatings improve light transmission: each uncoated glass surface reflects about 4% of incoming light and the light loss may become significant over a number of glass surfaces. “Multi-coated” usually means some, but not all glass surfaces are coated with anti-reflection coatings. In “fully mutli-coated” binoculars all glass surfaces are coated. If you are buying new, make sure you get fully multi-coated ones (most are). Anti-reflection coatings produce distinct colour hues on the front (objective) lenses. Good coatings are usually faint greenish, bluish or violet. Avoid binoculars with bright red (‘ruby’) coatings, they are often used in binoculars with lesser optical quality to hide or filter out optical errors.
Greenish coatings on the objective lenses of 11×70 binoculars.
Another fairly important parameter in binoculars is the exit pupil. That is the diameter of the light beams exiting the binocular eyepieces and entering your eyes. It is calculated simply by dividing the diameter of the objective lenses in millimetres with the magnification of the binoculars: 10×50 binoculars therefore have 50 / 10 = 5 mm exit pupil. Larger exit pupils mean smaller magnification, a larger field of view and a brighter image. Smaller exit pupils, on the other hand, mean a higher magnification, a smaller field of view and a somewhat dimmer image. As our eyes age the irises do not open as wide as they had. In young people the irises open to about 7 mm on average, while in older people they may open to ‘only’ 5 mm or so. This means the iris can act as a light baffle, cutting off some of the incoming light if the exit pupil of the binoculars is larger than the iris diameter. That is, if the exit pupil is 7 mm in diameter and the iris 5 mm, it cuts off some light. There is a silver lining though: even if that happens, you are still getting the best possible view at that magnification.
Handheld or on a mount?
Binoculars can be comfortably used handheld up to certain magnifications (and binocular weight). Up to 10x or 11x magnification and 70 mm lens size most binoculars can be used handheld, even for fairly extended periods of time. The weight if such binoculars is usually relatively small and the magnification sufficiently low that hands provide a sufficiently steady viewing platform. Leaning against a solid support, like the side and roof of a car, can help a lot. A mount / tripod will make the view much steadier and fainter objects will be visible. For stronger individuals 80 mm binoculars and up to 15x magnification will still be useable handheld. 100 mm and larger binoculars will require a strong and steady mount and can not be used well handheld.
Smaller and medium sized binoculars (50 to 80 mm range) can be mounted on stronger photo tripods using a suitable adapter. Larger binoculars – 100 mm and larger – require heavy duty mounts and tripods.
Scanning the sky near the horizon for comet C/2006 P1 (McNaught) in bright twilight with handheld 11×70 binoculars in late December 2006.
Tripod mounted 11×70 binoculars and a camera awaiting comet C/2021 A1 (Leonard) in December 2021.
In my experience
Here are some impressions about various binoculars of various sizes and powers I have used over the past three decades:
- 50 mm binoculars (7×50, 10×50): are great all round performers, small & light, but with a big light grasp. The offer very wide views (5-7° field of view) of bright comets. 7×50 usually offer wonderful very wide fields of view around ~7° in diameter, 10×50 around 5°. I would avoid higher magnifications in 50 mm binoculars (at least for astronomy). My personal favourite are the now discontinued Pentax PCF V series 7×50, but there are numerous fine 50 mm binoculars out there.
- 70 mm binoculars (10×70, 10.5×70, 11×70, 15×70, 16×70, 18×70): gather much more light and are better for astronomical observations. My current (since 2006) binoculars are 11×70 – the more or less first model from China that hit the market in 2002 under the Oberwerk brand and is now available almost everywhere under various brands (like Telescope Express, Helios and others). Inexpensive, but very capable (and maybe a bit fragile). They are good optically and very light (1.3 kg), easily used handheld for extended periods. Large field of view (4.6°). The Chinese 10.5×70 (the waterproof version) that is ~3x as expensive is noticeably better – sharper and with better transmission. Better overall, unless you want to do visual comet photometry (estimating the brightness of comets), where you will need center focus instead of individual focus. There are some expensive high end options of 70 mm binoculars out there, including APM, Nikon and Fujinon.
- 80 mm binoculars (11×80, 15×80, 20×80): go even deeper, but approach the limit of handheld observing due to weight. The 11×80 and 20×80 I have used were the same brand and class as the 11×70. The 11×80 is a monster, it goes deep – but probably better for young eyes as the exit pupil is huge. For older observers it probably doesn’t do much (any) better than the 11×70. 11×80 can be used handheld but they are heavier. The model appears to be discontinued. 20×80 is great for a closer look, but you need super steady hands or better, a good mount. The 15×80 I used were on an extended loan from another observer. A Japanese brand, can’t remember which one, but excellent binoculars both optically and mechanically. Loved them. Could be used handheld, better on a mount. 15×80 maybe hits a sweet spot with the exit pupil. There are many 80 mm options on the market. Just for fun: I also came across some German WW2 10×80 Flak binoculars that were used in anti aircraft artillery. No coatings on the lenses and it showed in the reflections on glass surfaces, but under dark Bortle class 2-3 skies the views were not bad at all.
- 100 mm and larger (25×100, 20×110, …): these are big and heavy binoculars that require good mounts. They go very deep and are excellent for astronomical observations. The 25×100 I used were the widespread Chinese variant. Big & heavy (4.5 kg), needs a mount. 20×110 are a space ship, our local astronomy club had them. Very heavy (7 kg!), needs a big and sturdy mount, but absolutely and totally worth it. Field of view is 2.6° – 5x larger in diameter than a full Moon. The best binoculars I have ever used (and it’s not even close).
For astronomy I would avoid binoculars with smaller objective lenses, that is below 50 mm. The light gathering ability of such binoculars is limited, as are the views.
So – now what? Which ones do I pick?
That is up to you. The good news is you can’t really go wrong. It will depend on several factors. Your budget. Obviously. There are numerous options for just about any budget. Your use. Do you want quick grab & go binoculars? Are you going to use them for other purposes too? Are you willing to carry a tripod / mount? My suggestion: once you get your binoculars forget all others and go and observe the sky. That’s what they are for!
In the upcoming Pt.2 I will talk about observing comets with binoculars – how to find them, where to look, timing, tips for detecting faint tails …
