One of the things we try to do with this What’s Up segment is tell you about things you can see with binoculars or the unaided eye rather than just a telescope, but we haven’t really talked about the different types of binoculars useful for astronomy and when you should get one instead of a telescope.

The Royal Astronomical Society of Canada has advice for which type of binoculars to get depending on what you want to see. If your budget is limited, get a good pair of binoculars rather than a cheap telescope because a cheap pair of binoculars will be better than a cheap telescope. Binoculars are also more user-friendly because they have wider fields of view than a telescope and provide the same orientation as your unaided eye, unlike most telescopes. Binoculars also provide a brighter field of view and not just because you use both eyes but because of the bigger entrance pupil (diameter of the light beam that comes out the back), which the low magnification provides.

There is no single binocular type best for astronomy. We’ll get to what the RASC advises for binoculars, but first some terms. 

Binoculars are listed by magnification and aperture, for example, 7×50 where 7 is the magnification and 50 is the aperture of each lens. There are other binocular features such as different prism types, Porro vs Roof. Most astronomy binoculars use the Porro prism because it provides a wider field of view and better image quality. It’s also cheaper. Roof prisms are more expensive and have features that are more useful for terrestrial binoculars, like compact size and durability.

Magnifications between 6 and 10 are ideal for scanning the sky, which can help with learning the constellations and other things. This class is also good for comets and large star clusters. Finally, low magnification binoculars can be handheld easier.

Binoculars with higher magnification, between 10 and 20, are more useful for smaller clusters such as the Pleiades and Hyades; however, they are harder to use, and you may need a tripod to use them comfortably, taking away some of the portability. Over about 10 times magnification, it may be easier to use a small telescope with a wide field eyepiece to provide a more comfortable and stable mounting but still get the wide field of view.

Another consideration for a larger pair of binoculars is just how much more you can see with them. Brightness increases as the aperture gets bigger, but brightness decreases as the magnification gets higher. The effect of these two factors is bigger binoculars, those with high magnification and large aperture, let you see dimmer things in more detail because the background will be darker. In general, doubling either the aperture or magnification will let you see four times as much. Sounds pretty obvious, but there is some detail that needs to be taken into account. Bigger optics will be affected more by the sky quality than smaller optics, so depending on your average weather, a smaller-aperture, lower-magnification binocular may outclass a larger one in resolution for your situation.

One upcoming event this month is what’s known as a Messier Marathon. Because of where and how ya boi Charles compiled his list of objects, it is possible to see all 110 objects in a single night from mid-March to April. Depending on your stamina, it may take you a few nights to get all of them in, and you may not get to all of them at all. That’s fine too; astronomy is a hobby, and you shouldn’t burn yourself out trying a challenge if you aren’t ready for it.

The Royal Astronomical Society of Canada has an observing program dedicated to the Moon for both binoculars and telescopes, and you don’t need to be a member to download it. The binocular edition has forty features to look for, and the telescope edition has one hundred. We’ll have a link to it in the show notes for this episode.

So grab some binoculars and a sky map or three, head outside, and look up.

More Information

Explore the Moon (RASC)

Porro Prism vs Roof Prism Binoculars: Which is Best? (Optics Mag)