Binoculars For Astronomy

Binoculars are a possible "small telescope" for astronomical use. They have distinct advantages: portable, let you use both eyes, low powers and wide fields. They also have some disadvantages. I briefly discussed astronomical binoculars in an article about Rich Field Telescopes at this site and in this article I extend that discussion and talk about my binocular usages.

Binoculars as Low Power Telescopes

Binoculars are convenient low-power telescopes: small, handy, and you may have them laying around the house. And "binocular astronomy" can be satisfying: there is a binocular astronomy column every month in Sky and Telescope magazine.

And binoculars offer an intrinsic advantage: they feed light from the object to both eyes. This can make it easier to see faint objects - the benefit varies from person to person. (It is easy to ascertain how much you benefit: just use binoculars some night on faint targets and compare using both eyes versus one eye closed or one of the objectives capped.)

But the typical binoculars that are in fact "laying around the house" are usually about 6-8 power. I find this low for astronomy. And if we move to "astronomical or big binoculars" we lose some of the advantages: the binoculars become expensive, heavy, and single-purpose.

Much interesting observing can be done at 6-8 power. It is just that your targets are limited, and many interesting targets will be too small to see well. If you already have suitable binoculars, by all means give them a try.

Binocular Nomenclature

The aperture and power of binoculars is specified by a pair of numbers, written thus: 7x42. That means the binoculars magnify 7 times and the objectives are 42mm in diameter.

For astronomy, you want binoculars with (relatively) large aperture, because you will be looking at dim objects. (Birders and casual daytime users generally prefer smaller aperture, because it makes the binoculars lighter and easier to carry, and makes them less expensive -- and a big "exit pupil" is not adventageous in the daytime.)

At my Rich Field Telescope article I discuss aperture vs power in the context of "exit pupil" size. Briefly: for faint astronomical objects and especially for use with nebula filters, you want an "exit pupil" of from about 5mm to 7mm. To determine the exit pupil, just divide the aperture by the power. Thus 7x42 binoculars have 42/7 = 6mm exit pupils.

Large Binoculars

Binoculars that are considerably larger than casual or birding binoculars are sold by several companies for astronomical observing. Sometimes these look like ordinary binoculars scaled-up to be giants. Other times they are really two small refractor telescopes, mounted side-by-side.

Big binoculars can be fun, but they are a serious investment. I advise to try regular binoculars and also try a regular small telescope before investing.

For a somewhat different view that suggests 10x50 as an astronomy binocular and discusses particular binoculars in some detail, see Steve Tonkin's page on astronomical binoculars. Steve Tonkin's main page also discusses binoculars and has a pointer to his book "Binocular Astronomy" (I have not read).

Binoculars and Nebula Filters

As you may have noted from other pages at this web-site, I like to use modern nebula filters for astronomy.

With binoculars, you have two problems: 1) you need two filters 2) you need to be able to fit the filters to the optics. The first problem is simply one of doubled cost.

But fitting filters to binoculars can be difficult. Most telescopes have one or more convenient ways to insert a filter into the optical-path: you can place a filter-slide in the optical path, as I do, or you can screw-in filters to the scope-end of most eyepieces.

But ordinary binoculars are a sealed unit. They do not come apart for the addition of filters near the eyepieces. Fortunately, you can attach filters at the objective-ends. Unfortunately, that can mean large filters. The clear-aperture of a Lumicon "two inch" nebula filter is about 43mm. (The filter has 48mm threads.) It is not very difficult to arrange a pair of filters in front of the objectives. Some binoculars come threaded for filters and then camera-store up-adapters and down-adapters will size match. Binoculars without such threads can often be given threads via a camera down-adapter that is held in place at the objective, say by the rubber protective coating.

The real problem is the aperture. If you buy, say, 70mm aperture astro-binoculars, you would like 70mm aperture filters. This is a expensive custom purchase. I own two pairs of binoculars that I use for astronomy: 10x42 Canons and 7x42 Bushnells. The 42mm apertures are not a coincidence - they are the biggest I can use with my "two inch" filters.

Some large binoculars are not a big version of small binoculars, rather they are two small refractors mounted side-by-side. Usually these binoculars allow for eyepiece switching etc just like telescopes, and you can use filters at the eye-end like you would with a small refractor. A few binocular-shaped astro-binoculars come with removable eyepieces that will take 1.25 inch filters.

Objects to Observe

For a small discussion of binocular-observing targets, see my discussion of RFT targets and also my discussion of RFT nebula-filter targets.

Binocular Mounts

If you observe the night sky with heavy binoculars for any length of time, or if you use binoculars of more than about six power, you will appreciate a bino-mount.

A device that just attaches the binoculars to a photo-tripod is awkward for astronomy, since you are looking up rather than out, and if the binoculars are on top of a tripod, the tripod legs are very-much in your way. What you want is a gadget that holds a counterweighted arm out to the side. You can attach your binoculars (some have tripod sockets, or improvise with a clamp) to the arm and the binoculars will hang where you put them.

Such an arm is typically an articulated "parallelogram arm", so that you can move the binoculars up and down while they stay pointed at the same place. This is useful. It would be more useful if the "stay pointed" also covered rotating the whole assembly about the tripod vertical axis -- that is moving the binoculars in azimuth. But it does not, so if you push the binoculars away so you can get out of the chair and let another observer replace you, the binoculars will probably not be correctly pointed for the next person.

I find a bino-mount very useful. The ones that you see retailed, however, are fairly expensive for what they are - you might consider looking at some and then making a bino-mount as a home project. You don't really need a parallelogram mount, a simple counterbalanced arm with a bracket that allows pointing will do.



I would be happy to correspond about binocular usage(s) with interested individuals.

I can be reached via email at