The second number is the aperture of the main lenses (the side the light comes in) in millimeters. The first number is the magnification provided by the binoculars. Binoculars are described using two numbers, usually in a "X x XX" format. (Want the TL DR version? Skip to the end.) What to look for in astronomy binocularsįirst, let's talk numbers. Go the other route of cheaper lenses, less coatings, BK-7 prisms, poorly collimated optics and you have a recipe for difficult to focus binoculars with soft / fuzzy star images out the outer edge of the field, vignetting of the field of view, and an instrument that may make your eyes / head hurt! Yes, they are inexpensive (sometimes as low as $60), but is that really worth it? Good, high-quality, anti-reflection coated lenses, BAK-4 prisms, expertly-collimated binoculars make great, specialty astronomy binoculars, but observers with instruments such as these will also need a beefy, strong tripod, because their weight will make them heavy to hold even after short periods of time, and the magnification will require it. We need more light through the lenses to see more, and higher magnification is always a good thing, right? Well. These seem like a great idea as well: 60 and 70 millimeter lenses with 15, 20 and 25 times magnification. That leads to the flip side the binocular world: The large aperture, high magnification ones. And though these may be perfectly acceptable for quick views of the occasional bird or squirrels, the sheer physics of their limited light gathering make them impractical for usage in astronomy.
Miniature, pocket-binocs can be had for $15 or $20, which is an astonishing price point when you stop to consider how many lenses and prisms are in them.