Above about 60mm, binoculars stop being something you hold and become something you mount, and the reward for that inconvenience is a view that sits between a handheld pair and a small telescope with two eyes instead of one.
What a giant pair adds
A 15x70 gathers about twice the light of a 10x50 and resolves noticeably more. Globular clusters that are fuzzy dots at 10x start to show texture, the brighter galaxies show shape, and the moon becomes genuinely detailed. Using two eyes makes faint objects easier to see than the same aperture in a single telescope, an effect experienced observers consistently report.
What it demands
A mount. Fifteen times magnification is impossible to hold steady, and a 70mm pair weighs enough that arms tire in a minute. A parallelogram mount is the traditional answer because it lets the binoculars swing to any position while staying at eye height, and it costs about as much as the binoculars do.
The neck problem, which is real
Mounted binoculars pointed near the zenith put your head in an awkward position, and this is the single most common reason large binoculars stop being used. A reclining chair solves it completely and is the accessory owners recommend most.
How to choose the size
Fifteen by seventy is the usual entry point to mounted binoculars: about twice the light of a 10x50, still light enough to carry in one hand, and cheap enough that the mount costs more than the optics. Twenty by eighty is the next step and needs a serious mount. Beyond that, the market thins quickly and prices rise steeply, because large binoculars need two matched objectives and two matched prism assemblies where a telescope needs one mirror.
Questions people ask about binoculars
What are astronomical binoculars?
Large binoculars, usually 15x70 and up, intended for mounted use on the night sky rather than handheld daylight use.
Do I need a mount for 15x70 binoculars?
Yes. At that magnification and weight the shake removes most of the detail the extra aperture gained you.
Are big binoculars better than a small telescope?
Different. Two eyes and a wide field beat a small telescope for sweeping and for large faint objects; the telescope wins on magnification and on planets.