1. astronomy A telescope whose lens (the "bucket") has a large aperture so as to gather and focus a large amount of photons, thereby producing brightened images of normally imperceptible objects. I'm always left amazed after using the light bucket at the university and glimpsing images of the cosmos way out in the dark of space.
2. By extension, such a lens (as on a camera or telescope) that has a large enough aperture to enhance images in low-level light. What's truly special about the light bucket on this camera is that it maintains a relatively high shutter speed while still capturing clear shots in low-light environments.