The objective of a telescope is its light gathering element. The size of the objective is the major characteristic to consider when deciding which telescope to buy. A major purpose for a telescope is to collect as much light as possible and concentrate the light so the object appears brighter. Light gathering power depends on the area of the objective or the square of the objective's diameter. Although many factors influence resolution, resolving power depends on the diameter of the objective.
Many astronomical applications use relatively low magnifications.
Theoretically, magnification above 50x per inch of objective diameter is useless. Magnifications in the range of 15x - 30x per inch are more commonly used. Therefore, usable magnification depends on the diameter of the objective. Buy the largest diameter objective you can afford. Telescope Selection Binoculars Often overlooked for astronomical use, binoculars provide an opportunity to explore the night sky with low magnification and wide field of view.