^Resolving power a telescope
For the two stars to be just resolved,
Implying, Resolving limit of a telescope is,
Resolving power of a telescope is,
Thus Δθ will be small if the diameter (2 a) of the objective is large. This implies that the telescope will have better resolving power if a is large. It is for this reason that for better resolution, a telescope must have a large diameter objective. Also a bigger objective collects more light & hence produces more bright images. We can also say by increasing the diameter of the objective lens we can increase the range of a telescope. The stars which are not resolved in the image produced by the objective can’t be resolved by any further magnification produced by the eyepiece. The primary purpose of the eyepiece is to provide magnification of the image produced by the objective.