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In my book it is written that "The angular resolution of the telescope is determined by the objective of the telescope. The stars which are not resolved in the image produced by the objective cannot 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."

I can't understand why eyepiece cannot resolve image of objective (lens) furthur.

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  • $\begingroup$ You can never increase (improve) the resolution limit of upstream optics. It's the primary aperture which controls the resolution. $\endgroup$ – Carl Witthoft Feb 4 '16 at 14:13
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Here is a series of images from two point objects (stars) to illustrate the idea of resolving power.
The objective lens of the telescope produces the diffraction.
If the distant stars are closer together the diffraction patterns come closer to one another.

enter image description here

Diagram (a) shows to diffraction patterns (the intermediate image in a telescope) well spaced whereas diagram (c) shows the stars were so close to one another so that the patterns overlap so much that it looks like one diffraction pattern. Diagram (b) illustrate a condition where the images from the two stars can be distinguished.

enter image description here

The eyepiece magnifies the intermediate images. So if the (c) is magnified you still cannot resolve the two stars all you get is a larger image the lack of resolution was due to the aperture of the objective lens.

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