Q.Discuss about the astronomical telescope.

Step 1. An astronomical telescope, used to magnify the apparent angular size of distant objects, has an objective of long focal length $f_o$ and large aperture, and an eyepiece of comparatively short focal length $f_e$.

Step 2. Parallel rays from a distant object enter the objective and form a real, inverted image at the objective's own second focal point (very nearly coinciding with the eyepiece's own first focal point, for the standard normal-adjustment configuration).

Step 3. The eyepiece, positioned so this real intermediate image sits at its own focal point, then acts exactly as a simple microscope in normal focusing, producing a final, further-magnified, still-inverted image at infinity.

Step 4. The telescope's angular magnification $m$ is the ratio of the angle $\beta$ the final image subtends at the eye to the angle $\alpha$ the original distant object itself subtends (at the unaided eye, or equivalently at the objective); both angles relate to the same intermediate image height $h$ via $\alpha\approx h/f_o$ (from the objective's own focal-length geometry) and $\beta\approx h/f_e$ (from the eyepiece's), so $m=\beta/\alpha=\boxed{\dfrac{f_o}{f_e}}$. …

Compare the angle the distant object subtends with the angle the final magnified image subtends, both express …

• Inverting the ratio fo/fe (using fe/fo instead), which would give a magnification less than 1 -- physi …