If $F_o$ and $F_e$ are the focal lengths of the objective and eyepiece respectively of a telescope,then its magnifying power will be

The magnifying power of a telescope can be increased by

In an astronomical telescope,the focal lengths of two lenses are $180 \, cm$ and $6 \, cm$ respectively. In normal adjustment,the magnifying power will be

$A$ lens of large focal length and large aperture is best suited as an objective of an astronomical telescope since :

An astronomical telescope has an angular magnification of magnitude $5$ for distant objects. The separation between the objective and the eyepiece is $36 \, cm$. The final image is formed at infinity. The focal length $f_o$ of the objective and $f_e$ of the eyepiece are

The focal length of the objective of a terrestrial telescope is $80 \, cm$ and it is adjusted for parallel rays. If its magnifying power is $20$ and the focal length of the erecting lens is $20 \, cm$,then the full length of the telescope will be......$cm$.