# Compound microscope vs astronomical telescope

In compound microscope, we take such an objective lens which has small focal length. While in astronomical telescope, we take such an objective lens which has large focal length.

Why don't we use same objective lens in both? The function of both the devices is to enlarge an object at infinity.

• What do you mean by "enlarge an object infinity?" Do you mean enlarge an object at infinity, or enlarge an object infinitely (i.e., maximize magnification)? – Ben Crowell Aug 14 '13 at 13:46
• eddited......:) – Rafique Aug 14 '13 at 13:56
• OK, thanks for the clarification. As explained in Kyle Kanos's answer, the microscope doesn't work on objects at infinity. – Ben Crowell Aug 14 '13 at 15:11

So it basically depends on the fact that most of our astronomical sources are at distances larger than $\sim10^{18}$ cm away from us, while our microscopic objects are less than $\sim100$ cm away from us.