The power of a telescope is typically expressed in terms of its resolution, which is the smallest angular separation between two objects that it can distinguish. This resolution is often expressed in arcseconds (").

Therefore, a power of 0.5 seconds of arc (0.5") would indeed be a valid expression for the resolving power of a telescope. This means that the telescope can distinguish between two objects that are separated by an angle of 0.5 arcseconds.

Note: The term "power" is sometimes used colloquially to refer to the magnification of a telescope. However, the actual resolving power of a telescope is determined by the diameter of its objective lens or mirror and is not directly related to its magnification.