# Calculating Field of View of a Telescope Given Time of Another Object

Given the celestial coordinates of a certain star and the time it takes to pass across your telescope's eyepiece, how can you calculate the field of view of the telescope?

This technique for determining the field of view of a telescope and eyepiece is called drift timing. Since the sidereal day is about 4 minutes shorter than the usual solar day, a fixed point on the celestial equator rotates at about 15.04 arc seconds per second of time. Thus the true field of view in arc seconds is given by $$TFOV=15.04 \hspace{2pt} t \cos(\delta)$$ where $$t$$ is the time in seconds for the star to drift across the field, and $$\delta$$ is the declination of the star. Divide by 60 to convert to arc minutes, or by 3600 to degrees.