The plate scale of a telescope connects the angular separation of an object with the linear separation of its image at the focal plane.

If focal length ${\displaystyle f}$ is measured in mm, the plate scale in radians per mm is given by angular separation θ and the linear separation of the image at the focal plane s, or by simply the focal length f:

${\displaystyle p={\frac {\theta }{s}}={\frac {1}{f}}\ ,}$

since

${\displaystyle s=f\theta \ .}$

Plate scale is usually expressed in arcseconds per mm:

${\displaystyle p\approx {\frac {206265}{f}},}$

where f is in mm,^[1] or expressed in arcseconds per pixel after further division through the pixel scale.