Plate scale

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 f is measured in millimeters, the plate scale in radians per millimeter 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 millimeter: $${\displaystyle p\approx {\frac {206265}{f}},}$$ where f is in mm,^[1] or expressed in arcseconds per pixel after further division through the pixel scale.