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

since


 * $$s =f \theta\ .$$

Plate scale is usually expressed in arcseconds per mm:
 * $$p \approx\frac{206265}{f},$$

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

Plate scale on JWST FGS/NIRISS
The plate scale of the James Webb Space Telescope component Fine Guidance Sensor and Near Infrared Imager and Slitless Spectrograph is about 0.065 arcsec/pixel. It uses a 2048 x 2048 pixel array with a pixel size of 18 microns a side with a field of view of 2.2' x 2.2'