Plate scale

The plate scale p of a telescope connects the angular separation θ of an object with the linear separation s 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 p=θ / s = 1 / F, since s=Fθ.

Plate scale is usually expressed in arc-seconds per mm p=206265 / F, where F is in mm, or expressed in arc-seconds per pixel after further division through the pixel scale.

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'

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