Plate scale can be derived using this image assuming that the angle is very small (so that tan(theta) is approximately theta).

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.

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.^[2] 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' ^[3]

See also

• Photographic plate

Notes