Plate scale explained

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 = (\theta)/(= s) (1)/(f) ,

since

s = f \theta .

Plate scale is usually expressed in arcseconds per mm:

p ≈ (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 and References

1. Book: Carroll, Bradley . An Introduction to Modern Astrophysics . 1st . Ostlie, Dale . 163 .
2. http://www.stsci.edu/jwst/instruments/niriss/instrumentdesign
3. http://www.stsci.edu/jwst/instruments/niriss/instrumentdesign