Plate scale

File:Plate Scale.gif

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 Template:Mvar is measured in millimeters, the plate scale in radians per millimeter is given by angular separation Template:Mvar, and the linear separation of the image at the focal plane Template:Mvar, or by simply the focal length Template:Mvar: $$p=\frac{\theta }{s}=\frac{1}{f},$$ since $$s=f\theta .$$

Plate scale is usually expressed in arcseconds per millimeter: $$p\approx \frac{206265}{f},$$ where Template:Mvar is in mm, or expressed in arcseconds per pixel after further division through the pixel scale.

Plate scale on JWST FGS/NIRISS

The pixel scale of the James Webb Space Telescope component Fine Guidance Sensor and Near Infrared Imager and Slitless Spectrograph is about 0.066 arcsec/pixel. It uses a Template:Resx pixel array with a pixel size of per side with a field of view of Template:Resx

See also

• Photographic plate