Plate scale

The plate scale of a <a href="/facts/Telescope/BDwYahhR">telescope</a> connects the <a href="/facts/Angular_separation/6ob2n0Wb">angular separation</a> of an object with the linear separation of its image at the <a href="/facts/Focal_plane/7vqmsXJy">focal plane</a>.
If focal length 
 
 
 
 f
 
 
 {\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:

p
        =
        
          
            θ
            s
          
        
        =
        
          
            1
            f
          
        
         
        ,
      
    
    {\displaystyle p={\frac {\theta }{s}}={\frac {1}{f}}\ ,}

since

s
        =
        f
        θ
         
        .
      
    
    {\displaystyle s=f\theta \ .}

Plate scale is usually expressed in <a href="/facts/Minute_and_second_of_arc/tybs6YGC">arcseconds</a> per mm:

p
        ≈
        
          
            206265
            f
          
        
        ,
      
    
    {\displaystyle p\approx {\frac {206265}{f}},}

where f is in mm,
or expressed in <a href="/facts/Minute_and_second_of_arc/tybs6YGC">arcseconds</a> per pixel after further division through the <a href="/facts/Pixel_density/0UbaSzsw">pixel scale</a>.
Plate scale is not changed when telescopes zoom in or out because the same amount of sky will be in the pixel whether it is enlarged or shrunk.

Plate scale open-in-new

Plate scale