plate scale

Plate scale (or image scale) is a measure of the scale of a telescope's image on its focal plane. The term dates from the era of astronomical photography, i.e., on astronomical photographic plates. The plate scale is the relation between angle across the celestial sphere and distance across the image. Effectively, it is a measure of magnification or what's termed power for binoculars and backyard telescopes, but oriented toward imaging, etc., and is proportional to the reciprocal of the telescope's focal length. Equations:

p = θ/s ∝ 1/f

p(arcsec/mm) = 206265/fmm

• p - plate scale.
• θ - angle across the celestial sphere.
• s - distance across the image.
• f - focal length.
• p_(arcsec/mm) - plate scale in arcseconds per millimeter.
• f_mm - focal length in millimeters.

The plate scale does not determine the telescope's angular resolution, which depends upon the size of its Airy disks, in turn depending upon its aperture: a reasonable telescope design incorporates a complementary aperture and plate scale, presumably based upon its purpose and the resources available. The telescope's (sensing) instrument also affects angular resolution, e.g., CCD pixel size, (or in the photography era, the resolution of the photographic plates). Angular resolution is also affected by seeing.