Resolution
Spatial resolution is a quality criterion of an optical instrument. More descriptive however is the
reciprocal value of that quantity, the resolution limit. It designates the minimum distance two points of an object may have, to be separated in the picture.
The resolution limit ‘d‘ of a telescope depends on its lens aperture D, its focal length f and
the wavelength of light λ (lambda), following the equation:

d = λ x 2f/D

Substituting the data of my telescope, i.e.
aperture D = 4" (90 mm),
focal length f = 40" (1000 mm) and the
smallest visible wavelength λ >= 17.7 µInch (450 nm for blue light)

you calculate d = .0004" (1/100 mm).

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