Working and Principle :
Resolving power of a telescope is defined as the reciprocal of the smallest angle subtended at the objective lens of the telescope by two point objects which can be just distinguished as separate.
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Resolving power of telescope is given by :
Resolving Power = D/d = a/λ
Resolving power for rectangular aperture = a/λ
Where, λ = 589 nm mean wavelength of light used
D = is distance of the objects from objective of the telescope.
d - is distance between two consecutive slits or objects
a - is critical width of the rectangular slit for just resolution of two slits or objects.