Minimum distance $$d_{\text{min}}$$ of two planets at which the planets can still be perceived as two separate objects through a telescope. This distance depends on the refractive index $$n$$ of the medium between the telescope and the objects, but also on the wavelength $$\lambda$$ of the light used for observation. The aperture angle of the telescope also has an influence on the resolution limit. The minimum distance can be calculated with the Rayleigh criterion.