The telescope: Resolving power
 
Starting with the diaphragm at full aperture, slowly close it down until it is no longer possible to resolve the stars as being separate points of light. Measure the diameter of the aperture with the selected diaphragm at this condition. Repeat the procedure and take the mean from five results. By assuming λ = 5500 A˚ , determine the resolving power for the setting of the diaphragm.
Repeat the experiment but this time commence with minimum aperture and slowly enlarge the diaphragm until the stars are resolved. Measure the diameter of the aperture required to resolve the stars under this operation.
Take five resolving power measurements with the blue, green and then the red filters in the beam. According to the effective wavelengths of the filter–eye combination, evaluate the resolving power of the telescope and see how it changes with wavelength.
Compare the observed values of resolving power with the actual angular separation of the double star.
These experiments serve to illustrate the difference between the resolving power as established by diffraction theory, using some chosen criterion, and the actual resolving power as determined with a telescope in combination with the eye.
In practice, when real double stars are being observed, the resolving power depends on the observing conditions, on the relative brightnesses of the stars and on their relative colours. The amount of scattered or background light may also affect the value of the resolving power. Apparent resolution can sometimes be achieved by partial knowledge of the objects such as the position angle of the line joining the two stars.
Some people are better than others at being able to resolve star pairs and may be able to resolve stars which are, in fact, closer than the Rayleigh limit.