The latter defect may be detected by cutting out three circular cards of suitable size with a small aperture at the centre of each, and inserting one at each end of the eye-tube, and one over the object-glass.

If the tube is rightly placed the apertures will of course lie in a right line, so that it will be possible to look through all three at once.

If not, it will be easy to determine towards what part of the object-glass the eye-tube is directed, and to correct the position of the tube accordingly.
The best tests for determining the defining power of a telescope are close double or multiple stars, the components of which are not very unequal.

The illuminating power should be tested by directing the telescope towards double or multiple stars having one or more minute components.

Many of the nebulae serve as tests both for illumination and defining power.