Although it is simple, it can reveal the craters of the Moon and the satellites of Jupiter. It is also very useful for demonstrating lens aberrations. You really need to build this telescope as a necessary step towards understanding the solutions employed in the second and improved telescope model. In the section “From Lenses to Optical Instruments”, you saw how a telescope works; here I simply remind you that the objective lens produces an image of the object observed, and this image is magnified by the eyepiece. Link.