Newtonian telescopes are the cheapest ones in a given aperture. No colour aberration but the image is less contrasty because of the central obstruction caused by the secondary mirror. The fast ones are recommended for deep-sky observations and astrophotography, but can also be used for planetary observations. Somewhat more compact telescopes than the traditional refractors but require a precise alignment of the optical elements. With some limitation they can be used for daytime observations as well. General purpose telescopes for both beginners and advanced amateur astronomers.