Newtonian telescope secondary mirror. This design reduces the size and cost of the telescope with a shorter overall telescope tube length (with the corrector extending the focal length in a "telephoto" type layout) combined with a less costly spherical mirror. Commercially produced versions of this design have been noted to be optically compromised due to the difficulty of producing a correctly shaped sub-aperture corrector in a telescope targeted at the inexpensive end of the telescope market. Newtonian telescope The Newtonian telescope, also called the Newtonian reflector or just the Newtonian, is a type of reflecting telescope invented by the English scientist Sir
Folk instrument Folk instrument A folk instrument is a musical instrument that developed among common people and usually does not have a known inventor. It can be made from wood, metal or other material. Such an instrument is played in performances of folk music. The instruments can be percussion instruments, or different types of flutes or trumpets, or string instruments that are plucked, hammered or use a form of bow. Some instruments are referred to as folk instruments because they commonly appear in folk music, even though they do not meet the criteria defining a folk instrument; an example is the harmonica.