with consumer telescope manufacturers because it combines easy-to-manufacture spherical optical surfaces to create an instrument with the long focal length of a refracting telescope with the lower cost per aperture of a reflecting telescope. The compact design makes it very portable for its given aperture, which adds to its marketability. Their high f-ratio means they are not a wide-field telescope like their Schmidt camera predecessor, but they are good for more narrow-field deep sky and planetary viewing. Consumer version of this design typically achieve focus by adjusting the position of the primary mirror rather than a traditional eye-piece. This means that small changes in the position of the mirror are magnified by the focal length of the telescope. As the mirror is not permanently fixed in place, it is possible for it to move by a small amount and cause the image to shift. This is otherwise known as "mirror flop". Some Schmidt-Cassegrain telescopes are equipped with mirror locks to fix the primary mirror in place once focus has been achieved. See also Argunov–Cassegrain telescope Dobsonian telescope List of telescope types Maksutov telescope Newtonian telescope Ritchey–Chrétien telescope Schmidt camera Schmidt–Newtonian telescope References External links Telescope types