the parabolic mirror

Small and cheap telescopes, based on the Newton’s optical scheme, normally employ a concave mirror with spherical shape as objective. Such mirrors can be used without intolerable effects on the images, within right limits (accordingly correct focal lengths).
In fact, spherical mirrors suffer just from the aberration their name says: spherical one.
Spherical aberration does not let each reflected ray to fall into the right focal point, but marginal ones will be more advanced than others.
As the diameter of the mirror raises, and unchanged focal length, this kind of aberration exponentially grows.
As an example, in order to maintain optical tolerance, a mirror with 400 mm diameter should have its focal length equal or higher than 5 meters: that means high difficulties when building and using that telescope.
The surface should be parabolic to avoid this kind of deficiency. The optical property of parabolic middle section mirrors, lets the incoming flat wave to be reflected into a single point: its focus. Every component ray performs identical distance inside the same media, the air (i.e. all the rays will arrive in phase to the focus). Beyond astronomic usage, parabolic mirrors are employed in other areas, for instance, holograms making, that requires a beam of monochromatic parallel rays.
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