For high magnifications, the diameter of the objective — how much light it collects — matters as much as the focal length. Building a lens a metre or more across is technologically difficult: thick glass absorbs light and chromatic aberrations become enormous. In 1672 Newton replaced the objective lens with a parabolic concave mirror: long focal length, no chromatism (reflection is independent of ), much lighter structures.
All large modern telescopes (Hubble, JWST, VLT, ELT) are reflectors: the objective is a concave mirror that concentrates light at the primary focus, and a secondary mirror redirects the beam to an eyepiece (Newtonian mount) or to a scientific instrument (CCD, spectrograph).
Key formula
The angular magnification remains that of the refractor:
Note
The decisive advantage of the mirror is that the law of reflection does not depend on wavelength: all colours are focused at the same point, without the chromatic aberration that afflicts lenses.
Collegamenti
Argomenti: Ottica Concetti: Lenti e specchi · Riflessione
Esercizi collegati: Problema — Raggi per oggetto oltre 2f (lente) · Problema — Raggi per specchio concavo con p minore di f · Esercizio svolto — Specchio concavo