Mirrors and lenses are the critical light-bending components of a telescope.
- Objective: The first lens or curved mirror that collects and focuses the incoming light.
- Corrector plate: A full aperture negative lens placed before a primary mirror designed to correct the optical aberrations of the mirror.
- Focusing mask: A full aperture mask temporarily placed before the primary mirror to aid in focusing the telescope.
- Sub-aperture corrector: One or a series of corrective lens (sometimes combined with a corrective curved mirror) placed after (near the focus) a primary mirror designed to correct the optical aberrations of the mirror. These can be just a small version of the corrector plate, but since they are usually used in a Cassegrain configuration in front of the secondary mirror they require additional modification since the light passes through them twice.
- Secondary mirror
- Curved mirror
- Honeycomb mirror
- Liquid mirror
- Parabolic reflector
Subsequent (sometimes optional) components realign, segment, or in some way modify the light of an incoming image:
- Field lens: A correcting lens placed just before the image plane of a telescope.
- Telecompressor or focal reducer: Optical element to decrease the telescope's focal length and magnification (usually by a fixed percentage) and widen the field of view, providing opposite effects of a Barlow lens.
- Star Diagonal: Used to change the angle of the light coming out of a telescope, for easier viewing.
- Herschel Wedge: Similar to a star diagonal with a wedge-shaped unsilvered prism reflector that reduces incoming light by up to 95% for solar viewing.
- Coma corrector a correcting lens used to reduce coma distortion in fast reflecting telescopes.
- Field flattener a correcting lens used to reduce field curvature in refracting telescopes for astrophotography.
- Barlow lens: Optical element to increase the telescope's focal length and magnification, narrow the field of view and reduce coma distortion, providing opposite effects of a telecompressor.
- Astronomical filter: Used to select specific colors (or light frequencies) for astrophotography.
- Filter wheel: One manner to easily insert filters into the optical train. Mostly used for photography.
- Focuser: Allows the user to adjust the focus by moving the eyepiece along the optical axis.
- Eyepiece: Performs the final focus correction before the light reaches the eye.
- Charge-coupled device (CCD): A light-sensitive integrated circuit digital sensor (commonly used in digital cameras) that turns light into an electrical charge used to collection image data.
Generally applicable to all items:
- Metallizing: A way of coating mirrors for high-efficiency light reflection.
- Optical coating: Thin layers applied to mirrors, filters, and lenses to avoid reflections, as well as absorb certain colors.