Telescope – Frequently Asked Questions (FAQs)

Refraction of different colour components of white light by different amounts. This causes white light to split into a rainbow and produces false colour fringing.

Achromatic refractor telescopes that have basic quality glass and only one or two elements in the objective group can produce chromatic aberration. This means that a beam of white light coming through the lens will be split into a rainbow, which means that the different colours will focus at different distances from the lens. From the point of view of the camera sensor, a star imaged through an achromatic telescope will have a false colour fringe around it, often red or blue.

Apochromatic telescopes are made to resist chromatic aberration. They may do this by adding lenses to ensure that the different colour components of light focus at the same point, or they may do this by using extra low dispersion glass that produces less dispersion in the first place. High quality apochromatic telescopes will use both strategies.

The aperture or diameter of the primary lens or mirror of a telescope determines the light gathering capacity of the telescope.

In Newtonian reflector telescopes that have paraboloidal mirrors, off-axis light (that is, from a point near the edge of the image) will come to focus slightly behind the focal plane, and this image will also not form a point. This can be managed using a correcting lens (coma corrector) near the focuser.