Glossary | Telescope accessories | General | Type of build | Telecentric
Increases the focal length and creates a parallel beam of light
A telecentric system extends the focal length in the same ways as a Barlow lens, but combines this with a parallel light path. As a result, the working distance (back focus) does not have to be exactly maintained, the quality and image scale remain the same. At the same time, telecentrics deliver an image with much higher contrast.
The parallel light path and high contrast are a great advantage for both visual observation and for photographing the Moon and planets.
In the case of H-alpha systems for solar observation, a telecentric is actually a must-have, since the Etalon filter depends on a parallel beam path. This also applies to other narrowband interference filters, such as etalon filters for the calcium-K line.