Back focus (astronomy)
In observational astronomy , back focus is the length by which the focal plane lies behind the fully retracted focuser . The focal length of a camera, for example, must be smaller than the back focus of the instrument in order to be able to focus. In Newtonian telescopes , the back focus is often relatively small in order to be able to keep the secondary mirror in the aperture small. With Cassegrain telescopes, on the other hand, the secondary mirror extends the focal length so that, as with refractors, there is enough back focus available.
Web links
- Peter Oden: What actually is ... back focus? Adventure Astronomy , June 15, 2018 .