I have built a DIY focal length reducer (focal reducer) by inserting a converging lens from an old telescope along the optical system of a Sky-Watcher Equinox 100ED . The telescope’s native focal length is 900 mm at f/9. With the DIY reducer, the focal length is reduced to 628 mm at f/6.28 using the objective lens of a Vixen 80 mm f/11 achromat.
Focal reducers are optical elements (usually a convex lens or lens group) that converge light from a telescope’s objective. It shortens the focal length and in effect, produces a faster telescope (lower f/ratio) and widens the field of view (larger portion of the sky is imaged). Any decent quality converging lens should work as a focal reducer (in this use case, a lens from a telescope I no longer use). It works opposite to a Barlow lens which increases the focal length by using a concave lens or diverging lens. DIY focal reducers may introduce aberration and must be considered when attempting this modification.
To reach focus, I had to shorten the optical tube by about 200 mm, and then reattach the focuser. The focuser’s draw tube was also shortened by 55 mm to prevent it from obstructing the light and stopping down the objective lens when the draw tube moves inward. The telescope’s optical tube has an inner diameter of about 100 mm which has enough space to accommodate various lenses. Only the central 60 mm part of the reducer is used to refract the light cone due to the presence of a light baffle in the telescope’s optical tube assembly.
Related link: View all home-brewed DIY astronomy equipment
Night Sky in Focus © Anthony Urbano | Bacoor, Philippines