Focal reducers are optical elements (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 objective lens from an achromat telescope or a finder scope could work as a focal reducer. DIY focal reducers may introduce aberration (coma) and must be considered when attempting this modification.
Using a Vixen 90 mm Objective Achromatic Lens
I have built a DIY focal length reducer (focal reducer) by inserting an objective lens from a 90 mm Vixen achromat along the optical system of a Sky-Watcher Equinox 100ED . The telescope’s native focal length is 900 mm at f/9. Using the objective lens of a Vixen 90 mm f/14 achromat, the focal length of the Sky-Watcher Equinox 100ED is reduced to 557 mm at f/5.57. To reach focus, I had to shorten the optical tube to about 55 mm. The focuser’s draw tube was also shortened to prevent it from obstructing the light and stopping down the objective lens when the draw tube moves inward. Varying the distance between the main lens and the reducer lens affects the resulting effective focal length. From 2020 to 2024, I have tried placing the DIY reducer at various distances until I finally found one configuration that produces stars with acceptable sharpness. To view a sample photo with a DIY reducer using a Vixen 90 mm achromat objective lens, click here.
Using a Threekor 40 mm or 50 mm Finder Scope Lens
Finder scope lenses can also be used a reducers. I have used the objective lens of a 40 mm Threekor finder scope as DIY reducer for my 8 inch Celestron SCT. I just removed the finder scope’s lens and then placed it at the threaded end of the 8 inch SCT, where you would attach a visual back adapter. I used a spare tube extender (I use M42 spacers) to mount the lens and insert it into the optical path. Upon testing, it reduced the focal length from 2000 mm to 1140 mm. I have also successfully used a 50 mm finder scope achromat lens as a DIY reducer. To view a sample photo with a DIY reducer using a 40 mm finder scope achromat objective lens, click here.
Related link: Images taken with a DIY Focal Reducer
Night Sky in Focus © Anthony Urbano | Bacoor, Philippines
Night Sky in Focus 