The Askar 3" f/3.9 full frame reducer features 4 elements optical design specifically for the FRA600, offers a 0.7X reduction in focal ratio, when used with the FRA600 astrograph,
This reducer effectively shortens the exposure time - the FRA600 focal length is reduced to 420MM, dramatically increasing your field of view and bringing more convenience and possibilities for your visual use or astrophotography.
The backfocus is the standard 55mm. The reducer also has a built-in M48X0.75 (2 inch) filter thread that allows you to thread in filters conveniently without additional hardware needed.
YouTube Video on the f/3.9 FRA600 Reducer