The Askar 3 inch F3.9 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, its focal length is reduced to 420MM, dramatically increases your field of view and bringing more convenience and possibilities for your visual use or astrophotography. The backfocus is the standard 55MM. It also has a built-in M48X0.75 (2 inch) filter thread that allows you to screw on filters conveniently.