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The Askar new FRA400 F3.9 full-frame reducer reduces the telescope’s effective focal ratio by a factor of 0.7x, thus an original f/5.6 becomes an f/3.9 system, your exposure time will significantly shorten, providing a shorter, more optically “fast” focal ratio to make astrophotography more efficient. It designed for use with FRA400 refractor, providing a 280MM focal length for a wider imaging field of view. It achieves the 55MM back focus, and conveniently threaded for use with M48X0.75 (2") filters.